Two repeaters, one frequency (part 4) - Voting receivers

This is an ongoing series of posts:  For the most recent, previous post in this series, see Part Three (link).

In the other three parts of this string of posts I described the two repeaters, how their operation is greater than the sum of their parts and how the two transmitters can share the same frequency without causing mutual interference.

This time, it's the "Voting Receivers".

In order to achieve reasonable parity with a multiple-transmitter system, we also need to duplicate the effect on receive which means that we have to take the users' signals from both sites and then decide how to use them on transmit and for this, we have a "Voter" system.

As its name implies, the voter "looks" at all of the signals that it is getting from the various receivers and somehow decides which one is best - and that one is sent to all of the transmitters in the system.  The result of this is that during a user's transmission, it's possible that the receiver being used might change several times as the signal fades from one receiver and is picked up by another - particularly in areas where coverage overlaps!

Now, then, does one "vote" on a signal?

First, let's talk a bit about what happens to an FM signal as it gets weaker.

Unlike AM, in which the audio can get weaker as the signal gets closer to the noise, the audio from an FM signal stays constant no matter how weak or strong the signal might be - although something interesting happens when it gets to be too weak:  It doesn't get quieter, it gets noisier!

What is happening is that as the signal gets weaker, the detector - which looks for the "frequency wobble" of an FM signal - starts to have trouble distinguishing between the desired signal and the background noise, so that noise starts to creep into the audio.  There is a threshold at which a slight decrease in signal starts to result in a very marked increase in noise and for a number of reasons, this is often referred to as the "limiting threshold."

Figure 1:
The front panel of the voting controller showing the indicators
of the input channel status (green) as well as various
status/health indicators of the unit itself (red.)  A DE-9
jack provides access to the RS-485 bus for configuration
and status monitoring.
Click on the image for a larger version.

What this means is that if you have two copies of the same signal from two different receivers, you can tell which one is the weaker signal NOT by how loud or quiet its audio might be, but looking at how much extra noise it contains:  If the receivers are otherwise identical, of the two versions of the same signal, the noisier one will always be the weaker one!

How, then, does one compare noise?

There are two common techniques:

• Compare "quiet-ness".  In the pauses and spaces between words, there will be brief pauses of silence - or, at the very least, portions that aren't as loud as the peaks of the audio.  The weaker signal will have more noise, so it won't be as "quiet" during these pauses as the strong signal.  The receiver that has the highest "minimum" audio level will be the one that is weaker.

• Compare total loudness.  As it turns out, as a signal gets weaker, the noise gets added to the audio which means that if you look at the total amplitude of the two signals, the one that's louder overall (that is, there's the audio plus noise) will be the one that is weaker.

Comment: 

One can also look at the level of ultrasonic noise from the discriminator, but this is not easily done on sites connected only via a voice-bandwidth audio link and may be discussed in a later blog entry.

For a number of reasons, we chose the second method for our voting system.

It makes the job a bit easier if one looks only at the higher-frequency audio - say, that above about 2 kHz - as this has less speech energy in it overall, yet the noise will increase at the high frequencies of the de-emphasized audio just as readily as it will at the lower frequencies.  By "looking" only at these higher frequencies it becomes a bit easier to ignore the voice energy when trying to make comparisons of quality between multiple channels of the same audio.

There are some complications, however:

As you can imagine, no matter what method of voting you pick, it's important that all of the audio sources are as identical to each other as possible in terms of amplitude (loudness) and frequency response.  Consider the following example.

• One receiver is louder than the other due to a misadjustment.  The louder receiver - even if it has a perfectly good signal - will look "worse" than a quieter one since more total audio energy will be detected from it.  In other words, compared to the the other, quieter receiver, the loud one will be considered noisier.  Because of this, the voter will "prefer" the quieter signal, even if it is a bit noisier.

• One receiver has better "highs" than the other.  Since the noise from a weak signal will readily appear in the "highs", if two equally-noisy signals appear at once, the receiver with the stronger "highs" will be considered the worse one and the voter will prefer the other signal.  Since our system looks at the audio above 2 kHz, a receiver with extra "highs" makes things even worse!

When we first installed the voter, we had all of these problems, but we expected that we'd have to tweak a few things, but several things caught us off-guard:

• The first one was that the "local" receiver was connected to the voter via copper wire, while the other receiver was connected via a UHF radio link.  The local receiver had better "highs" in its audio than the receiver on the UHF radio link and the "highs" in the voices of the users on the local receiver caused the voter to "think" that the signal was worse than it really was and it would jump back-and-forth while people were talking.

• Due to a design flaw in the local receiver it was discovered that it would start to clip if the received signal's deviation was greater than about +/-3 kHz.  Interestingly, this flaw had gone unnoticed in the 30 years that this receiver had been used because no-one had looked for it, and it really didn't sound that bad since the transmitter removed the speech harmonics, anyway.  Coupled to this was the fact that pre-emphasized audio - common on our VHF and UHF bands - boosts the highs as compared to the lows, further masking the clipping.  With all of that distortion, the voter was "seeing" plenty of audio energy above 2 kHz and voting "away" from it in favor of the receiver connected via the UHF radio link.

The first problem required that we put a low-pass filter on the audio from the local receiver to make it "look" like the audio coming across the UHF link and for this, a filter was installed that scraped off the audio above about 3 kHz.

The second problem - the receiver distorting - was a bit trickier.  Once on the workbench, I noticed that the clipping was asymmetrical and I traced it to an emitter-follower audio stage coupling from the discriminator chip with insufficient headroom, trying to go "below" ground.  Poking around with an oscilloscope, I noticed that the input signal to this emitter-follower stage was "clean", so I modified it by turning it "upside-down" and using a PNP with its collector connected to the ground side instead of an NPN with its collector connected to the V+.  Doing this moved its clipping threshold "upwards", toward the V+ supply instead of ground.

That problem solved, I then noticed that I still couldn't go above about +/- 3 kHz deviation without significant distortion - but this time I traced it down to the 10.7 MHz IF crystal filters.  As it turned, one or more of these had gone bad and the filtering was no longer "flat" across the required 15 kHz IF bandwidth of this single-conversion receiver.

Fortunately - because of another project - I had on hand some extra 10.7 MHz crystal filters and I was able to replace those in the receiver and nicely flatten out the response.  The ultimate result was that the receiver could accept about +/-7 kHz of deviation modulated with a 1 kHz tone before it started to distort badly.  The measured IF bandwidth was now about 16 kHz at the -3dB points and much more than 20dB down at 20 kHz.

Re-installing the receiver, the audio levels from the remote receiver (via the UHF link) and the local receiver were very carefully matched and the voter behaved a bit better - but it was still favoring the "remote" receiver slightly.  Fortunately, when I designed the voter I included a number of software parameters that could be tweaked and I was able to "bias" the voter slightly toward the local receiver.

The voting controller:

Figure 2:
The main voter board.  As you can see, it is all hand-wired on
glass-epoxy prototyping board.
Click on the image for a larger version.

The voter itself is homebrew and uses a PIC18F4620 processor at its heart.  Capable of accepting up to 8 receivers, it has two voting channels:  The "current" receiver - the one that is being selected for on-air, and the "other" channel that is being compared to the on-air receiver.


If there is just one receiver active, the choice is simple:  Select it - no voting required!

If there are two receivers active they are compared against each other, the "better"one being put on-air and the worse one on the "other" channel:  If, at any time the signal on the "other" receiver gets to be better of quality than the one that is on-air, they flip positions.

If more than two receiver go active at the same instant it will grab the lowest-number receiver first and then compare, one-at-a-time, the quality of the other receivers against it.  If it finds that one of these other receivers has a better signal, it will put that receiver on the air and the process repeats itself with the processor constantly seeing if any of the other receivers have a better signal than the one that's currently on-air.

In the audio chain of the voter is a digitally-programmable potentiometer that is used to set the gain of each selected receiver (on both the main, on-air channel and the "other" channel) to the levels that allow the audio of each receiver to be perfectly matched.  There is also a "phase selector" switch that allows either a 0 or 180 degree phase change of the receiver to account for the fact that there may be an audio inversion somewhere earlier in the audio chain.

Both the selected "on-air" and the "other" receiver are passed through a 2 kHz high-pass filter which then goes to a rectifier circuit, the result being that a DC voltage is produced that is proportional to the amount of audio energy.  While some of the energy above 2 kHz is going to be voice energy, some of it will also be the added noise on a weaker signal, so that "noisy" channel (e.g. the one with the poorer signal) will have a higher voltage than the other.  The two voltages are digitized by the computer and used to make the decision as to which receiver has the better signal.

Cleaning up the squelch noise:

One issue with using a link receiver for connecting receivers for voting is the fact that if there's a "choppy" signal, the remote receiver's squelch will constantly open and close - but how do you convey that fact to the voter?

One way is to simply turn the UHF transmitter on and off and follow receiver squelch, but this gets a bit messy, particularly since there will be a burst of noise every time that happens (e.g. the "ker" in "kerchunk"!)

To avoid this, the instant the squelch of the remote receiver closes, a 3.2 kHz "voting tone" is sent down the link.  At the voter, this tone is detected and treated in the same way as dropping of the link transmitter and this causes a "squelch" indication from that remote receiver to show that the input signal has gone away.  After the tone is detected, the link transmitter can shut off and that extra "ker" (e.g. burst of noise) won't ever be heard by the user!

This tone also has another purpose:  Since it is a very strong 3.2 kHz tone it will be intercepted by the voter's noise detector as a full-scale indication of "bad signal noise" and the voter will instantly "see" it as a signal of inferior quality and jump away from it in favor of another signal, if available.

Other circuits:

In order to meet FCC rules, the UHF link transmitter must occasionally ID and this is done using a 2.44 kHz tone frequency, a frequency above most of the audio content.  2.44 kHz notch filters are used in front of the tone detector (but NOT in the audio path to the audio output of the voter) in each of the voter's channels (e.g. the one that is on-air, and the "other" one) to remove the link IDer and keep it from affecting the voting as well as a notch filter to remove the 3.2 kHz "voting tone" that is sent on the UHF link.  If the link IDs with no-one talking, the 3.2 kHz voting tone will already be on, keeping an "open squelch" indication from being passed to the voter and prevent that ID from even keying up the repeater.  If an ID happens to occur while someone is talking through the voter, there's no harm as the pitch of the ID is fairly high and most people don't ever notice it!

Figure 3:
Inside the voter's die-cast metal box with the circuit board removed.
All input/output signal lines go through feedthrough capacitors
and ferrites to remove any RF energy that might be coupled
onto them - an absolute necessity since the repeater site
has extremely high levels of RF energy!
Click on the image for a larger version.

Another circuit an 8-channel multiplexed input that is very rapidly scanned to determine which COS (squelch) output lines from the receivers are active, this being done so that the voter knows which receivers are actually active!  The voter also has the capability of producing "courtesy beep" tones which are used to identify the various remote receivers by pitch allowing users to determine into which a particular user is being received.  Since most of our users come across the main receiver, this one doesn't have a beep!

The voter - like the disciplined oscillator - has an RS-485 bus on it which allows it to be configured via a serial port as well as be queried and configured remotely.  As it turns out, one can kludge an RS-485 interface to talk to most RS-232 ports by using just one of the receive/transmit lines and this is brought out on a DE-9 front-panel connector.  In this way, one can look at the configuration of the voter - even make adjustments - while on-site as well as talk to any other device that is also hanging on the RS-485 bus!

Finally, the voter has 16 front-panel LEDs, 8 of which indicate which receiver is (or recently was) active while the other 8 LED indicate the various states of the hardware for monitoring and diagnostic purposes.


Final comments:

This voter was installed in October of 2009 and has been in continuous service ever since.  In the first month, the code was tweaked slightly to allow more-detailed tweaking of the noise detection algorithm to minimize the likelihood of "false voting" and to permit one receiver to be favored slightly more than the other.  The only "major" change was to add a minimum "dwell" time to the voting - that is, once a particular receiver was selected, it had to stay there for a minimum amount of time before another receiver was allowed to be selected unless, of course, that receiver dropped out!

The only hardware tweak was increasing the gain of the noise detectors to allow the voter to better-determine the relative quality of signals that were already nearly "full-quieting":  Before doing this the voter couldn't tell the difference between signals that were better than about 18dB quieting, but now this has been extended to about 25-27dB quieting - a noise level below that which most users even notice!

[End]

This page stolen from ka7oei.blogspot.com

Two repeaters, one frequency (part 2)

In Part One I'd described why it might be advantageous to place multiple repeaters of a linked system on the same frequency.  In short:

• A single frequency conserves spectrum.
• Being on the same frequency over the system's coverage area is more convenient to the user as it eliminates the need to try to figure out which frequency might works best for a given area.
• The whole system is greater than the sum of its parts because of the probability that brief periods of poor coverage may be augmented by another site.

 In the first part only the implementation of the receive portion of the system was discussed in which multiple receivers were used in a voting scheme - that is, a system in which the signals from the various receivers in the system were analyzed and the best one at that instant was sent on all transmitter.

How, then, does one implement multiple transmitters on the same frequency without their clobbering each other?

This comes again to one of the peculiar aspects of Frequency Modulation (FM) mentioned in the first part of this series:  The Capture Effect.  Briefly, this is the tendency for the stronger of two FM signals to override the weaker - and if they are of sufficiently different signal strength, there may not even be evidence of the weaker signal.

As it turns out, for a number of reasons this effect is more obvious on wideband FM as used in broadcast and you may have even observed a different FM station to suddenly "pop in" in an area where there was overlap.  On the narrowband FM used on amateur radio this effect is somewhat less dramatic and "doubling" (two stations inadvertently transmitting at once) is typically detectable by there being a rather obvious squeal and distorted speech behind the stronger station transmitting or, in cases where the signals are almost exactly of equal strength, neither party wins as the two obliterate each other in an unintelligible mess of noise.

What is worth noting in the above example is that the two transmitters involved are:

Figure 1:
Inside the frequency control/crossband repeater unit at Scott's.  There is
an identical unit at the other site at Farnsworth Peak.
The 10 MHz oven-controlled oscillator is in the upper-left corner
while the standard GE "EC" channel element is in the upper-
right corner.  This unit - like its twin - is hand-wired on glass-
epoxy prototyping board.
Click on the image for a larger version.

• On different frequencies.  It's likely that the two transmitters that operated at the same time were on slightly different frequencies - even several hundred Hertz apart.  This frequency difference resulted in a heterodyne (squeal) that decreased intelligibility.
• The two transmitters were definitely not carrying the same audio.

As it turns out if there are two transmitters that are both held to very tight frequency standards (within a few 10's of Hz at most) and they carry exactly the same audio, they tend not to clobber each other to nearly the same degree if they are of similar signal strength. What's more is that these "similar" transmissions seem to bother each other less as the difference in their respective signal strengths become greater.

Again, the system is laid out thusly:

•  Farnsworth Peak is the "hub" and the audio for all transmitters in the system originates from there.  The audio to the auxiliary sites (such as Scott's) is conveyed via a UHF link and retransmitted on VHF.
• All audio from all receivers ends up at Farnsworth and the "best" audio is what is transmitted to all sites.
• The auxiliary sites (such as Scott's) are essentially crossband repeaters:  2 meter audio is received and relayed to Farnworth on UHF where it is voted upon and this audio is transmitted from Farnsworth  on UHF where it is repeated on VHF at the auxiliary sites.

What this means is that at Scott's, there's a box called the "Disciplined Oscillator" that contains a precision, oven-controlled 10 MHz oscillator that is capable of holding the VHF transmit frequency to within 1-2 Hz of where it is intended to be.  As it so-happens, this same box also contains the intelligence to function as a controller for a pair of crossband repeaters that goes from VHF to UHF for signals that are received and then again from UHF to VHF as the master audio from Farnsworth is transmitted.  This box also provides a few other basic functions such as timeout timer (in the event a link gets "stuck") as well as providing a Morse ID on the UHF link from Scott's to Farnsworth - just to keep things legal.  This same box also has an RS-485 serial interface to allow it to be connected on a bus with other devices so that it may be remotely controlled, configured and polled as needed.

 When we originally designed the system we anticipated that we may need to adjust a few parameters in order to successfully have two transmitters operating on the same frequency without their causing objectionable mutual interference.  The first - and most obvious - of these was frequency control.

Because we use independent oven-controlled crystal oscillators, we couldn't nail the frequencies of the transmitters down precisely to match each other as would be possible were we to have used a GPS or Rubidium-based reference, but we could count on their being within 1-2 Hz of where we had parked them.  Once the system was put on the air we solicited the help of someone who happened to live in an area where the strength of the two transmitters was precisely equal and then tweaked the frequency offsets and then made a subjective analysis as to what was "least annoying."

As it turned out, there were two ranges that seemed to be reasonable in terms of frequency offset:

• 3-6 Hz offset.  This caused a bit of a "whooshing" sound if the two signal strengths were fairly close and fairly weak.  If the signals were exactly the same strength then the periodic nulls could cause it to drop out briefly and make the signal unintelligible, but even a slight reposition of the receive antenna could mitigate this, however.
• 40-60 Hz offset.  This caused a buzzing somewhat akin to the sound of a subaudible tone as heard on a signal with severe multipath distortion.

Ultimately, we settled for the 3-6 Hz offset as it was deemed to be the most "user friendly" overall - especially when one considered that one was by far more likely to be traveling mobile through the overlap areas than stationary and that the Doppler shift of a moving vehicle might not only exceed the amount of frequency offset anyway (if it was only 3-6 Hz, at least) but that the "dwell" time in a precise null where the signals of multiple transmitters canceled each other out was going to be extremely short.

Another factor often considered in multiple-transmitter systems is that of audio delay to match the time-of-arrival of the different distances between transmitters - plus additional delay in the audio links used to tie the disparate systems together.  Before we were to go through any hassle of adding an audio delay somewhere, we first wanted to see if it was really going to be a problem in the overlap areas, anyway.

It wasn't.

The only thing that we did do was observe the audio phase at and below 1 kHz and then, using the ability to select either a 0 or 180 degree audio source, set them as close as we could.

So, what does it sound like in the overlap areas?

First of all, the coverage of the sites and their geography meant that about the only significant overlap areas were in canyons to the east of the Salt Lake area where signals from either transmitter would already be subject to multipath, anyway.  As it turns out, traversing these area it's rather difficult to tell where the coverage of one transmitter begins and the other ends - and it often goes both ways.  In those area that do have severe overlap the contention between the two transmitters sounds little different than typical mobile flutter - perhaps slightly "faster" than typical 2-meter flutter but not as fast as what might be heard on a 70cm repeater in an area with severe multipath!

In Part 3, a bit of "nerdy" technical information about how the various parts work...

[End]

This page stolen from ka7oei.blogspot.com
 

Two repeaters, one frequency (part 1)

These days, finding a frequency to expand ones repeater system can be a challenge - even in "rural" parts of the country such as Utah where the Salt Lake area is about the only large population center for hundreds of miles.

Figure 1:
The Scott's Hill site, part of the UARC 146.620 system
Click on the image for a larger version.

Typically, a linked repeater system consists of several repeaters tied together on a backbone frequency and each of these individual repeaters is usually on its very own frequency:  About the only time that frequency re-use is implemented is if several of these individual repeaters are located far enough apart that they won't bother each other and it is often the case that different subaudible tones are used to prevent mutual interference should a user be in an area with potential overlap.

More than a decade ago the Utah Amateur Radio Club decided to expand the coverage of its 146.620 repeater and a mountaintop site was secured - a story in and of itself to be told another day, perhaps.  As things often happen the project lay fallow for several years until a set of circumstances provided the ambition and impetus to push it along farther.

From the beginning, the intent was to have a "Synchronous" and "Voting" repeater on this other site, Scott's Hill, that was to share the same frequency as the original repeater on Farnsworth Peak, but putting together such a system was understandably more involved than the typical linked (but each site using a different frequency) repeater system.

The original repeater on Farnsworth Peak provides impressive coverage, from north of the Utah/Idaho border, west beyond the Utah/Nevada border, to the south into parts of central Utah but pretty much stopping at the Wasatch range to the east of the Salt Lake metro area.  For the most part, the coverage of Salt Lake area repeaters is limited eastward by the abrupt rise of an 11,000 foot mountain range along the east side of the populated areas and unless a repeater is located atop those mountains, coverage to the east is minimal.  Unfortunately - or fortunately - repeaters located in the Wasatch intended to provide coverage to the high valley areas east of the Salt Lake Valley tend not to provide good coverage into the Salt Lake valley itself owing to the shielding effects of the mountains themselves - that is, the taller peaks on which repeaters are placed are generally set back a bit and the somewhat lower "front" peaks to their west tend to block the view of the valley.

Scott's Hill is such a site:  It sees well from the East through the Northwest but it can actually see none of the Salt Lake valley to the south and west.  It does, however, have a good, line-of-sight view of Farnworth Peak, so the linking between the two sites is pretty easy.  This general exclusivity of coverage also means that having the two repeaters effectively sharing the same frequency would be simplified as there were relatively few places where the two would overlap with comparable signal levels.

Figure 2:
Voting controller for the 146.620 system.
Click on the image for a larger version.

Now, how does one go about putting two repeaters on the air, on the same frequency, without their clobbering each other?

Multiple receivers on the same frequency:

For receive, the answer is pretty easy:  Voting receivers.

On a "Voting" system, one typically brings the audio from all of the separate receivers to one central location and there, they are all analyzed for signal quality and the best of the lot is selected and used as the audio source for the entire system.

Compared to the typical linked system where the user selects which repeater/frequency is to be used, there are advantages to having ONE frequency with multiple (voting) receivers:

• Easier to use.  If there is only ONE frequency, the users don't have to constantly change to the best frequency for the area from which they are transmitting - assuming that they know which is the best for their specific location!
• Frequency re-use.  With a voting system, only ONE frequency is required which can save a bit of spectrum.
• The whole is greater than sum of the parts.  On a multi-receiver system, it's typical that while one particular receiver works best for a specific area, it's also likely that the less-optimal receivers will also provide a degree of coverage in that same area.  If one enters an area where coverage is a bit "spotty" on the primary-coverage receiver, there's a reasonable chance that one of the other receivers may be able to still hear the mobile and "fill in" - all of this without the user having to worry about it!
• The addition of even more receivers.  Once the "base" voting system is installed, it's practical to install additional "fill" receivers for those areas where better coverage might be desired:  These extra receivers need only be a simple receiver and link transmitter rather than a full-blown repeater requiring a lot of expensive filters.

While there are a number of ways that voting systems can work, pretty much all of them exploiting a "feature" of the frequency modulation (FM) that we use on our VHF and UHF bands:  Quieting.

You have probably noticed that as an FM signal sounds the same whether it is very strong or weak - at least until the signal gets to be really weak - at which point it starts to sound noisy but NOT quieter!  If one were to listen carefully, it might also be observed that the noise tends to start out at the higher frequencies first - and this is how a radio's squelch works:  It listens for the high-pitched hiss that starts to show up as the signal gets weak.

Most voters listen for this "hiss."  On a typical system, since all of the receivers are listening to the same audio being transmitted, the one with the least amount of hiss is, in fact, the one receiving the best signal.  If you think about it, all one really needs to determine is which one has the least amount of "audio plus hiss" as the only thing that will be different among the receivers with different-quality signals will be the amount of hiss on them.

Ideally, one would do this comparison at the receiver itself where one has access to the "guts" of the receiver and can look at the "discriminator audio" where the spectral content can go into the 10's of kHz.  Practically speaking, however, we have to link these individual receivers back to one site for the voting and conventional FM link radios can't pass the 10's of kHz of audio necessary to do this so the "audio plus hiss" scheme is used.  The voter on the 146.620 system works this way, mostly looking at the higher-frequency audio (e.g. above 2.5-3 kHz) to determine which of the inputs has the "best" signal (e.g. least "audio plus hiss.")

There are other ways to do this - including digital means where precise signal quality measurements are telemetered to the main controller - but our intent was to construct the entire system using "off the shelf" radio modules that were available on the surplus market so that there would be a reasonable hope of it being maintained in the future.

This article - including more details on two transmitters sharing the same frequency - continues in Part Two.

[End]

This page stolen from ka7oei.blogspot.com