Two repeaters, One frequency (Part 3)

For a follow-on article in this series, see Part Four (link)  for a discussion of how the voting receiver system works.

In parts One and Two the general overview of a "synchronous" (or "simulcasting") and voting repeater system was discussed.  In a nutshell:

• Both repeaters operate on the same frequency saving spectrum and simplifying the system's use since the user doesn't have to remember which particular frequency of a "normal" linked system covers a certain area best.
• The coverage of the two repeaters overlaps to a degree.
• Because of precise frequency control, the two transmitters don't really clobber each other in overlap areas, particularly in a moving vehicle.
• Because of voting receivers and multiple transmitters, the users can seamlessly move between coverage areas with no intervention on their part.
• The total coverage is greater than the sum of the parts owing to the increased likelihood of one or another site hearing the user and/or being heard - particularly if in an area where coverage is spotty to one or both sites.

 Originally (back in the late 90's) the idea was to frequency-convert the received signals from the 2-meter frequency to a subcarrier-baseband and send them to the main site where they could be voted upon and then a master modulator would then ship back (via a microwave link) a subcarrier which was then up-converted to the transmitter frequency.

The details were worked out and some of the equipment was actually built and tested - and it worked!  However, the magnitude of the task bogged things down and one thing led to another and the project languished - until 2009.

By then I'd already put together 2 voting systems and one multi-transmitter synchronous system (using GPS frequency references) and had other ideas on how to do things a bit more simply which translated to "being more likely to get completed!" The project got underway in earnest in mid-July of 2009 where the plans were re-draw and tasks divided as appropriate.

Instead of building the transmit and receive gear from the ground up it was, instead, decided to modify off-the-shelf GE MastrII radio gear to fit the bill.  This equipment is readily available on the surplus market and the individual pieces could be used with little or no modification - which meant that spares of those same pieces (receiver, transmitter, power amplifier, etc.) could be kept on hand as spares!  What's more, for the most part these units used common, off-the-shelf parts (resistors, capacitors, transistors) and were thus field-repairable now and for the foreseeable future.  Finally, a lot of information is available on these radios on the web so if, in the future some trouble shooting is required, there's plenty of advice to be had online.

What modifications were required to the radios were fairly simple:

• Instead of a standard crystal module (called an "ICOM" by GE) a simple, plug-in module (using a "gutted" ICOM) was plugged into the exciter instead.  This was connected via coax to an external module that provided the low frequency (at 1/12th of the transmit frequency) at the precise frequency.
• Transmit audio was fed into the subaudible tone input port.  This was done because it did not have the highpass and lowpass filters that the normal microphone inputs had:  We would do the high/low pass filtering externally!
• The receiver modification (for the Scott's Hill site) simply involved obtaining discriminator audio.

There were some additional modifications done to provide interfacing to the rest of the system - namely an outboard de-emphasis, a low-pass filter and a switchable notch filter (for a "quirk" we later discovered) but these were mounted on the backplane - a more-or-less passive board that would likely never require replacement!  Pretty much everything else was "stock" and could be tuned up and adjusted according to the original manuals!

Transmit frequency control:

The most important aspect of a multi-transmitter (simulcasting) repeater system is that the transmitters be where they are supposed to be, frequency-wise!  While there are several ways of doing this, we took a somewhat unique approach.

A standard transmit crystal (at 1/12th of the VHF transmit frequency) was ordered and placed into an "EC" type ICOM.  This is, in effect, a self-contained oscillator module that has provisions to be frequency-controlled with an external voltage.  This module is completely standard and off-the-shelf and it could be plugged into any GE MastrII VHF transmitter and work normally.

In our case, however, this "EC" module is plugged an external module - called a "Disciplined Oscillator" - that takes the crystal frequency (which is 12.2183333 MHz for a 146.620 MHz transmit frequency) and locks it to a reference based on a 10.0 MHz oven-controlled crystal oscillator.  This is done by synthesizing an audio frequency, using a PIC microcontroller clocked to the 10 MHz oscillator, that has a resolution of a few parts per billion and with a bit of dividing, mixing and comparison has the result of locking the 12.2183333 MHz oscillator to the 10 MHz reference to within a tiny fraction of a Hz.  Essentially, the frequency accuracy is that of the 10 MHz oscillator!

The 10 MHz oscillator is an oven-controlled crystal oscillator (OCXO) pulled from scrapped satellite gear and is well-aged (made in about 1990) and has a stability of about 10E-8 - within 1 Hz or so at the 2 meter transmit frequency.  This OCXO also has an external voltage control tuning line that is under control of the PIC microcontroller and with it, the 10 MHz frequency (and thus the transmit frequency) can be tweaked to set each transmitter on the desired frequency - which also means that the frequency difference between the two transmitters may be precisely controlled.  In the nearly 3 years since the system was made operational we've observed that the transmitters have stayed within about 1 Hz of their intended frequencies relative to each other over the course of the temperature excursions during the year!

This "Disciplined Oscillator" module also has another function that, since it was computer-based, was easy to implement, and that's as a simple dual cross-band repeater.  On Scott's (the remote site) it simply cross-bands the 2 meter receiver to the 70cm link transmitter - taking care of thinks like proper IDing, timeout timers, etc. and it also takes the 70cm link receiver and controls the 2 meter transmitter coming back the other direction:  Both operate independently of each other...

Squelch control and voting:

It also does one more thing:  COS (Carrier Operated Squelch) signalling.  The 146.620 repeater is one of the few repeaters in the area that does not have a subaudible tone requirement, this being because it's an "open" repeater and that extreme care is taken at all receiver sites to keep the receive frequency as clean as possible - a task that is arguably easier since the demise of analog television in the U.S.!

Since the Scott's Hill transmissions are relay to/from the master site via a UHF link there would be an extra squelch tail (the "ker" in "ker-chunk") if the loss of a signal at Scott's were signalled simply by its UHF transmitter being keyed/unkeyed.  Instead, the loss of squelch is signalled by the appearance of a strong, 3.2 kHz tone sent over the link which performs two functions:

• It signals to a decoder at the master site that the squelch as closed at the other end.
• It signals to the voting controller at the master site that the signal being received is "bad" and should NOT be used.

(This 3.2 kHz tone is "notched" out and its brief appearances in the system audio are not heard by the users.)

So, what happens if a user's signal into Scott's is dropping rapidly in and out?  As the squelch opens and closes, the tone is turned off and on (tone on = squelch closed/signal dropout.)  When the tone is turned on the voter disqualifies this tone, but if that same user is getting into the other receiver (at the master site) then this tone will guarantee that the signal from that receiver will be used, instead.

There is a short "hang time" on the link transmitter which means that when an input signal disappears from the 2 meter receiver at Scott's, the tone will turn on instantly, making the master site ignore the input, and then the UHF link transmitter signal will drop and in this way, the extra squelch tail from the UHF link transmitter dropping is never heard by the user.

As it happens, the signals coming the other way (from the master site to Scott's over the UHF link to be retransmitted on 2 meters) also use this 3.2 kHz tone - this time, to control the Scott's VHF transmitter.  In this case, however, the activation of the tone starts the "Unkey" sequence at Scott's allowing time for the disciplined oscillator to put an extra "beep" on the transmitter (so that users know which transmitter they are hearing!) and then unkey the VHF transmitter.

Since the 3.2 kHz tone being sent to Scott's occurs just before the master site's 2 meter transmitter unkeys, it's possible to set the timing so that both sites unkey at precisely the same time:  If both site's didn't unkey simultaneously, many users would be annoyed by the presence of an extra squelch tail if they could, in fact, hear the "other" transmitter hanging in there for a short time!

At the master site:

As it turns out, the master site's interfacing a was bit easier... sort of...  This repeater's master site is actually split, with the receiver and antenna being several hundred feet away from the transmitter, this being done to put it farther away from the megawatt of RF being emitted from all of the TV and Radio transmitters on the main site!  It is connected via transformer-coupled cables (for lightning protection) and has operated with minimum maintenance since the early 1980's.

Since we already had on-hand local COS (squelch) and audio from the receiver, there was no need for the tone signalling schemes of the remote site but, instead, the audio and COS lines could be input to the voter.  There was a problem:  The "local" receive audio was "too" good!

The way the voter works is that it analyzes mostly the audio above 2.5 kHz and of the receivers being compared, it is the receiver with the MOST audio above 2.5 kHz that is considered as being the one with the worst signal.  The reason for this is pretty simple:  As an FM signal gets weaker, it gets noisier, so it stands to reason that given two otherwise identical signals, the one that is also noisier will have a total signal level that is higher - particularly at higher audio frequencies.

The problem was that the audio from Scott's had already passed through a radio link which tended to scrape off the audio above about 3.5 kHz or so while the "local" audio, being coupled via wire, had no such low-pass filtering, so we had to add some.  What was happening is that the "local" audio - with its additional "highs" (as compared to the audio from Scott's) was being considered to be "bad".  By removing those extra high-frequency components and making the two audio signals pretty much equal we were able to make the more-or-less directly comparable.

Next time -Part Four:  A bit more about the voting controller and some of the remote control/monitoring capabilities.

[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