Ritron RPM Series Application Note

APPLICATION NOTE JULY 4, 1996

1200/9600 BPS AND POCSAG MODEMS USING THE RITRON RPM SERIES TRANSCEIVERS

The Ritron RPM-150 VHF-FM 30 Watt Programable Synthesized Radio Transceiver is

a great radio for use with a 9600 bps packet modem.  It is even more suited

for use with a dual mode 1200/9600 bps modem with POCSAG encode/decode

capabilities.

This is because the RPM-150 has about a 60 millisecond TX/RX synthesizer

settling time from application or release of the PTT signal.  There aren’t any

synthesized amateur radios that are any faster and you can’t use a TNC TXDELAY

setting that is faster than the slowest user you want to support on the

system.  If you want really fast TX/RX transitions Ritron makes the RM series

crystal controlled telemetry modules that do a full TX/RX transition in less

than 1.5 milliseconds.  But then with who would you be able to connect?

Also the connection for the 1200/9600 bps and POCSAG modulation can be made to

an internal op amp summing node. This means that the signals from the internal

microphone speech processor, internal sub-audible tones, 1200 bps and 9600 bps

modems do not interact or load each other and change modulation levels.

Finally the RPM-150 uses a two point modulation technique.  This provides

essentially flat frequency response from a few Hertz to 5000 Hz.  This really

helps provide good 9600 bps and POCSAG modulation response.

And of course, the RPM-150 is proudly designed and manufactured in the United

States of America.

Here is a description of what is required for a typical installation.

Using the RPM’s PC programming software, disable the PTT Debounce function for

minimum TX/RX delay.  This feature is available in RPM’s with the “04”

microcontroller or later.  Earlier units with an “02” or “03” microcontroller

cannot eliminate the 16 to 32 millisecond debounce time and will therefore run

slower.

Short R608 to ground on the volume control / display board. It enables the

receive audio to be set all the way to zero.

The microphone gain is set for commercial applications. Change C237 from

1000pF to 100pF and R248 from 56K to 120K for more microphone gain.  This is

optional since the modem signals do not go through this path.

Install the optional Ritron 9 pin accessory connector on the RPM-150 rear

panel wired per Figure 1.

The popular Kantronics KPC-9612 has been successfully used with the RPM-150 as

follows:

1) Remove R5. It overloads RXA discriminator output.

2) Set the 9600 bps TXDELAY for 9 or 10.

3) No special TX or RX equalization is required.

4) Remove J6 to decouple the 9600 bps modulation.

5) Set J16 and J17 to position 2. This allows the RPM-150 to power the

   KPC-9612 via DB-9 pin 7.

6) Install J7 for proper output level.

7) Install J1 in for proper 1200 bps RX equalization.

8) Remove J2 for proper 1200 bps modulation.

9) Adjust for proper 1200 & 9600 bps deviation per the manual.