Characterizing the RTL-SDR Blog (Version 3) for HF reception using the "direct" input.

An inexpensive option for SDR (Software Defined Radio) reception on the HF (low frequency) bands is a device sold by "RTL-SDR Blog" - the current iteration being Version 3.  Originally intended for digital VHF/UHF TV reception - and that of FM broadcast - the hardware is also capable of tuning much lower frequencies.

Figure 1:  An RTL-SDR Blog V3 USB receiver "dongle".

Unlike most other inexpensive RTL-SDR dongles, this has - via a single SMA port - the ability to operate in "direct" mode where RF below the VHF frequencies is passed straight to the A/D converter rather than via a down-converter, allowing reception from (theoretically) a few hundred kHz to around 30 MHz.

How does it do this?

The typical RTL-SDR Dongle actually consists of two tunable devices:

• The Rafael R820T.  This is simply a frequency converter, capable of handling an input signal from somewhere below 60 MHz into the GHz range and converting it to what we'll call an "IF" (Intermediate Frequency) which is much will likely be below 12 MHz.  In addition to having a programmable oscillator and mixer for frequency conversion, his device has some built-in filtering that provides some protection to strong-ish off-frequency signals, and it has an AGC (Automatic Gain Control) that can adjust the level being output from it to prevent overload of the A/D converter as well as some front-end attenuation control to reduce the likelihood of overload on the input.
  

• The Realtek RTL2832U.  The down-converted output of the R820 chip is passed to this device, which consists of two 8 bit A/D (Analog-to-Digital) converters that are clocked at 28.8 MHz (meaning that signals above 14.4 MHz will be "aliased"), a USB interface, a (reported) 8051-type microcontroller and a digital frequency "converter" that is also capable of being "tuned" to produce a quadrature "baseband" signal that is output onto the USB port - the rate being programmable from around 250 ksps to 2880 ksps.  This device does NOT have an AGC or gain/attenuation control.
  

VHF/UHF operation:

Briefly, reception on the VHF/UHF frequencies is done the following way:

• A simple high pass filter/diplexer passes only the VHF/UHF signals (e.g. those above approximately 40 MHz) directly to the R820T.
  
• The filtering of the R820T is programmed for the desired characteristics at the operating frequency.
• The frequency converter in the R820T is offset from the input signal to provide an output (IF) signal in the 1-14 MHz range - more likely somewhere between 3 and 6 MHz.
• The level of this output signal may be automatically controlled by the AGC system of the R820T to keep it within the optimal range of the A/D converter.  Similarly, input gain adjustment on the R820T can prevent it from being overloaded by strong signals.
  
• The tuner within the RTL2832U is set to about the same frequency being output by the R820T (likely in the 5-12 MHz range) so that it is within the range of output ("baseband") sample rate, and output via the USB port.

HF operation:

• The signal to be received is applied via the RF antenna port and diverted from the R820T to a very simple low-pass filter/diplexer to an RF amplifier.  This diplexer's effect begins at approximately 22 MHz - See the discussion near table 1, below.
  
• The output of from the RF amplifier is passed through additional filtering and applied to one of the two A/D inputs of the RTL2832U - typically the "Q" input.
• The tuner within the RTL2832U is set near the frequency to be received so that it is within the range of the output ("baseband") sample rate, and output via the USB port.
• Because the sample rate of the RTL2832U is 28.8 MHz, any signal above 14.4 MHz will also appear "below" it as well - see the discussion below.
  

There's a penalty to pay for simplicity:

For those familiar with receiver topology - digital or analog - several things the above description of the HF operation of the RTL-SDR dongle without proper measures (e.g. filtering, gain control) should give cause for concern - namely:

• There is NO bandpass filtering at all.  Whatever is being intercepted by the antenna system will be input directly to the A/D converter via the preamplifier.
  

• ANY signal applied to the antenna input - no matter the frequency - can contribute to overload.  Because the inputted RF goes into the A/D converter, a strong signal well away from where you are listening can cause overload.  For example, if you are listening to 14 MHz, a strong AM broadcast signal (e.g. mediumwave) could well be the culprit if you are experiencing overload.
  

• There is no AGC in the HF ("direct") signal path.  Considering that the A/D converter is only 8 bits, this means that compared to even low-end shortwave receivers, the range of signal levels over which this device will operate is very narrow.  The lack of an "AGC" (automatic gain conttrol) means that there will likely not be enough gain when signals are very weak and overload of the A/D converter is likely if the signals are very strong.
  

• The A/D converter's sample rate is 28.8 MHz.  What this means is that the Nyquist limit - the frequency above which the digitized output can no longer represent the input signal - is half this, or 14.4 MHz, uncomfortably close to the 20 meter amateur band, and entirely below the 17, 15, 12 and 10 meter bands - and this does not even consider the fact that the sample frequency is within the 10 meter band itself. 

What this means is that there are unsuppressed image responses all across the HF spectrum.  For any frequency you tune below 14.4 MHz, you can also hear any signal above 14.4 MHz, the frequency of which is calculated by subtracting its frequency from the sample rate (e.g. 28.8 MHz).  For example, 15 meter signals will also appear, spectrally inverted (e.g. USB = LSB) 7.80-7.35 MHz.  Image frequencies appear in table 1 - along with MDS and clipping values - below.

In other words, if you are tuned to any HF frequency, you are actually "hearing" TWO frequencies at the same time unless you have specific filtering to prevent such image response.  See the right-hand column of Table 1 (below) for the frequency at which you will see an image.

The above puts strict limits on the performance of the RTL-SDR dongle as an HF receiver and these realities must be considered when the configuring any system that might use them.

The usable dynamic range:

In theory, 8 bits of A/D sampling would indicate about 48dB of useful signal range, but the reality is more complicated than this.  Compared to the bandwidth of the narrow signals typically sought on HF, the overall sample rate of the A/D converter - and even the rate after the RTL2832U's converter has reduced the signal to the sample rate being sent to the USB port - are very much higher, effectively improving the bit depth via oversampling - and the fact that the HF spectrum is backgrounded with what amounts to white noise works to our benefit, helping to spread discrete, spectral artifacts that are the inevitable result of the imperfect signal acquisition.

This also works the other way:  Very strong, discrete signals (e.g. SWBC transmitters) can also cause mixing products which can find their way in other signals - particularly weak ones - but this effect may be minimized if one manages to keep the input level "high enough" so that at least several bits of A/D conversion are involved and also limited such that A/D converter overloads are minimized as much as practical - a balancing act that makes it difficult to handle both weak and strong signals at the same time.

All of this makes the actual, usable range a bit difficult to divine.  To this end, an RTL-SDR (V3) unit was put on the workbench, using the "HDSDR" program as a receiver, and its operation was analyzed by observing CW (unmodulated) signals on narrow (SSB) bandwidths.

Methodology:

Ideally, we would be able to determine the RF level at which the A/D convert clipped directly, but the HDSDR program does not provide a means to see the peak A/D level, requiring us to infer it by noting the level from a known-accurate signal generator at which the S-meter starts to decrease at the same rate that the signal level is reduced - and also by noting the disappearance from the waterfall many of spurious signal caused by overload.  Typically, this is about 3dB below the "maximum" S-meter reading.

For sensitivity, DL4YHF's "Spectrum Lab" program was used to measure the SINAD in a 500 Hz bandwidth set by the HDSDR program, the "minimum discernible signal" being equivalent, in this test, to 3dB S/N.

Table 1:  Measured signal levels for A/D clipping and MDS, along with corresponding image frequencies on HF.  Because the sample rate of the RTL-SDR is 28.8 MHz, ALL signals above half this frequency (e.g. 14.4 MHz) are, by definition, Nyquist images.  The far-right column is "apparent" dynamic range - see the discussion below.

Frequency (MHz)	Clipping (dBm)	MDS (dBm)	Image  (MHz)	Apparent DR (dB)
0.5	-17	-97	28.3	80
1.0	-24	-107	27.8	83
2.0	-27	-109	26.8	82
4.0	-27	-116	24.8	89
7.0	-29	-117	21.8	88
10.0	-30	-117	18.8	87
14.0	-30	-117	14.8	87
15.0	-30	-116	13.8	86
18.0	-30	-116	10.8	86
21.0	-29	-114	7.8	85
24.0	-25	-109	4.8	84
28.0	-14	-95	0.8	81
29.0	-16	-94	0.2	78
35.0	-16	-90	6.2	74
40.0	-2	-84	11.2	82

Discussion:

Table 1 tells us several things:

1. As mentioned in the documentation found at the RTL-SDR Blog web site, signals below 2 MHz - and especially below 1 MHz - are rolled off by the "Bias Tee" blocking choke which has insufficient inductance at frequencies below the AM broadcast band.  To a degree, this effect can be mitigated by removal of the RF choke which will remove the capability to inject DC onto the cable.  This table includes the effect of this choke.
   
2. The RTL-SDR has a low-pass filter to diplex the HF and VHF and above frequencies to separate signal paths, and the effects of this filter are becoming evident above 15 meters (21 MHz).  This also means that by itself, the RTL-SDR becomes "deaf as a post" on the 12 and 10 meter bands.
   
3. Note that at the very low frequencies, the sensitivity appears to be somewhat reduced.  The effects of high-pass roll-off - likely caused by coupling capacitors and the input bias inductor - appear to be evident through at least 7 MHz.
4. There is clearly no image rejection at all, considering that the sensitivity through 21 MHz is comparable to that below 14 MHz.  For example, 15 MHz WWV will also appear on a receiver program at 13.8 MHz, at the same apparent signal strength.
   
5. Low-pass effects are evident by 24 MHz, most likely a result of the diplexer used to split the HF (direct) and VHF/UHF signal paths.  This limits sensitivity on the 12 and 10 meter amateur bands.
6. With only 8 bits of quantization, additional noise will be generated due to the imprecise nature of the process.  This "noise" will show up as spurious signals and, less obviously, as a rise in the overall noise floor - depending on the nature of what is being digitized.  In short, the fewer the number of bits, the less likely it is that weak signals will coexist (and be audible) in the presence of strong signals.
7. Across most of the HF spectrum, the RTL-SDR will overload signal at about -30dBm - which is approximately equal to an S-meter reading of "40 over S-9".  While this seems like a fairly strong signal, this power level represents the total amount of RF energy - no matter the frequency!
8. Note that the power at which the A/D converter starts to overload is approximately -30dBm across much of the HF spectrum.  This represents the TOTAL amount of RF power required, at all frequencies combined, that will result in overload.
9. Across much of the HF and MF spectrum (0.5-28 MHz) the apparent dynamic range is on the order of 80 dB.  This number should be taken with a grain of salt as it was measured in the absence of any other signals -  hardly a real-world equivalent.  Practically speaking, this number is indicative of the best possible performance under ideal conditions.
   

Points 7 and 8, above, should be considered very carefully in terms of its implications:

• You cannot simply connect an RTL-SDR Dongle to even an "average" performing HF antenna and expect reasonable results as the total power from ALL signals reaching the receiver are likely to exceed the -30dBm signal level - particularly if you have any AM (mediumwave) transmitters anywhere nearby (e.g. within 20 miles/30km).
  

• Particularly on the lower bands (80, 40, 30 meters) the signal levels of amateur and especially shortwave broadcast signals can, by themselves, exceed the -30dBm overload level - particularly in Europe and the eastern U.S. On some bands, the shortwave broadcast band adjacent to the amateur band (e.g. 41 and 40 meters) are too close to effectively filter out and it may be that an RTL-SDR is simply not usable on these bands when propagation favors reception on these frequencies. 
  

• As seen in Table 1 (above) the RTL-SDR dongle becomes increasingly deaf on the higher HF bands (particularly 12 and 10 meters) making them unusable at these frequencies without additional amplification AND filtering.  What's worse is that the lower HF frequencies (e.g. below 10 MHz) are typically very noisy while the higher frequencies are quiet by comparison.  If you connect an antenna to the RTL-SDR dongle with no band-pass filtering and try to tune in, say, the 15 meter band, you will likely hear only noise (and signals) around 4.8 MHz, which will likely overwhelm any weak, 15 meter signals.
  

• If you plan to use an RTL-SDR for HF reception and expect even mediocre performance, you should precede it with a band-pass filter for the frequency band of interest.  For the highest HF bands (15, 12 and 10 meters) the typical noise floor in a quiet location is around -120 dBm (in a 500 Hz bandwidth) - which is well below the noise floor of the RTL-SDR at these frequency meaning that a preamplifier (along with a bandpass filter) will be required for reasonable performance.
  

• It's worth remembering that unlike an analog receive system, one cannot always use all 8 bits for digitization:  The signal input must be kept well below the "full scale" level (at and above which "clipping" will occur, causing distortion and signal degradation everywhere else) to accommodate for random fluctuations that are ever-present on signals input from the antenna.  What this means is that the A/D must be under-driven overall and that fewer bits are actually being used most of the time.  In order to maintain suitable margin, it's typical to drive the A/D converter at between 1/4 and 1/2 full scale, meaning that for 8 bits of A/D conversion, 2-4 bits are typically being used - and fewer, still, when the band is "dead" and signals are weak.
  

Conclusions:

• DO NOT simply connect an RTL-SDR to your HF antenna and expect it to work as well as even a low-end shortwave receiver:  If it doesn't get overloaded by local AM broadcast (mediumwave) signals, it will get overloaded by strong shortwave broadcast signals when conditions are favorable on certain bands.  This isn't to say that you won't hear anything if you do so, but know that normal signal levels present on even an "average" antenna will be enough to overload the RTL-SDR dongle.
  

• ALWAYS precede an RTL-SDR with a band-pass filter that is specific for the frequency range of interest. For example, if you are interested in 40 meter reception (7.0-7.3 MHz for ITU region 2) your filter should pass only frequencies in this range, and this filtering will prevent unwanted reception of signals at the image frequency around 21.8 MHz.   Unfortunately, the use of a band-pass filter precludes reception outside its design range, but this is necessary considering the limited capability of the RTL-SDR dongle in terms of handling both strong and weak signals at the same time and its unfettered response to unwanted images.
  

• In some cases - even with a mediocre antenna - the signals in the desired frequency range may exceed the signal handling capability of the RTL-SDR and cause overload.  As noted above, a single signal stronger than -30dBm can do this, but so could a number of signals whose total power can exceed this.  Typically, this overload is manifest as intermittent distortion across the entire receiver as signals fade in and out.  In such cases, it might be beneficial to attenuate the signals reaching the RTL-SDR as the degradation caused by overload is more "destructive" across the entire receive frequency range than too-little signal.
  

• Because of image response and roll-off, the HF port of the RTL-SDR is really not well-suited for 12 and 10 meter (24-30 MHz) reception.

Comment about HF upconverters:

Specifically to address some of these issues, there are upconverters available for the RTL-SDR that will upconvert the HF spectrum to VHF (typically in the 100-130 MHz range) to allow the use of that signal path, making use of the Raphael R820T converter.  This converter has the advantage of having a degree of band-pass filtering and the ability to use AGC (automatic gain control) on the signal path.

This method can be useful, but there are several caveats:

• If frequency stability is of importance, the addition of the upconverter introduces two additional frequency stability issues:  Drift of the upconverter itself, and the fact that any existing drift in the dongle itself will be multiplied because of its operation at the higher frequency.  This can be an issue in environments where the temperature is not stable and/or when a frequency sensitive mode like SSB or (especially) digital modes are used.
  

• If multiple bands and receivers are to be used, there may be the temptation to upconvert the output of the upconverter to VHF and distribute this signal to the receivers.  While this may work in many cases, it's worth noting that if the entire HF spectrum is converted, the total signal power level can be significant, potentially overloading the upconverter itself, any RF amplifiers that might be used at VHF, and/or the front end of the RTL-SDR dongles themselves.
• While a "fix" might normally be to filter out just the HF band(s) of interest, this can become impractical if the 40 meter band (7.0-7.3 MHz) is upconverted to, say, 137 MHz where it can become difficult to make an effective band-pass filter at that frequency.
• It is possible to filter for a specific HF band before the upconversion, but this means that each RTL-SDR would require its very own upconverter.  While effective, the cost of an upconverter for each band may be prohibitive.

* * *

Successfully using an RTL-SDR on HF:

As mentioned earlier, one must precede the RTL-SDR with a band-pass filter to obtain reasonable performance on HF - or any other frequency range where very strong and very weak signals will be simultaneously present at the antenna input:  Remember that, especially in the "direct" mode, all signals applied to the RF input - even those MHz away from where the receiver is tuned - will count "against" you in terms of the total amount of RF power that may be applied to the A/D converter before it clips/overloads.

To see some RTL-SDR based HF receive systems in operation - and to be able to directly compare them with higher-performance receivers using the 16 bit signal path, visit the Northern Utah WebSDR site at sdrutah.org (link).  This will take you to a "landing page" where you can select several receivers - specifically:

• WebSDR #3:  This server uses RTL-SDRs for both the 80 and 40 meter bands.
  

• WebSDR #1:  This server uses SDRPlay RSP1a receivers which have both an RF AGC and around 14 bits of A/D converter depth - plus external band-pass filtering.
  

Both of these systems use the same antenna for 80 and 40 meters and it is possible to directly compare signals between the two in side-by-side windows.  Generally, the 8 bit RTL-SDRs used on WebSDR #3 hold their own compared to those on WebSDR #1, and this is possible only because the WebSDRs are preceded with both band-pass filtering and AGC as described in the link below.

* * *

Additional resources:

• Band-pass filters are readily available in kit form from QRP Labs http://qrp-labs.com/bpfkit.html  
  

• An article on using band-pass filtering and AGC (Automatic Gain Control) to improve the performance of an RTL-SDR when used for amateur band service may be found here:  http://www.sdrutah.org/info/rtl_sdr_agc.html .  Several "test" receivers are currently in operation at the Northern Utah WebSDR that demonstrate the efficacy of doing this.

 This page stolen from ka7oei.blogspot.com

[End]

Using the "RTL.SDR" dongle (with HF port) as found on Ebay

Some months ago I was perusing the wares of a seller on EvilBay and having decided to purchase one of those all-purpose component testers, I was tempting myself with other items on sale.  One of the items that I could not resist was the "RTL.SDR" that was billed as having an onboard frequency upconverter to allow HF operation (it actually doesn't - see below) plus a more stable reference oscillator.

Figure 1: 
The EvilBay "RTL.SDR" with extra connector
for HF reception.
According to the pictures on EvilBay, the units
currently sold host different picture, with
 a graphical representation of the schematic
diagram showing the mixer.
Click on the image for a larger version.

"What are these 'dongles' of which you speak?"

Stepping back for a moment, the "RTL" SDR dongles have been around for a few years, so-called because they typically contain as one of their active devices the Realtek RTL2832U.  These devices - costing roughly $10 (more or less) also contain a device (a Rafael Micro R820T in the case of the one pictured) with an onboard frequency synthesizer, I/Q mixer and (8 bit) A/D converter that allows one to receive signals from 50 MHz (or lower) to well above 700 MHz - sometimes way above 700 MHz, just using a computer with reasonable horsepower and FREE software!

While the thought of an 8 bit A/D converter and its rather limited dynamic range (40-ish dB) might make it sound like these devices may be useless when connected to an antenna, the fact that they convert only a limited range of frequencies (approximately +/- 2.4 MHz at most) around the tuned frequency - plus the "oversampling" nature of the raw A/D sampling rate of the 8 bit converter means that relatively narrowband signals can have quite good ultimate quantization range.  What can suffer is when, within the sample-rate passband there are signals that range widely in signal strength:  Under such conditions it is recommended that one "finesse" the gain controls for optimum input signal levels to the A/D converter.  Another deficiency from which they suffer is quantization noise - that is, the inability to "see" weak signal that are down in the mud, plus the tendency for this noise to somewhat "spread" the energy of nearby, stronger signals around and raise that noise floor.

Perhaps the most popular software to do this is "SDR#" (a.k.a. "SDR-'Sharp'").  Easily found via a web search, this software is compatible with many software defined radio packages and it supports many modes and features, also offering many optional plug-ins.

These low-cost USB dongles were intended to allow the reception of digital TV and FM/Digital Audio broadcast (but not the sort of TV or digital audio broadcast that is used in North America) on a computer so they were designed to tune from something around the low-medium VHF range - say, 20's to 50's of MHz and up, almost completely leaving out the HF frequency bands.

Several schemes have been devised to allow the use of these handy devices to receive HF and these typically involve the use of some sort of upconverter to translate the HF frequencies from as low as the AM broadcast band up to a range that these devices will tune.  Another scheme - used here - is to feed HF directly into the "Q" input of the RTL chip - this input not normally being used with its frequency converter.

So, I soon found myself with the device pictured in Figure 1 in my hands.  Getting busy with life and other things, it was a few months before I got around to playing with it and while I quite easily got it working on the "UV" input which, on this particular unit tunes from just below 25 MHz to over 1 GHz (although I'm not sure exactly how far above 1 GHz...) but I was puzzled as to how to configure it operate at HF.

How to make it work on HF:

Back then, a web search didn't help as there was a plethora of hits on "RTL SDR Upconverter" so I removed the unit from its case and found the callsign "BA5SBA" on the board that you see in Figure 2 and got a hit with Google on a German page (this one - LINK - you may need to run it through a translator, so try THIS LINK which may or may not work...)  While this web page didn't describe exactly what I needed to do, it did provide enough clues to get it working!

Comment:

• More recently, the some of the sellers on EvilBay are including in their postings some basic instructions on how to get these devices to work on their "HF" ports, but what follows below contains more information - and is hopefully easier to find and more permanent - than an EvilBay posting!

Figure 2: 
Inside the BA5SBA "RTL.SDR" with the "HF" port.
The jumper in the upper-left corner marked "A" and
"P" appears to be intended for putting 5 volts on
the "UV" antenna connector to power an external
RF amplifier.  ("A" = active, "P" = passive).
Click on the image for a larger version.

With a bit of reverse engineering I determined that the "HF" input was connected, via a transformer, some low-pass filtering and some protection, to one of the inputs of the RTL chip - specifically, the "Q" input - which not used in its normal role when converting from higher (VHF/UHF) frequencies.

Using a signal generator set to 5 MHz I connected it to the "HF" input of the device and, just for kicks, tuned SDR# to 5.000 MHz as well.  After a bit of fiddling about, I finally found how to configure the unit to receive at HF.

• STOP the SDR receiver first by hitting the square "stop" button at the top.
• Bring up "RTL-SDR Controller" window (accessible by clicking on the "gear" symbol)
• Under "Sampling Mode" set it to "Direct Sampling (Q Branch)".  Note:  You cannot change the sampling mode without first STOPPING the SDR.
• Restart the SDR by pressing the "start" button (the right-pointing triangle resembling a "PLAY" button)
  

Once I had done this I saw the 5 MHz signal on the display!


Comments: 

• By default, SDR# will set the "RF Gain" setting in this same "RTL-SDR Controller" window (see Figure 6) to minimum gain (all the way to the left.)  If you leave it there, the SDR dongle will be EXTREMELY DEAF when it is in the normal "Quadrature Sampling" mode.  When you have it in either "Direct Sampling" mode this control is disabled.
• I would suggest checking the box that says "RTL AGC" and moving the RF Gain control in either "Quadrature Sampling" or "Direct Sampling" modes:  If you do not do this your receiver may be very deaf!
• I experimented with using this device with the "HDSDR" program and found that at HF, its sensitivity was noticeably worse than that obtained when using it with SDR#.  I'm not sure why this might be, but it seems to have something to do with the way the "ExtIO" driver - or HDSDR itself - handled the internal gain settings of the dongle.

Figure 3: 
A screenshot of SDR# showing the configured "Shift" for the mixer.
Click on the image for a larger version.

Now I am very surprised that I can tune SDR# to "5.0" MHz and see the 5 MHz signal that I am inputting as the chip on this dongle should not function at that frequency!  What I expected to have to do was as follows as shown in Figure 3:

• Knowing that the onboard mixer uses the 28.8 reference oscillator, check the "Shift" button on SDR#.
• Enter a frequency of "-28800000" (yes, negative 28.8 million) into the box as shown in Figure 3.
• Note:  There is a bug in some versions of SDR# that, if you are already tuned to a frequency lower than the "shift" frequency, the tuning may not work properly until one sets a much higher frequency with the 100 MHz digit and tunes back down.
• Having done this, one need not mentally add the desired HF receive frequency to 28.8 MHz:  The receive frequency is simply input to the main frequency tuning as one would any other frequency!
• If the 28.8 MHz frequency reference oscillator is somewhat off frequency one must first set the frequency using the "normal" VHF/UHF mode and calibrate it in the "RTL-SDR Controller" window and THEN, after switching to HF mode as described above, one can input a frequency that is slightly different from 28.8 MHz as needed to correct the offset.  I did not have to make any correction at all!

Comments: 

• The 28.8 MHz reference oscillator supplied with this unit appears to be quite good and stable and was only off a few PPM:  Your mileage may vary.

• If you want to use the VHF/UHF input again, remember to reset the "Sampling Mode" back to "Quadrature Sampling" and UN-CHECK the "Shift" box that is visible in Figure 3.

Does it matter whether one uses the "0 MHz" method or the "-28.8 MHz" method with the offset?  Not really:  I could see no difference in the sensitivity of the SDR - but it is good to be aware of both methods just in case the "0 MHz" method doesn't work!


Comments on the design of the HF mixer and its limitations:

Sensitivity (or the lack of...)

Figure 4:
The low-pass filter in front of the mixer.
Click on the image for a larger version.

The very simple mixer on this SDR Dongle has its limitations, the first of which is that being entirely passive, the ultimate sensitivity at HF is not particularly good:  Using SSB (2400 Hz) bandwidth, signals down to around -90dBm were audible, but that was about it.  While this represents a signal of around 7 microvolts and may seem horrible (and it is, compared to modern HF receivers!) it should be remembered that if this is connected to anything resembling a full-sized HF antenna, it "hears" just fine since the background noise on HF bands - and thus the signals - will typically be above this level - at least on the "lower" bands of 10 MHz and below.

Nyquist rules!

The other problem is that the sampling rate on this input is 28.8 MHz.  What this means is that there is a Nyquist limit of 14.4 MHz - and that means that any signal above 14.4 MHz will reappear again as an "image" at a lower frequency, approaching Zero Hz as it approached 28.8 MHz.  In other words, if you were tuned to 7.1 MHz in the 40 meter band, you would also hear a signal at 7.1 MHz below 28.8 MHz, or 21.7 MHz!  This also means that if you were trying to listen on 21.1 MHz in the 15 meter band, you would hear a signal at its 7.7 MHz image!  The situation is worse on 10 meters since the 28.8 MHz local oscillator is nearly in the middle of this band:  An FM signal on 29.5 MHz would also appear on 28.1 MHz!

Figure 5:
The response of the low-pass filter depicted in Figure 4.
The bandpass response is the solid line.
Click on the image for a larger version.

If you look at the circuit board in Figure 2 you'll notice several components in the foreground, namely a number of inductors and surface-mount capacitors, all with the values marked on the silkscreen.  Presuming that the values of these components are as-marked, I drew the schematic in LT Spice that is shown in Figure 4 and simulated it with the result in Figure 5.  As can be seen, this low-pass filter effectively blocks signals much above 40 MHz or so, offering more than 40 dB of attenuation above 60 MHz.

Other than keeping signals from low-band TV and FM broadcast out of the "HF" input, this filter does not help us at all in our image problem.  Because this filter does not block signals immediately above 28.8 MHz, we have another set of images that occur which means that a signal that is at 33.8 MHz - 5 MHz above 28.8 MHz - will also show up at 5 MHz! 

If there are no signals (or noise!) on these "image" frequencies, there should be no problem, but this is likely not to be the case during the daytime on a large wire antenna - which means that if you really want to use this dongle for serious HF reception, you need some sort of filter in front of it, either a separate Low Pass (below 14.4 MHz) and High Pass (above 14.4 MHz) to be used individually, or you could use a tunable preselector to pass only the frequencies of interest.  (There would be a problem with 10 and 20 meters as these bands are close to or include the Nyquist frequency!)


Possible modifications:

If you look at Figure 2 you'll note that there are several small "prototype" areas on the circuit board.  These are large enough to allow the construction of some simple circuits, namely a bit of additional gain (amplifiers, filters) to improve the sensitivity of the receiver.

As mentioned before another useful modification - one external to this box - would be the addition of a preselector to filter a narrower range of frequencies.  Another would be to simply add another oscillator of a much higher frequency (say, 100-150 MHz) and feed it into the mixer to put the Nyquist limit completely above the HF band.

Is this "HF" input useful as-is?

In general, yes, with a good antenna.  Despite its flaws, when one casually tunes around the HF bands its problems do not immediately jump out, at least on the lower bands.

The biggest advantage of this particular SDR Dongle is that it seems to have quite a stable oscillator on board - much better than the $8 USB "stick" that I have experimented with previously:  This one seems to move only a few 100 Hz from cold start even at UHF frequencies whereas the very cheap ones seemed to move all over the place with temperature!

To be sure, one could probably get better HF performance by building (or buying) an upconverter for this sort of SDR that uses a much higher local oscillator frequency - say, 100 MHz -  but if one upconverts all of the HF frequencies en-masse, there are a few things that one must remember:

• If you use a chip like the NE602, it may simply overload when connected to a full sized HF antenna unless you are prepared to add switchable attenuation to it and/or a preselector!

• On a full-sized antenna, intercepting and converting the entire HF spectrum can represent quite a bit of signal energy.  If the HF upconverter that you are using does not, itself, overload, there is a good change the the converter chip in the RTL-SDR itself will overload with so much energy from such a wide variety of frequencies!  Remember:  The receiver chips in these dongles were intended to work with the relatively weak and sparse broadcast signals intercepted on small, indoor antennas, not the entirety of the HF spectrum when the bands are open!

• If you use a separate upconverter circuit - say, to 100 MHz - the frequency stability will be dependent on that 100 MHz local oscillator and the reference used by the dongle itself for its normal operation.  With two oscillators, there is the greater potential for drift, a fact exacerbated by these oscillators effectively operating at rather high frequencies.

Figure 6: 
The screen for configuring the dongle's
hardware.
Click on the image for a larger version.

A few brief comments on using it at VHF/UHF:

Remembering to set it to the "Quadrature Sampling" mode when using the "U/V" input, there are a few things to know when using this device - and possibly others like it - with SDR#.

Clicking on the "gear" symbol will open a screen such as that depicted in Figure 6 that allows modification of settings related to the dongle's hardware.  In particular, note the check in the box for RTL AGC and the lack of a check in the Tuner AGC box:  The RTL AGC box is necessary in both the Quadrature Sampling mode to get the most gain out of the device.

The Tuner AGC box - which is only available when Quadrature Sampling mode is active - if checked, does not seem to work properly in the the version of driver and/or SDR# at the time of initial posting of this blog entry (March, 2015) as it seems to increase the gain too much, causing overload of the A/D converter and actually reducing performance - particularly if a decent antenna is connected!  (This is really where the deficiencies related to just having 8 bits of A/D conversion start to show up:  With higher resolution - and higher dynamic range - the "sweet spot" in which the performance would be good would be wider.)

Figure 7: 
 Top:  Properly adjusted RF gain setting
Bottom:  The degradation from too much RF gain!
Click on the image for a larger version.

Figure 7 demonstrates the problem.  The top half of the image shows an optimally adjusted amount of gain - a setting of 12.5 dB for this particular antenna (yours will vary!) while the bottom shows a gain setting of 37.0 dB.

Even though more signal is reaching the A/D converter in the bottom example, it is way too much and the converter is being badly overloaded, causing the noise floor to rise from below, submerging weaker signals!  If the RF gain is sett too low, degradation can also occur, but this time from too little signal.

The idea is to adjust the RF gain so that there is there is the maximum difference between the highest signal and the noise floor seen between stations - even if the absolute level of the signals may be lower, overall!



[End]

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