Saturday, March 28, 2020

Setting/calibrating signal and noise levels for signal/noise level monitoring using the "wsprdaemon" script

The WSPRDaemon script, a work largely of Rob, AI6VN, is designed to automate WSPRNet reporting of signals transmitted using the WSPR protocol - (The script may be found on GitHub, here).  While the WSJT-X program, itself, can report the signals that it hears in terms of callsign, Maidenhead grid locator and apparent signal-noise ratio, the WSPRDaemon script leverages the multi-receiver capability that can exist in various hardware configurations - including the KiwiSDR - which, with the appropriate platform, can receive as many as 15 signals on 15 different frequencies simultaneously.

Comments:
While this discussion is focused on methods and measurements exclusively using the KiwiSDR as that is the hardware with which I am most familiar, where similar capabilities exist, these same methods can be applied to other receive platforms.
Similarly, the methods discussed on this page need not be specific to the software mentioned and may be applied any time receiver signal level measurement is needed.
Care has been taken such that at the Northern Utah WebSDR, all of the WebSDR's receivers' S-meters are calibrated to within a dB or so in the center of their target band using the methods described below.
HF noise floor measurements:

In addition to facilitating the decoding and reporting of WSPR signals, the WSPRDaemon script can also divine the apparent noise floor within the receive bandwidth on some of these platforms.  In the case of the KiwiSDR - where an "S-Meter" reading is available via the remote interface - this reading can be used to derive absolute measurements and such data, taken from multiple locations over disparate geographical locations, can be used to glean information about the noise on the HF bands over time.

In practice, accurate, absolute signal level readings are quite difficult to obtain owing to the issue of meaningful calibration - and the more complicated the antenna system is, the more difficult it may be to get such measurements as one must consider things like:
  • Cabling losses
  • Splitter losses
  • Filter losses
  • Amplification
  • Calibration of the receiver
  • Gain of the antenna (e.g. dBi, dBd)
Fortunately, all but the last on the list may be accounted with a single measurement using a known-accurate signal source.

The "short" version:

The ultimate goal of this exercise is quite simple:
Input a signal of known amplitude into your antenna system - say, -60 dBm - and have the noise logger report that same signal level, +/- your known antenna gain.
The steps below are included in the event you may not be familiar with the methods/techniques involved.

An example receive system:

As an example, the KiwiSDR-based portion of the system in use at the Northern Utah WebSDR (a browser-enabled remote HF receiver found at sdrutah.org) will be discussed and a simplified block diagram of the signal path may be seen below:

Figure 1:
Simplified KiwiSDR Signal Path used at the Northern Utah WebSDR showing several gain blocks and filters.
As noted, the directional coupler is connected in "reverse" as it is being used to inject rather than extract a signal from the signal path.
Click on the image for a larger version.
The components of the receive system depicted above in detail:

As with any antenna system, it begins with an antenna - but for the moment, we will ignore any intrinsic gain - or loss - that the antenna may have.
  • Directional coupler:  At the Northern Utah WebSDR, the Mini-Circuit Labs ZFDC-20-3 is used at the main antenna input.  This device has minimal insertion loss (<0.25dB) on the "through" line and there is a separate port (labeled "CPL") that is coupled at 20dB from the through line.  Typically, this sort of device is typically used to extract a small sample present on the line while minimally affecting it and "in" and "out" ports are labeled for such purposes, but here it is being used in reverse to inject a signal into the receive signal path.
This device is convenient in that it has the advantage that it may be left in line all of the time with negligible effects at HF:  It can put test signals into the receive signal path without affecting off-air performance.  This device is optional in that the same measurements may be obtained without it by connecting the signal source directly to the antenna port - the obvious disadvantage being that the signal path is interrupted - something that we try to avoid on a busy, multi-user system such as a WebSDR.
  • Calibrated signal source:   It should be no surprise that having a known-accurate signal source is an absolute necessity when it comes to calibrating the signal level readings with available levels in the range of -60 to -30dBm being the most useful.  Having access to such devices (such as a Communications Test Set or RF Service Monitor) is helpful -but other means of obtaining a usefully-accurate signal include:
    • An "inexpensive" calibrated signal source like the Elecraft XG-3 - the >$300 price tag (when fully equipped) being the reason for the quotes.
    • A "transfer" measurement from a signal source with a consistent level.  Some antenna analyzers - including the popular "NanoVNA" - can output a level that is fairly consistent.  Coupled with outboard attenuators (a switchable step attenuator and/or an assortment of fixed attenuators) one can use known-accurate test equipment (an RF power meter, signal level meter, spectrum analyzer) to obtain a table of signal power readings for each of the HF amateur bands.
      • Because of the harmonic content of the NanoVNA, it is preferred that a selective means of measuring RF power like a spectrum analyzer be used rather than radiometric detector like a power meter - but the difference is unlikely to be significant.
      • Important note:  The amplitude of the NanoVNA's output port is too high to be directly used as a calibration signal - especially if amplification is used in the signal path.  Expect to need attenuation in the range of 20 to 50 dB to provide a usable signal level:  It's recommended that one use a switchable step attenuator and/or a series of fixed attenuators to get reasonable signal levels.
    • A transfer measurement from a piece of equipment that is likely to be "pretty close" - such as the KiwiSDR itself:  With the default "factory" S-meter calibration level of -13dB the KiwiSDR to which I have access have typically been within a couple of dB when comparisons were made.
      • For calibrating, a signal source (e.g. NanoVNA) would be connected directly to the input of the receiver - via the attenuator(s) - and the S-meter levels noted for different settings on each of the amateur bands.
  • AM BCB Filtering:  In many cases, strong AM broadcast band (mediumwave) signals will be present on the antenna port.  Even if these signals are not strong enough to overload the RF amplifier that follows, such signals can overload the receiving device downstream.  As mentioned later, there are advantages to strategically attenuating strong signals to minimize the total power impinging on the A/D converter of any direct-sampling receiver.
The AM broadcast-band filter used at the Northern Utah WebSDR is a rather complicated affair (and is described in this article:  Managing HF Signal Dyanmics and preventing overload... (link)).  Its main purpose is to reduce the amplitude of several very strong (50kW) local transmitters to prevent the "wasting of amplifier power and A/D bits" on very few signals.
  • Amplification (post AM/BCB filtering):  In general, a 15 dB system noise figure is sufficient to allow the detection of a 10 meter signal above even the lowest expected level of background noise found at this frequency.  For this reason it is suggested that an RF amplifier be placed after the AM BCB filter (to minimize the probability of overload and intermodulation/mixing products)Remember:  Once loss appears in a receive system due to loss/attenuation, no amount of amplification can restore the system noise figure beyond that point.
Such an amplifier should also be placed prior to any RF splitting as much as is practical as any loss - which can be from the feedline, in filters or in a splitter - will directly contribute to the the system noise figure.  In cases where very strong signals may be present on the antenna system (local AM BCB, strong Shortwave Broadcast signals - when the bands open) it is preferable to have an amplifier at this point that has only modest gain - in the area of 12-15 dB:  A high-gain amplifier (>20dB) may be more prone to overload in such conditions and the typical means of preventing this (addition of an attenuator in front of the amplifier) is not recommended as this will increase system noise figure.
  • Limited attenuation high-pass filter:  Any receive system is capable of handling only a certain amount of total signal input power - and the direct-sampling receiver of the KiwiSDR is no exception.  The overload level of the KiwiSDR's 14 bit A/D converter is approximately -13dBm, so the total power arriving at its antenna port should be kept below that level and strategic, selective attenuation of local, strong carriers - often those of AM/mediumwave broadcast transmitters - should be considered - but there is another problem:
The sensitivity of a KiwiSDR is approximately -155dBm (in a 1 Hz bandwidth) over its frequency range which means that in a unity-gain HF antenna system that at 10 meters, the KiwiSDR will be "short" by at least 6-8dB from hearing the textbook "rural quiet" noise floor on that band, requiring 10-15 dB of overall amplification to place the 10 meter noise floor comfortably above that of the receiver's in that environment.  This amplifications has a cost:  With the addition of a 12 dB RF amplifier, the KiwiSDR will now overload at -25dBm.  It is not uncommon for the absolute power level of a single high-power shortwave broadcast station to exceed this level under good band conditions - and there will be many such signals on the bands - not including the wide-band energy from lightning static - that can combine in overall signal power and exceed this overload level.

Were the KiwiSDR being used over a very narrow range of frequencies - such as the case for most commercial direct-sampling receivers - this could be managed by gain adjustment in the signal path, but if the KiwiSDR is to be used over the entire HF spectrum this cannot readily be done.  Instead, one can selectively insert attenuation where the signals+noise are likely to be the highest, namely below approximately 10 MHz and adjust calibration accordingly.
The use of this "limited attenuation" high-pass filter leaves signals above 12 MHz alone but offers significant attenuation (about 12 dB - around 2 "S" units) to signals below 8 MHz.  Because of the relatively high noise levels in the lower HF bands, additional attenuation - and the commensurate increase in system noise figure - can be tolerated without the loss of useful sensitivity.
Note:  This same issue of overload will occur with similar devices - such as the Red Pitaya - if operated as wideband HF receivers, without input band-pass filtering.  Simple math will indicate that even if the KiwiSDR had a 16 bit A/D converter - which would theoretically yield another 12 dB of dynamic range - one would still need to take these same precautions.  It is for this reason why all well-designed direct-sampling receive gear has narrowband filtering that precedes its A/D converter.
Unfortunately, the "limited attenuation" filter fits a niche requirement as is not a commercially-available device, but one may be built without too much difficulty:  Two versions are described here:
    • Revisiting the limited-attenuation high-pass filter - link.  This version is similar to that above, but its attenuation pass range is limited  to the range of approximately 1700 kHz to 12 MHz, allowing the passage of signals in the AM broadcast band and below for the reasons discussed in the article.
    • Because of the simplicity of these filters they exhibit a low return loss (high reflectance) at lower frequencies and  it is recommended that they be preceded with an amplifier to present the input feedline with a more consistent "50 ohm" impedance.
  • Amplification (post high-pass filter):  Where high signal levels are likely to exist it is prudent to distribute the signal gain across several blocks rather than have a single high-gain amplifier near the "front" of the signal path.  An amplifier placed after the "limited attenuation" high-pass filter will see lower signal levels from the very strong lower HF-band signals and be less subject to overload.
  • Splitter:  A 4-way splitter is depicted in figure 1.  A real-world 4-way splitter will have 6.5-7.5 dB loss between the input and any of its output ports which means that for the higher-frequency bands it is particularly important to have established the system gain and noise figure prior to the splitter.  Like any other block of gain or loss, the impact of such a device must be considered when evaluating the system overall.
Analyzing your receive system:
The procedure following this point assumes the availability of a known-accurate signal source:  See the heading "Calibrated Signal Source", above, for more information.

Again, we are characterizing the signal path beyond the antenna and ignoring any gain/loss of the antenna itself for the moment.

A procedure for obtaining the calibration of the receive signal path is as follows:
  • Input a known signal level at the input of the receive signal path.  This signal would be input at the antenna port - or via the "coupling" port of the directional coupler.  This signal should be input at a frequency in the band of interest as calibration values should be obtained for every amateur band to be used.  For WSPR operation it is desirable that one generates test signals within the WSPR passband.
  • Tune in the signal and note the signal level on the receiver.  A known signal level of at least 20dB (30dB is better) above the receiver noise floor and other signals in the receiver passband is suggested to minimize effects of the noise and signals - but the level should be no higher than necessary to avoid overload/compression of any amplifiers - or the receiver itself.
    • If using a directional coupler as suggested, make sure that it is oriented correctly by comparing the signal levels when the generator output is fed directly into the receive signal path and again with signals fed via the coupler:  If its "in/out" is reversed the coupling level will be inaccurate. (Again, for a coupler used for extracting a signal from a coax, the connection will be reversed.)
    • In the case of the 20 dB coupler suggested above, the signal levels reaching the receiver will be 20 dB lower due to the coupling loss than that if the signal generator coupled directly and this should be remembered when taking measurements.
    • If the levels of the test signal are low (less than 20dB S/N) and cannot be increased, narrowing of the receive passband can minimize the effects of noise - just observe the S-meter while making such adjustments to verify that the test signal remains within the passband.
    • Because we are measuring the amplitude of a coherent signal within a finite bandwidth rather than trying to measure the noise power in a specific bandwidth, we must make sure our test signal is strong enough that the power of the background noise is a small percentage of the total.
    • While it is necessary to compensate for detection bandwidth when measuring the noise within that bandwidth, one does not do so for coherent signals as all of the power of that signal is (theoretically) all on a single frequency.  It is for this reason that the measurement obtained of the coherent signal should match the actual power of that signal.
  • Knowing the signal level being input to the signal path, observe the S-meter reading on the KiwiSDR on a per-band basis.  It is not important that the S-meter read correctly - except for the benefit of users connected to the KiwiSDR and looking at the meter - as the levels reported by wsprdaemon will be adjusted in its calibration file.
    • If you are running wsprdaemon and are producing a signal within the WSPR passband, the signal will show up on the graphs after several minutes as a constant level - and this level can be used to verify calibration.
      • The level displayed on the graph will be that of the KiwiSDR plus the amount of attenuation/amplification specified in the wsprdaemon.conf file for that band.  Use either raw data or the interactive Grafana data to read the signal levels precisely.
    • At the Northern Utah WebSDR, the KiwiSDR S-meter calibration setting has been adjusted so that for bands above the roll-off of the high-pass filter (e.g. 20-10 meters) the S-meter read the same as the input signal level.  At lower HF bands, the S-meter reads low by the amount of attenuation of the filter.  Since there is currently only one S-meter calibration point that applies to all frequencies, there is nothing to be done to correct this.
Any adjustment of the KiwiSDR's S-meter calibration should be done before completing the steps below.  If it is changed, the final per-band calibration values will need to be adjusted accordingly.
  • Record, for each band, the amount of RF you are applying at the antenna port (taking into account the coupler, if used) and the S-meter reading on the KiwiSDR.
  • The difference between these two readings will be the amount of gain - or loss - in your signal path.  Again, we are not concerned about the absolute gain of the antenna itself at this point.
    • If the reading on the KiwiSDR is above that of the absolute input level, signal gain is indicated at that frequency.
    • If the reading on the KiwiSDR is below that of the absolute input level, signal loss is indicated at that frequency.
  • We have now characterized the gain/loss of the signal path.  If our antenna had unity (0 dB) gain we would use the numbers that we'd just obtained as an offset in the wsprdaemon.conf file.  For example:
    • If -50dBm were present at the input of the signal path and the KiwiSDR read -38dBm on 10 meters, we would know that we had 12 dB gain at that frequency.  To compensate we would put -12 db in the entry for 10 meters in the wsprdaemon.conf file.
    • If -50dBm were present at the input of the signal path and the KiwiSDR read -52 dBm on 80 meters we would know that we had 2dB attenuation at that frequency.  To compensate we would put 2 dB in the entry for 80 meters in the wsprdaemon.conf file.
    • Again, to verify our calculations we can place a carrier of known signal level within the WSPR passband for the band being tested.  If the measurements have been done correctly the levels on the graph should match those of the level of the RF at the input of the signal path.
Compensating for coax cable losses:

The next step would be to include the losses of any coaxial cable in the system before the point in the signal path where we injected our test signal.  This can be directly measured with a wattmeter and dummy load being used to measure the power lost over its length or - presuming a fairly low VSWR - can be estimated using the manufacturer's loss values on a per-band basis.

These loss values would be added to the per-band numbers in the wsprdaemon.conf file.

Compensating for antenna gain:

With simple equipment, calibrating the signal path for an absolute level is pretty easy - but the gain of the receive antenna at a given frequency is more difficult to determine as very few people have access to a known-accurate antenna range - so we will have to make an educated guess.  Because many HF antennas have definite patterns of of lobes and nulls one will have to decide if the peak gain of the pattern will be used, or some sort of averaged gain value.

Passive antennas:

For simple wire antennas, the gain can be estimated with reasonable (+/- a few dB) accuracy.  For example, a half-wave dipole above typical ground will likely be in the area of 1-2 dBi gain peak while the gain of a commercially made vertical or Yagi antenna might be available from the manufacturer's data sheet.

For multi-wavelength wire antennas such as long wires or rhombics, antenna simulation programs are suggested and may be the only reasonable means of estimating gain - but again, one will have to make a decision on what gain value to apply in light of the likely existence of lobes and nulls.

Active HF antennas:

If the feed from a conventional antenna (beam, dipole) includes an amplifier, the steps mentioned above may be used to estimate its gain and one need only add the loss of the feedline and gain of an amplifier to obtain the desired offset for the wsprdaemon.conf file.

Electrically-short active antennas such as the PA0RDT mini-whip pose a particular problem.  They are essentially a capacitive plate (or wire) that couples RF from the "aether" - but the amount of coupling (which correlates with the apparent gain of the antenna) can vary widely, the determining factors including:
  • Height above "ground".  In general, the higher above the ground - earth or metal - the more signal such an antenna will intercept.
  • Type of "ground".  The ground above which the antenna is mounted could be earth (dirt. soil) which is lossy or it could be a metal roof (much preferred!) to which the feedline's ground is bonded.
  • Size of the "antenna".  The traditional PA0RDT antenna uses a PC board plate for coupling - but there are many variations on what size of coupling conductor - wire or plate - is used, all of which can affect the apparent gain.
  • Gain response of the amplifier with frequency.  In most - if not all - implementations of an E-field whip are subject to gain drop-off with higher frequency and this is not easy to measure directly and determining a suitable number is almost entirely guesswork.
    • Unfortunately, such drop-off conspires with the high RF levels at low-medium HF frequencies.  With a broadband, direct-sampling receiver like the KiwiSDR, overload will surely occur if enough gain is added to the system to attain the natural noise floor at the higher HF bands (e.g. 10 meters) unless strong compensation (e.g. something like the high-pass filter described above) is used.
However one might obtain the antenna gain, this value would be applied to the wsprdaemon.conf file:  Positive gain would mean a reduction of this number by the dB value while negative gain would mean a commensurate increase in this value.

What to expect:

In theory, the noise floor one sees should generally reflect that predicted in the ITU-R data, reproduced in figure 2, below:

Figure 2:  "Typical" noise floor for various radio environments.  Because the above chart is based on a 500 Hz bandwidth, one would subtract 27dB from its vertical axis to scale to an equivalent 1 Hz noise bandwidth when measuring noise power.
Click on the image for a larger version.
To be sure, the values depicted in Figure 2 are very generalized - and one would be lucky to approach the values of "quiet rural".  In general, these value depicted above would be most representative when the respective frequency band is "dead" (e.g. daytime for the lower bands, nighttime for the higher bands) with a "quiet sun".  While usable only as a general guideline, one should be suspect of their calibration settings if the "quiet band" noise floor values they measure on their system is markedly lower than those of the chart above.

Cross-checking noise level measurements with the receiver:

It is possible to check the noise level measurement provided by wsprdaemon using the receiver in question, but to do this two conditions must be satisfied:
  • You must know the receive bandwidth.  For most SDRs this is pretty easy:  The "bandwidth" setting - which is typically at the -3dB or -6dB points - is "fairly close".
  • The passband must contain ONLY noise and no other signals.  On the WSPR frequencies this may not be possible, but measuring on a "nearby" frequency (up to a few 10s of kHz away, provided that such measurements aren't affected by narrow band-pass filters that might be present).
    • Wider (SSB) bandwidths are fine, but it may be necessary to user narrow bandwidths to avoid signals within the passband.
 The method of making this measurement is approximately thus:
  • Note the average signal level reading (in dBm).
    • Since we are looking for what amounts to a rolling average over time, the readings of occasional noise bursts must be considered which means that a "fast" AGC should be used and one should not be looking at any "peak" readings.
    • Since we are likely doing a "sanity check" simply staring at the signal meter and guesstimating the average.
    • It is best to do this during "quiet" conditions on a given band - that is, when it is dead:  Trying it on 80 meters during the night where there may be a lot of summer static is not recommended.
  •  Note the receive bandwidth.
    • On an SDR, this is likely to be the "bandwidth" figure which is probably taken from either the -3dB or -6dB points - which will be "close enough.
  • Calculate the bandwidth noise power ratio.
    • Because wsprdaemon's noise power measurements are scaled for 1 Hz detection bandwidth, the reading obtained on any receiver with a wider noise bandwidth than that will be much higher and the reading (in dB) must be offset by that amount.
    • As an example:  If you are using a 500 Hz receiver bandwidth (e.g. a CW filter) you need to account for a 500:1 power ratio which is:
      • 10 * log10(500) = 26.99 = 27 dB - that is, one would subtract 27 dB from the average level that you observed.
    • In other words, to scale to the 1 Hz wsprdaemon bandwidth:
      • Your signal level - (10 * log10(RX bandwidth in Hz))

Conclusion:

While it is unlikely that the typical amateur will be able to be absolutely confident of the accuracy of their absolute HF noise floor measurements, the steps above will greatly limit the uncertainty of the system's measurement to the the properties of the antenna itself.

[End]

 This page stolen from ka7oei.blogspot.com



Thursday, January 16, 2020

Improving the stability and performance of the FiFi SDR receiver

At the Northern Utah WebSDR link, among one of the several receiver configurations is that where a "SoftRock" receiver is used with a sound card.  This combination works very well - far exceeding in overall performance, especially dynamic range, almost any available "wideband" receiver including the SDRPlay, Red Pitaya and KiwiSDR Note 1 with the caveat that only a bandwidth equal to the sample rate of a sound card - 192 kHz or so maximum - can be covered "per band".
Figure 1:
One of the three FiFiSDRs obtained for use at the Northern Utah WebSDR.
Click on the image for a larger version.

Problems with the USB Sound Cards:

Up to now, we have been using a combination of plug-in (PCI, PCIE) sound cards, using USB sound cards when the number of such receivers exceeded the number of plug-in slots on the computer.  Among the few affordable USB audio devices that can sample at 192 kHz are the Asus Xonar U5 and U7 (including the MK 2).

These devices work very well for the task - when they work:  After nearly 2 years, only two of the ten U5 and U7 devices that we had acquired over that period still work, the majority having failed when the USB interfaces would fail to negotiate at full USB 2.0 speed (if they negotiated at all!) after a few months of operation - often after a reboot.  Unfortunately, an Internet search revealed that this is not an uncommon problem and a plausible explanation as to the reason for these failures - or a fix - was not to be found.

An alternative:

Rather than spend more money on the unreliable Asus U5 and U7 USB devices - most of which we had gotten on EvilBay, originally in proper working order - we decided to switch to the "Fifi SDR" device from Box73.com in Germany.  Originally introduced around 2010, the current "Version 2" increased the bandwidth from the original 96 kHz to 192 kHz - and rather than just a sound card in a box, the FiFiSDR includes an entire synthesized "Softrock" receiver with decent performance and best of all, they cost about the same as a brand new 192 kHz capable USB sound card.  Several popular WebSDR systems - including KFS in Half Moon Bay, CA - use these devices and have reported good performance and reliability.  If they had been available when we were acquiring the equipment for the Northern Utah WebSDR, we would have started using them earlier.

Three FiFiSDRs were ordered - exactly enough for our needs - and upon arrival, I assembled them (a bit of soldering and mechanical assembly) and began to test them.  For whatever reason my Windows 7 machine at my workbench steadfastly refused to recognize the Fifi's sound card interface, but my Windows 10 laptop did and after a few missteps - mostly related to the program I was using to interface with it (HDSDR) having been previously configured for different SDR hardware - I got all three up and running.

Initial impression:

My initial impressions of the performance of the three FiFiSDR were generally good - ignoring the "elephant in the room" discussed below:  The receiver sensitivity, although varying by 2-3 dB between receivers, was within advertised specifications and once they had been powered up for several minutes the frequency was quite stable (within a few Hz).  Immediately, I noticed a few low-level CW spurious signals, but these were at or below the microvolt level and would likely be submerged in the noise floor - at least on the lower bands.

Because the sound codec was integrated within the receiver itself, the center-frequency (so-called "Zero Hz") noise was quite low because a possible pick-up point (e.g. a cable going from the output of the SoftRock receiver to a sound card) has been eliminated.  I did notice a fairly strong artifact at or near zero Hz - likely a DC offset with a bit of 1/F noise - but this is typically removed by a low-frequency high-pass filter in software and is not likely to be an issue.

I did notice two artifacts typical of "SoftRock"+sound card receivers:
  • Under no-signal conditions, the noise floor would rise by several dB at "high audio" frequencies as manifest by a slightly "lighter" waterfall at the extreme low and high ends of the 192 kHz passband.  This is quite typical of sound cards and has been observed on nearly every sound card that I have used.
  • Under conditions where the external (ionospheric) noise exceeded that of the receiver's noise floor, there was a bit of "droop" at the extreme low and high ends of the 192 kHz passband.  I've noticed this effect on nearly every softrock-type receiver and attribute it largely to signal drop-off in the audio chain at high audio frequencies.
I did see something that alarmed me:  Unstable spurious signals that drifted about (the "elephant" mentioned previously) but a "fix" for this problem is pretty easy and is described later.

Static sensitivity!

I'd seen a mention or two that the Fifi SDRs would occasionally "lock up" - but it didn't seem to be a common theme in the groups online - but once I got the them up and running, I saw two things that concerned me:
  • When I touched the metal case and had no antenna connected the receiver's noise floor went way up.  This is bad news - particularly if it is to be installed in an electrically-noisy environment - like anywhere near a computer.
    Figure 2:
    The sole "official" case-to-board grounding point is at the corner near
    the 3.5mm jack
    Copper foil was wrapped over both the top and bottom and soldered
    to the board's ground on both sides with a bit of excess
    folded over on the end to provide a connection to the end plate.
    (The solder connection was re-done, but I didn't get a picture.)
    Click on the image for a larger version.
  • If I had even the slightest amount of static electricity on my body the Fifi SDR would crash when I touched it and refuse to come back to life until I unplugged the USB and plugged it back in again.
Upon observing either of these, my suspicion as to the problem was verified with an ohmmeter - the metal case was, in no way, connected to the internal PCB.

Inspection revealed why:  Not only was the case very heavily anodized, the two boards fit somewhat loosely in the slots inside - and there was only a single common ground in one corner of the main board.  Even if the board was snug, it probably would not have made electrical connection through the case's oxide coating.

Remedy:

Clearly, I needed to find a simple way to bond the board to the case.  The receiver's  main board's use of a single-point ground seemed reasonable - particularly when one pairs a computer with a very sensitive receiver as one must carefully avoid on-board ground loops - so I resisted any temptation to "bond it everywhere" - at least not without careful testing.
Figure 3:
The point where the copper foil makes contact with the end plate.  Note that
the ends of the drawn-aluminum case have had the oxide layer removed to
bare the metal:  The same was done on the end plates to allow the copper
to make contact.  The bared ends of the case and corresponding parts of
the end plates were coated with a light layer of anti-oxidant.
Click on the image for a larger version.

First, I used a rotary tool with a wire wheel to remove the anodization (clear oxide coating) from around the edges of the end panels as well as the very ends of the drawn aluminum case so that when the screws were installed, they would have metal-on-metal contact.

The next step was to provide a connection from that ground in the corner to the case - and I did this by wrapping the edge of the board in that corner  (Figure 2) with copper foil and soldering it, leaving a bit of excess to wrap around the end of the board (Figure 3) - the idea being that it would be compressed by the now-bare aluminum end panel and make connection to the rest of the case.

Because bare aluminum quickly forms its own insulator when exposed to atmospheric oxygen, a thin layer of anti-oxidant compound (e.g. "NoAlOx" or "DeOxIt" - both used in electrical wiring) was applied to the bared aluminum on the ends of the case as well as where the copper would press against it:  This would prevent re-oxidation and the loss of connection over time and with exposure to air and moisture.

Result:

Upon reassembly, the end of the board with the copper connection was tightly pressed against the inside of the end panel and there was a low-resistance connection between the boards inside and the case.  Because of this effort, not only does the receiver noise floor not go up when I touch the case, but I can give the unit a pretty good "zap" with a static spark and not have it affect the operation of the device!

If I'd had the time to do so, I would have tested the efficacy of additional board-to-case ground points, making sure that these additions did not reduce the performance of the receiver.

Comment:
Although not instructed to do so by the assembly guide, installing the nut on the 3.5mm audio connector may the connect the board to the case (via the end-plates) at that point. As they are, the connector's body is not is not long enough to protrude very far through the end plate and only a few threads were presented.
Figure 4:
On the top, an 80 volt gas-discharge tube.  Holes were drilled on the top to
provide connection and mounting.  The tube is bent away
from the case to prevent it being shorted to the case.
When drilling, take care to avoid intercepting any traces on either
side of the board.
Click on the image for a larger version.
Because of the thickness of the end plates, the installation of the nut will prevent proper insertion of a cable into the connector - but since I wasn't planning to use that jack, I installed it anyway.

The "floating" antenna jack:

Presumably to prevent circulating currents between the antenna system and the "ground" of the computer (via the USB cable) the antenna jack is coupled to the receiver via a 1:1 transformer.  Having a "floating" antenna connector made me nervous:  If one were to connect a FiFiSDR to an ungrounded wire antenna, wind static could easily cause high voltage to appear on the antenna connector which might not only cause a shock, but if it arced to ground somewhere - possibly within the receiver - it could damage the receiver's RF amplifier and/or be conducted to the USB interface to the computer where it could cause the FiFiSDR and/or computer to crash or worse, cause damage.
Figure 5:
The 150k resistor on the bottom of the board to drain static.
Any value between 47k and 220k would suffice.
For another modification (described below) 0.1 and 0.001
capacitors were soldered across this resistor for RF bypassing.
Click on the image for a larger version.

While I am not advocating using any antenna without appropriate grounding, I do know that it does happen and out of principle, I added circuitry to mitigate the risk:  The addition of a 150k resistor to prevent the accumulation of charge and an 80 volt gas-discharge tube between the RF and system ground.

Even if one does not add a gas-discharge tube, I would certainly advocate the addition of the drain resistor!


* * * * * * * *

Spurious signals in the receiver - the "elephant":

Having gotten all three receivers operational, I noticed something else that was alarming:  In two of the three receivers I could see, near the upper and lower edges of the passband (192 kHz sampling rate, 75 kHz and farther, symmetrical about the center frequency) some "ragged" signals that drifted about:  The third receiver also showed these same spurious signals, but they were much weaker, closer to the center frequency indicating that the same problem was evident, but likely farther out of the +/- 96 kHz passband of the receiver's sound card and showing up via aliasing.  Even if though the spurious signal on this third receiver was weak, I decided that was likely to mix with existing signals and cause additional, undesired signals to be produced within the passband, and would likely be the case with the other two receivers.

The spurious signals on the two worst receivers (example in the upper half of Figure 6) were fairly strong, about "S-9" in strength, making their existence unacceptable.  A quick check of the FiFiSDR wiki and trouble ticket system revealed that there were at least two tickets (#324 and #332reporting this issue - but both were years old and were still open with no suggested resolution.

Figure 6:
A screen shot from HDSDR showing the spurs before the modification (top half) and after the modification (bottom half).
These spurs are symmetrical about the center frequency (red line in the middle) indicating that they are NOT at RF, but rather at a point beyond the RF mixer in the audio chain.  The amplitude of these spurious signals make this receiver nearly unusable due to their strength.
Because of the rather low apparent signal strength, it is most probable that the actual frequency of these oscillations is not in the 0-96 kHz range, but much higher and being made visible because of the finite attenuation of the codec's low-pass filtering and aliasing of the A/D converter's sampling rate.  
Click on the image for a larger version.
The symmetrical nature of this signal - and the fact that its nature was completely independent of the receive frequency - indicated that the origin of this signal was not at RF, but was within the audio chain or related to the FiFiSDR's power supply.

Careful observation showed something else:  As an applied signal within the passband of the receiver (e.g. +/- 90 kHz or so of the tuned frequency) increased in amplitude above approximately -70dBm, these spurious signals would start to  "noise up" and disappear - finally vanishing by the time the signal achieved -30dBm.  Because the signal was affected by signals in the audio chain, this observation took the onus off the likelihood of the power supply oscillating, pointing directly at the audio chain indicating that whatever was causing it was directly in the audio signal path.

Figure 7:
The addition of the 470k resistor to the "ADC1LP" pin of the codec.
The ground plane was scraped and the resistor soldered between it and
the capacitor(s) as shown, taking care to avoid shorting the "ADC1LP"
line to ground.  The corresponding (original) resistor on the "ADC1RP" line
is the upper-most resistor at the left marked "474", upside-down.
Click on the image for a larger version.
Wielding an oscilloscope, I started probing the audio chain - but I could see nothing obvious in terms of unusual signals - but I noticed that when I touched the probe to pin 39 ("ADC1LP") of the audio codec - an Analog Devices AD1974 - the frequency would shift slightly. 
Touching a voltmeter probe to this pin I observed that this spurious signal would disperse widely - as if frequency-modulated by the AC mains field on the workbench - but the same did not happen when I touched the voltmeter probe to pin 41 ("ADC1RP") indicating that the problem was only on the "left" channel of the codec.  Disconnecting the DC blocking capacitor from IC5, the audio amplifier, the amplitude of this oscillation remained the same, shifting frequency very slightly:  This implied that the problem was the AD1974 itself.

At this point I noticed something else:  The voltage on pin 41 ("ADC1RP") was a few 10s of millivolts lower than that on pin 39 ("ADC1LP") - something that should not occur as both were presumably biased from the same internal voltage reference.  I then observed that there was a 470k resistor between the traces connecting pin 41 and ground - but this resistor was missing on pin 39.

On a hunch I added a 470k surface-mount resistor to ground at the bypass capacitors connected to pin 39 and found that the spurious signal disappeared.  Apparently, the designers of the Version 2 of the FiFiSDR had observed a similar problem and added the 470k resistor to "ADC1RP" - but did not do so on "ADC1LP".

This "fix" worked on all three receivers.


Improving common-mode (longitudinal) isolation:

(Sorry, no pictures at this time.)

One thing I noticed about this receiver was that its antenna input was transformer-isolated from the case.  Ideally, this is a good thing as it can reduce any ground loops that may contain circulating AC or DC currents which, unless everything is well-bonded to a common ground, may cause problems (e.g. hum on audio devices, potential USB instability).  Unfortunately, no transformer is or can be perfectly balanced - and this could be demonstrated on the FiFiSDRs by simply touching the outer shell of the BNC connector and observing a slight noise increase when in an indoor RF-noisy environment - even after the case grounding issue discussed above was solved.

To quantify this imbalance I took some measurements, applying an amount of signal between the FiFiSDR case after bonding it to the circuit board as described above and the shell of the BNC connector, observing the amount of signal that was required to achieve an "S9" reading.  Doing this at both 5 MHz and 29 MHz, I obtained the following results in terms of common-mode (longitudinal) isolation:
  • 5 MHz:
    • Isolation = 29dB unterminated
    • Isolation = 50dB terminated at 50 ohms
  • 29 MHz:
    • Isolation = 24dB unterminated
    • Isolation = 35dB terminated at 50 ohms
Whether or not this is acceptable in your situation is something that you will have to decide - but I chose to make a minor modification:  The addition of a 0.1uF and 0.001uF capacitor in parallel (two capacitors being used to provide low impedance from low to high frequencies) with each other (this was in parallel with the 150k resistor depicted in Figure 5) to bridge the "RF Ground" and the system ground:  Unlike connecting the two grounds together with a jumper, this would still provide low frequency DC and AC isolation.  The result was that there was no longer a significant difference between the readings when the BNC connector was terminated or unterminated.

With the addition of the capacitors, the isolation improved to about 50dB on both frequencies - terminated or not.  I was hoping for even greater improvement than 50dB, but I suspect that because the case-to-board mounting occurs in only one place, in a corner away from the RF connector, circulating currents were flowing across the board.  It is possible that bonding the system ground to the case near the antenna connector would have improved this - but I did not have time to test this and make sure that it did cause significant degradation.

* * * * * * * * * * * * * * * * * * * * * *

Note 1:
Unlike some of the inexpensive "Wideband" receivers based on the RTL chips (e.g. RTL-SDRS - which have just 8 bit A/D converters) higher-end receivers that have greater simultaneous bandwidth (SDRPlay, Red Pitaya, KiwiSDRs) have greater bit depths - typically 12-14 bits, offering greater dynamic range.

Even a higher bit-depth wideband receiver can be at a disadvantage compared to the combination of a SoftRock and sound card:  Not only does the 16 bit depth of a sound card offer more dynamic range, but the lower operational bandwidth (192 kHz maximum for a sound card based receiver) means that there is less overall "energy per Hz per bit" impinging on the A/D converter than a MHz-bandwidth A/D converter.
* * * * * * * * * * * * * * * * * * * * * *

Comment:  An attempt was made to post the solution to the problem of the spurious signals to the FiFiSDR ticket system, but the post was rejected by the system.  I have not had the time to register with the site.

This page stolen from ka7oei.blogspot.com

[End]


Thursday, December 26, 2019

Using TV (F-connector) 75 ohm splitters and taps in 50 ohm systems on the amateur HF, VHF and UHF bands

I recently posted several articles about using commercially-available splitters link - and making one's own splitters - link - particularly for the HF frequencies and below (e.g. 30 MHz, down to a few 10s of kHz).  A comment was posted asking about how useful inexpensive 75 ohm "TV and satellite" type splitters might be for amateur radio use.
Figure 1:
The assortment of 75 ohm TV and satellite splitters and
taps tested in this article.
Click on the image for a larger version.

Implied by this question is the use of these devices in receive-only or small signal applications:  They cannot be used for transmit purposes as putting even 100 milliwatts through one of these devices is likely pushing its power-handling capability.

I've used these devices in 50 ohm circuits before - typically for VHF and UHF (2 meters, 70cm) where, along with some attenuators, combined the outputs of multiple signal generators to do "multi-tone" testing of receivers - but the question seemed to be a good one.  Rummaging around, I gathered a bunch of devices of various manufacturers and decided to test them for insertion loss and port-to-port isolation.

Note:
Please do not ask questions like "How well does a 'brand X' splitter work over the [fill in the blank] frequency range?"
There have been thousands of makes and models of these devices sold around the world over the past several decades and I simply am not able to find, locate, and measure more than the tiniest fraction of devices that have been sold.  The information given here is expected to be generally representative of the devices available from reputable manufacturers and distributors - but your mileage may vary.
Limitations of the measurements taken:

Because my VNA (DG6SAQ WVNA) was constructed for use with 50 ohm systems (the changing of  both internal hardware components and software would be required for "proper" analysis of a 75 ohm system) I was able only to analyze them in that context - but because the question was about using them in amateur radio service - which presumes a nominal 50 ohm system - I believe that the results are still useful within the limits noted in this article.

Because the emphasis of the question was interpreted as being for amateur-band frequencies likely to be encountered by the average user, the measurement range was limited to frequencies below 1 GHz - in some cases down to 100 kHz.  The nature of the equipment and methods (e.g. 50 ohm test equipment and cabling, the use of inter-series adapters, etc.) used to test the splitters and taps increasingly limits the usefulness and accuracy of these measurements at frequencies above that of the 70cm amateur band (above 450 MHz).

The variety of splitters and taps available:

There are literally thousands of brands and models of TV/Satellite splitters and taps available on this planet - some of them from recognizable names, but most not.  For those devices from sources that might be suspect (e.g. not "name" brands from reputable suppliers) you are on your own to determine the suitability of those devices for your purpose.

Although not intended as an endorsement per se, it has been observed that devices marketed by Holland Electronics appear to consistently meet their stated specifications and is one of the few brands that is likely available worldwide from a number or different sellers - including Amazon - and major suppliers of electronic components and TV/satellite supplies.

Over the years I have seen many dozens of brands and models of these devices - and the vast majority of them are what they are purported to be, but I have run across some devices that claimed to be splitters, but were simply a box with wires connecting the ports together.  In many cases, the casual user would not have noticed anything amiss, but using several of these faux devices in a larger system would certainly result in cumulative signal degradation (e.g. "ghosting" of analog signals, degrading of quality - but not necessarily signal strength - of digital signals).

General types of devices:

There seem to be three general types of these devices out there:
  • "TV" and/or "VHF/FM/UHF" and/or "CATV" - These devices are typically designed to operate over the range of off-air TV stations across the world and the frequencies typically found on receive-only cable TV (with no Internet), encompassing the frequency range of about 40 MHz through 700 MHz, more or less.  While useful for use on the amateur bands from 6 meters through 70cm, inclusive, their usability on HF or above this range is limited as noted in the testing, below.
  • "Satellite" splitters - These devices are typically designed to operate starting at about 900 MHz, often extending to 1500 or as high as 2500 MHz, depending on the vintage and intended use.  These devices are not usable on the 70cm amateur band frequencies and below.
  • "TV/CATV/Satellite"- These devices are of a bit more recent vintage and are designed to accommodate a very wide range of frequencies - often from about 5 MHz through and above 2000 MHz - a band that includes off-air, cable and "L-Band" satellite signals - plus the "reverse" channels (sometimes called the "T" channels) often used by "cable Internet" modems that reside below 45 MHz.  These are the most useful to amateur service and can often be used on HF through 70cm.
If you do not see a specific frequency range noted on the device itself, assume the worst-case, smallest frequency range that covers that usage category - unless you can test them yourself.

* * *

General findings

For the TL;DR types, here is a summary of the results of the measurements described in more detail farther down the page.


Using 75 ohm devices in 50 ohm systems:

The most obvious issue is that TV-type consumer devices are almost universally equipped with type "F" connectors which means that one must use either an adapter or use a cable with an attached "F" connector.
Figure 2:
Left to right:  Two BNC female to male F connecitrs with an
F-type 75 ohm terminator on the right.
Click on the image for a larger version.

For receive-only systems, it's not too uncommon to simply use a 75 ohm cable like RG-6 - which is quite low loss and very inexpensive - to connect a 50 ohm antenna to a 50 ohm receiver.  The effects of this apparent "mismatch" are typically minimal as most receivers are only "approximately" 50 ohms, anyway.  In theory, the use of 75 ohm cable on 50 ohm devices will result in a 1.5:1 mismatch and commensurate losses, but this sort of mismatch is commonly observed on many antenna systems that are ostensibly designed to operate at 50 ohms and is usually of minor consequence.

When using an inexpensive cable like RG-6, it's worth noting that most of these cables use copper-coated steel (CCS) center conductors which may have implications for DC resistance of power is being sent on this cable (for a preamplifier, converter, controls) as this type of cable will have far more total resistance than one with a solid copper center conductor.  Copper-coated steel center conductors may also have implications in terms of skin effect at low frequencies (low HF and below) - but this is beyond the scope of this article - see, instead, this article by Owen Duffy.  There exist cables with copper-coated aluminum (CCA) center conductors that have lower DC resistance that CCS cables, but they tend to be more fragile due to the tendency of the aluminum center conductor to become brittle with flexure.

The device itself (splitter, tap) is designed primarily for 75 ohms and this means that its performance will be somewhat degraded in a system that is "completely" 50 ohms (e.g. 50 ohm cables with F-connector adapters) but these effects are largely as follows:
  • The "through" loss may be slightly higher.  In the case of a 2-way splitter, the ideal loss will be 3dB - but even at the proper impedance, it will be slightly higher than this due to component losses, typically in the area of 3.5 dB.  Practically speaking, the main effect of using a 75 ohm splitter in a 50 ohm system was a slight change (only a few tenths of a dB) in the loss.
  • Reduced isolation between ports.  The most obvious effect on splitters was that the isolation between ports (e.g. the "out" ports of a 2-way splitter) was reduced.  Compared to some specialized splitters, the isolation of inexpensive, consumer-grade "TV" splitters is lower overall.  As can be seen from the graphs, below, operating in a 75 ohm system resulted in better isolation - sometimes over 40dB at certain frequencies - but this assumes that all loads and sources are well-matched to 75 ohms, something that is not likely to be the case in a real-world installation.  Typically, isolation reduced to something in the 20dB area when operated in a 50 ohm system.  In many cases, this is "good enough".
  • In splitters and taps, resistors are major components in determining their "native" operating impedance.  For example, a 75 ohm splitter or tap, depending on design, may have a 150 ohm or 37.5 ohm (2 times and one-half 75 ohms, respectively) resistor contained internally.  In theory, changing this resistor to a value appropriate for 50 ohms (typically 100 or 25 ohms) would optimize performance at 50 ohms - but doing this may or may not be worth the trouble. 
In short:

Unless your situation requires precision, the use of inexpensive, TV-type splitters and taps of the types described on this page will yield "reasonable" performance over the design frequency range - provided that the device is constructed as described by a reputable manufacturer.

The use of a (nominally) 75 ohm device in a 50 ohm system will require using connectors that are not normally used in 50 ohms systems (typically "F" connectors) which means that adapters of some sort will be needed - the expense, bulk and inconvenience of which must be considered in the overall design.

Finally, note that the above comments are for the general case:  Remember that your needs, requirements and results may vary and that you must do your own analysis and testing to verify that such components are appropriate in your specific case.

* * *

Plots of various devices:

Below are selected plots of devices representative of the types on-hand.  In general, devices with similar stated ratings performed in the same manner.  In all of these plots, the insertion loss is represented by the blue line while the complex impedance data is depicted on a Smith chart in the middle:  Numerical data at the frequencies indicated by markers is seen in the lower-left corner of the screen.  Again, remember that at higher frequencies, the nature of the 50 ohm test system, connecting cables and adapters will increasingly skew the results - particularly those depicted by the Smith chart.

The interpretation of a Smith chart will not be covered here, but there are many online resources that describe its use including this video in a series on this topic by W2AEW on his YouTube page.

A "satellite" splitter:
Figure 3:
The "through" loss of the HFS-2 splitter represented by the blue line
across the top.
Click on the image for a larger version.

This device - a "Tru Spec HFS-2" is representative of those intended for use on an (older) L-band system found in satellite receive systems, having on its label a "900-1500" MHz frequency range.  As noted above, the limitation of the measurement set-up made measurements above the 70cm amateur band (in the 440 MHz area) suspect - but the object here was to see if it was usable below that range.

At initial glance, the "through loss" of this device below 900 MHz (Figure 3) might seem to indicate that it worked below this frequency, but notice that at lower frequencies (below 50 MHz) indicates a loss less than 3dB indicating that it is not working as a proper 2-way splitter.  A look at the isolation plot (Figure 4) tells more of the story.
Figure 4:
Isolation between ports of this splitter.
Click on the image for a larger version.

As can be seen, at about 900 MHz and above, the apparent isolation between ports is reasonable but at 2 meters (146 MHz) it is only 3dB verifying the fact that at these lower frequencies, it less a proper splitter, but more equivalent to a device where the three ports are connected with a piece of wire.  The apparent isolation increase at low HF is more likely an artifact of its construction - the insertion loss being below 1 dB (in Figure 3) verifies this.

In short, these "Satellite only" splitters aren't really useful on TV and CATV frequencies or the amateur bands 70cm and below.

A "TV" splitter:
Figure 5:
The "through" loss of the Archer splitter.
Click on the image for a larger version.


I tested several splitters that were intended for general VHF/UHF/FM use - one of these being an "Archer" (Radio Shack) two-way splitter being typical of that type.  The implied frequency range is from at least 54 MHz to 700 MHz - the extent of the cable TV, FM broadcast, and off-air VHF and UHF TV frequencies at the time it was made.

Figure 5 shows the measured "through" loss in a 50 ohm system.  Compared to a plot done at 75 ohms (not shown, using resistive matching) the insertion loss barely changes across the frequency range.  In both 75 and 50 ohm systems, at least at 2 meters, down to 20 meters (14 MHz) seems to be "ok" - but the "dip" in the 3-4 MHz area - and the fact that the attenuation below it drops below 3dB - indicates that it's not likely acting like a splitter at these lower frequencies.

Figure 6:
Port to port isolation at 75 ohms for this splitter.
Click on the image for a larger version.
Figure 6 shows the port-to-port isolation at 75 ohms and we note that in the "low" and "high" VHF band (U.S. channels 2-13 - which more or less includes the 6, 2 and U.S. 222 MHz amateur bands, that the isolation is quite decent - well above 20 dB.

From this plot we can see that the "dip" in the 3-4 MHz area seen on Figure 5 is quite telling as the port-to-port isolation is pretty much gone below this frequency

Figure 7:
Port to port isolation in a 50 ohm system for this splitter.
Click on the image for a larger version.
The plot of Figure 7 shows what happens if the splitter is operated in a 50 ohm system.  The main effect is that the port-to-port isolation is reduced - being on the order of 15 dB or so from the 20 meter band through the 2 meter band (14 MHz - 144 MHz).

From this we can conclude that this splitter is quite usable from the middle of the HF spectrum through at least 2 meters - and is probably usable through 70cm.


A "TV/CATV/Satellite" splitter - preferred for HF use:

Figure 8:
Holland HFS-2P through loss in a 50 ohm system.
Click on the image for a larger version.
I have on hand several splitters that have on their label a frequency range that starts at (typically) 5 MHz with a high end of between 600 MHz and 2450 MHz.  The reason for this extended "low end" is likely due to their being designed for use in systems that have "Cable Internet" where the return (upstream) signal from the user's modem to the cable system are likely to be in the 5-50 MHz (or, possibly, a bit higher) range.  The plots included are those of a Holland Electronics HFS-2P which is a 2-way splitter/combiner that has a stated range of 5-2050 MHz and the results of this device are typical of that type.)

Figure 8 shows the "through" loss in a 50 ohm system showing a reasonable insertion loss (4 dB or below) from below 40 meters (about 5 MHz) through at least 70cm (440 MHz) - but again, the limitations of the measurement set-up make readings higher than this a bit suspect.

Figure 9:
Port-to-port isolation at 50 ohms.
Click on the image for a larger version.
Again knowing that the "isolation" measurement is the way to get the "true" story, port-to-port isolation in a 50 ohm system is depicted in Figure 9.

This verifies - to the extent that the test set-up can - the 5-2050 MHz range showing that the port-to-port isolation from 5 MHz to 1 Ghz is well over 15dB.  A port-to-port isolation measurement at 75 ohms (not shown) is slightly better (by a few dB) over the same range.

The combination of Figure 8 and Figure 9 show that this device may be usable down to the 160 meter band (1.8 MHz) provided that a slight amount of extra insertion loss (about 1dB) and lower isolation (approximately 12dB) can be tolerated.    (The Holland HFS-2D has characteristics similar to the HFS-2P down to 1.8 MHz.)

Figure 10:
The through loss, the other 7 ports being terminated with 75 ohm F-type
connectors.  The insertion loss is reasonable - between 10.5 and
11.5 dB over the range of 1.8 to 450 MHz.
Click on the image for a larger version.
An 8-way splitter:

The final splitter to be tested was the Holland Electronics GHS-8 8-way splitter-combiner.  Often, splitters with an even number of outputs greater than two contain multiple two-way splitters which means that this 8-way splitter might contain seven such devices - but I didn't break it open to check.
Figure 11:
The port-to-port isolation between two adjacent ports with the "in/out"
port and unused ports terminated with 75 ohm "F" loads.  The apparent
isolation is on the order of 35dB from 1.8 through 450 MHz - but this would
likely drop to something closer to 20dB.
Click on the image for a larger version.

Figure 10 shows the typical "through" insertion loss with the seven unused ports being terminated with 75 ohm "F" type terminators:  I don't have enough F-male to BNC-female adapters on-hand to terminate the 7 ports at 50 ohms - but if one were going to use one of these devices, it's probably more convenient to use F-type terminators on the unused ports, anyway.  The typical "through" loss is measured to be about 10.5-11.5 dB - slightly higher than the predicted "ideal" 9dB insertion loss, but typical for these devices.

The port-to-port isolation was also measured and the use of 75 ohm terminations on the other ports and the "common" in/out port likely improved this:  The isolation would likely be significantly worse if all ports were at 50 ohms, for the same reason as the other splitters tested.

Based on these readings, this device is useful down to 1.8 MHz and up through 2 meters - and probably 70cm.

Figure 12:
Coupling coefficient at 50 ohms for this tap
Click on the image for a larger version.
A TV-type signal "tap":

Likely unfamiliar to many, a signal "tap" is a very useful device in multi-drop TV installations found in hotels, hospitals and other larger buildings.  Unlike a splitter - which usually divides a signal equally to its output ports - a "tap" will siphon only a certain amount of signal off the cable and leave the majority of it intact - which is very useful for systems such as those in a hotel or hospital to distribute and split a signal hundreds of times to serve all of the devices.

In some ways it can be considered to be similar to a part of an SWR bridge where only a small amount of signal is sampled - and in only one direction - allowing the majority of the original signal to pass with minimal loss.  Several taps - all from Holland Electronics - were tested as they were what was on-hand and the "DCG-6SB" is represented in the plots. 
Figure 13:
The "reverse isolation" loss of the tap (e.g. turned "backwards") with a
50 ohm termination.
The reverse isolation is described as being the absolute amount of isolation
(e.g. that in the chart above) minus the coupling coefficient which means
that the actual forward coupling loss - which means that using Figure
12, we know that the actual reverse isolation is about 7 dB lower than
indicated by the graph above.
Click on the image for a larger version.

Figure 12 shows the "coupled" energy in a 50 ohm system:  Compared to the coupling in 75 ohm system (now shown) the insertion loss was slightly higher (about 1dB) but the frequency loss/flatness was about the same, being pretty consistent from about 1.8 MHz through 1 GHz.

Figure 13 shows the reverse isolation of the tap:  Rather than 6dB of coupling from the main line for signals going the "other way", the absolute is closer to 20dB - about 13dB lower.  (The actual reverse isolation is the absolute isolation minus the forward loss).  In a 75 ohm system (not shown) the reverse isolation was quite a bit better (closer to 30dB over the 5 MHz-1GHz range) - but this result is completely expected:  The reverse isolation is akin to measuring VSWR, and operating a 75 ohm device at 50 ohms implies a VSWR of 1.5:1 - a "return loss" of 14dB - very close to the values depicted in Figure 13 over much of the frequency range when the "forward" loss is taken into account.

On a tap there is yet another measurement to be taken - the loss between the in and out port.  Because we are measuring a 6dB tap - a device which siphons off about 25% of the signal - we would expect at least that amount (theoretically 1.25dB for 6dB) to be lost as it is coupled to the "tap" port. Figure 14 we can see that the measured loss is slightly higher than this between 1.8 and 200 MHz- a bit over 2dB.  Some of this "extra" loss is due to the intrinsic losses of the device, but a smaller amount is a result of the use of a 75 ohm device on a 50 ohm system.
Figure 14:
Through loss of the 6dB tap in a 50 ohm system.
Click on the image for a larger version.

This device - which is rated down to 5 MHz - may be useful through at 160 meters (1.8 MHz) - but the insertion loss goes up rather quickly at lower frequencies.

This device is NOT suitable for passing DC (e.g. for amplifiers, control signals) as it has a DC short across it - but that is not true of all taps.  For example, the Holland Electronics "HDCS" series does allow low frequency RF down to DC to flow through it - but like the DCG-6SB, its coupling coefficient deteriorates quickly below about 1.8 MHz.

* * *
General conclusions:

If you are going to use TV-type splitters for HF, make sure that you get devices that are explicitly rated down to 5 MHz.  Based on the (limited!) sample of devices that were tested, these devices can be expected to work into the 160 meter amateur band (down to 1.8 MHz).  While these devices may be usable thoughout the entire AM broadcast band (down to 540 kHz) expect performance to drop quickly in terms of added "through" attenuation and worse port-to-port isolation.

A "TV" type device - one that may indicate a start frequency of 5 MHz, or just any device that is claimed to work at TV (VHF/UHF) and FM broadcast frequencies will likely work from 6 meters through 70cm (50 MHz - 450 MHz).

Again, for general signal splitting and combining, these 75 ohm devices, used at 50 ohms, are quite usable for non-critical applications - provided that they be used at low power levels (a few 10s of milliwatts at most) and where one need not have precise 50 ohm matching and high port-to-port isolation.  Remember that most 50 ohm devices (receivers, amplifiers, filters) have only "approximately" 50 ohm source/load impedances - and filters in particular will, out of their design frequency range (outside the band-pass, on a notch frequency, above the low-poss cut-off, below the high-pass cut-off) will likely have anything but a 50 ohm characteristic impedance, so even a "proper" 50 ohm splitter/tap device would not necessarily yield any better performance in those situations.

For information about the design and use of splitters/combiners in general, a good reference is Mini-Circuits AN10-006, "Understanding Power Splitters" - link.

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Far more data was gathered than was presented here, but that depicted above is representative of the devices that were on hand.

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This page stolen from ka7oei.blogspot.com

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