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  Using RF Links with
Voting Receivers and
Some History of
Motorola's Voting Systems

Compiled by Robert W. Meister WA1MIK
From Discussions with Greg Carttar
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Using RF Links with Voting Receivers:

Just doing a simple cross-band repeater will not give you satisfactory results, but it will work... sort of. What you really have to do is transmit raw unfiltered discriminator audio across the link.

One fire department system uses Motorola CDM750 radios at the remote sites. The VHF receiver is set for flat RX audio on the accessory pin output. That is coupled to the mike input of the UHF link radio and it's set for flat TX audio also. Therefore, you get a fairly accurate representation of the discriminator audio at the far end of the link without repeated pre-emphasis/de-emphasis cycles, which kill the high-frequency content and make the comparator less likely to vote correctly.

CSQ/PL detect (5v logic low) out of the VHF receiver keys the UHF link transmitter (PTT low).

The UHF link receiver at the comparator is set for "filtered" audio so that the required de-emphasis takes place once only, at the comparator end of the link. CSQ/PL detect at the accessory connector of the UHF link receiver (5v logic low) is level shifted and inverted to +12v logic high to interface with the SpectraTAC modules.

Alternatively, you can use a stock SpectraTAC receiver as the link receiver and de-emphasis and level shifting is taken care of. I have done both. The stock SpectraTAC receiver works fine in narrowband. The CDMs still limit the transmit deviation even with flat audio, so they won't over-deviate.

Regardless of which you use, you want a 1000 Hz tone deviated at 2.5 or 5 kHz at the VHF receiver to result in 0 dB (0.775v) out of the UHF link receiver, set with the audio level on the link receiver, and you want the 2175 Hz idle tone to be 13 dB below that. NOT 12, NOT 14... 13 dB exactly.

Note that the link DOES NOT transmit all the time. It only transmits when there is a signal. The link DOES NOT pass idle tone. Idle tone is generated at the link receiver co-located with the comparator.

It is a little tricky to get the deviation correct across the link. A 1000 Hz sine wave signal into the VHF receiver at 2.5 kHz deviation (narrowband) or 5 kHz deviation (wideband) through a simple level pot is then used to set the link transmitter to 1.5 kHz (NB) or 3 kHz (WB) deviation across the link. This keeps the audio below the limiting threshold of the link radios.

Depending on what you are using for link radios, you may have to experiment with levels a little. About the only thing that will let you work with flat audio are Maxtrac, Radius, and CDM mobiles. Maxtrac and Radius radios must have the 16 pin accessory connector to get flat audio in and out of the radio.

BUT, for an RF link to work correctly, it MUST transmit flat discriminator audio.

I walked someone through using MICOR and MSR2000 as linking radios. He had to do some minor modifications to get flat discriminator audio buffered and injected into the channel element, but he got it going and it works fine. (An MSR2000 is basically a MICOR with black panels. The modules are nearly identical.)

When you transmit flat audio across the link, the link PASSES PL which speeds the key-up time of the link quite a bit.

Motorola made a filter module (called a "roofing filter") QRN8498A (gray front) that plugged into a SpectraTAC comparator chassis next to the SQM that was being used on a local receiver co-located with the comparator. The filter provides 300-3000 Hz bandpass filtering to make the local receiver have similar audio characteristics to remote receivers whose audio passes through frequency-limited RF links or phone lines. The local receiver audio goes through the roofing filter and then into the SQM. This was necessary with MICOR and MSR repeaters. It is not necessary with MTR2000 repeaters, since they do that filtering when you program them for SpectraTAC mode.


In one RF linked system, the link receiver, co-located with the comparator, provides idle tone. We're "fooling" the comparator into thinking that it is listening to a remote receiver by transmitting flat audio across the link.

In a Part 90 public safety system, we have to use PL for repeater control.

So, the distant VHF receiver decodes PL and keys the UHF link transmitter. This VHF receiver/UHF link transmitter pair transmits raw discriminator audio INCLUDING PL. No pre-emphasis/de-emphasis takes place in the remote receiver or transmitter.

The co-located link receiver decodes the PL that is passed across the UHF link and provides de-emphasis processing. It also generates idle tone for the comparator. No constant transmission of idle tone is required. The entire link chain uses PL to keep it legal and protected.


Some History of Motorola's Voting Systems:

The original voting comparator was the "TAC" comparator, which used a combination of switched tones to indicate received signal quality. Some SpectraTAC receiver manuals have this module in them. "TAC" was very primitive. The comparator voted the receiver that reported the best signal.

This was followed by the "SpectraTAC" format, where voting is based on the signal with the least high-frequency noise. Thus we have the SpectraTAC Comparator and Receivers that we all know and love.

There was a DVP digital version of the SpectraTAC comparator, but it was a different chassis.

SpectraTAC remained popular, but the next generation comparator was the DigiTAC. It would do SpectraTAC format 2175 Hz voting AS WELL AS digital DVP encryption format. It was (is) a much more complex device, since in digital format it uses all the bit streams from all the receivers and combines them to construct a complete bit stream if there are bit errors in any receiver.

The DigiTAC was vastly superior to SpectraTAC in terms of console integration and audio in/out flexibility, and it is programmable in the field for dozens and dozens of parameters... but I like the voting action of the basic SpectraTAC more. That said, the flexibility of the DigiTAC makes it more attractive in complicated systems.

Motorola then designed the AstroTAC as a digital comparator, and DigiTAC became their analog comparator. DigiTAC was discontinued in 2013.

Credits and Acknowledgements:

TAC, SpectraTAC, AstroTAC, DigiTAC, MaxTrac, Radius, GM300, MICOR, MSR2000, PL, and a bunch of other terms are copyrighted by Motorola Inc.

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This page created 29-Jun-14 by Robert W Meister WA1MIK.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.