Data to Simulcast Transmitters

[RADIOMAN2002]

The question is, how does a transmitter in a wide area Type II trunking system get its low speed data sent to it. I imagine the voice goes over a 600ohm phone line or microwave link, but how does a data stream which has about the same average frequency as PL or DPL get sent to the transmitter. I would imagine there is some sort of modem, but is it internal to the station or is it separate. I am trying to extend the range of a LTR system and don't want to have to install a controller at every site.

[xmo]

In Motorola analog simulcast, the low speed and voice are combined at the prime site and distributed to the remote sites with T-1 links. The T-1's connect to channel banks located at each site. For simulcast, Motorola has developed a very specialized card for use in these channel banks. That card is known as a Digital Simulcast Modem [DSM-II]

The DSM has frequency response flat from virtually zero so it passes the entire simulcast modulation including the low speed. The DSM-II also connects to GPS 1PPS and automatically sets the simulcast timing. You never even have to phase optimize the system.

You can buy these items from Motorola and use the technology for other applications such as a simulcast voice paging system. The parts are pretty pricy. [about 10K per DSM]

[d119]

In Motorola analog simulcast, the low speed and voice are combined at the prime site and distributed to the remote sites with T-1 links. The T-1's connect to channel banks located at each site. For simulcast, Motorola has developed a very specialized card for use in these channel banks. That card is known as a Digital Simulcast Modem [DSM-II]

The DSM has frequency response flat from virtually zero so it passes the entire simulcast modulation including the low speed. The DSM-II also connects to GPS 1PPS and automatically sets the simulcast timing. You never even have to phase optimize the system.

You can buy these items from Motorola and use the technology for other applications such as a simulcast voice paging system. The parts are pretty pricy. [about 10K per DSM]

In addition to that, the low-speed data is actually summed from one central location via a "USCI" (Universal Simulcast Controller Interface), and distributed to the DSM-II cards from there. So the low speed data your subscriber units are hearing is actually centrally generated at the prime site (if I recall correctly from working with SZ 4.1).

The same thing is done with PL in conventional systems, with the interface being called a "CSCI" (Conventional Simulcast Controller Interface). The transmitter PL is centrally generated in the CSCI and then distributed out via channel banks. This keeps the PL at the same level and frequency across all sites.

I question the comment about never needing to phase optimize a system... If this were the case, why does every system include a $30k+ aglient DSA? And I also received extensive training on how to phase optimize systems that use DSM-II's when I went to Motorola University

[xmo]

For systems using the DSM-II the optimization procedure is contained in 6881098E65. The DSA is used to LEVEL optimize these systems using swept measurements to precisely adjust the modulation compensation of each base station in order to flatten their frequency response so that the audio modulation leaving each base station is as identical as possible.

Phase optimization is an entirely different issue. In systems using the DSM-II, phase optimization is a process of DSM programming and verification of the correct operating parameters [e.g. 1 PPS present, MARKER present]. The documentation does not contain a measurement procedure using the DSA or any other instrumentation to adjust or to verify the optimization.

Earlier Motorola simulcast systems required complex measurements to adjust and verify the timing of the transmissions from each site. The DSM's automatically insure simultaneous launch of the modulation from each site.

[d119]

Ah OK. In class they always referred to that as "Phasing" the system with the DSA.

[Wowbagger]

If this were the case, why does every system include a $30k+ aglient DSA?

Egad! They get 30K for the Agilent DSA? No wonder Motorola was so keen to have Aeroflex add the ability to do the same thing into the 2975 and 3900 family!

[xmo]

When GPS simulcast was introduced Motorola started drop-shipping either the 35665A or 35670A dynamic signal analyzer along with many of the systems. Perhaps this was initially done because phase optimization was necessary on the earlier simulcast systems and the System Engineers [the folks who put together the equipment list for each customer's requirements] hadn't been advised of the change.

The 35665A probably only cost a little over $10K back in the day but it was discontinued. The 35670A is Agilent's current replacement and yes - you can easily spent over $30K on a one of those depending on options.

The first service manuals for GPS simulcast systems made very little use of the DSA - hardly more than using it as an AC volmeter. The current procedures were developed after the importance of the precise matching of the station's mod comp became known.

Even this process only uses a fraction of the capabilities of a DSA so it certainly makes sense to incorporate swept audio measurement capability into your RF test sets so that the system's test equipment budget can yield a greater return than the purchase of an instrument that is rarely used.