Using GM350 to encode Pocsag ?

[oli2bo]

Hi all,


I'm trying to use a GM350 to encode Pocsag at 2400 bauds. I've the correct Tx freq, the correct RIC code, and I encode with a computer and the www.dsp4swls.de "sorfmon_pocsag_encoder" software. It's a screen version, with the accessory 16 pin plug.

I'm using the following accessory pinout : pin 3 for ext PTT, pin 5 flat Tx audio directly on my soundcard output, and pin 7 to the ground of the computer.

I sometimes have a result, but it's not reliable at all (I'm only at 5w power, just here in my office). Every times it works is when I wait 10-15 min between the tests.

When I listen to the the phrases with another radio, it seems to be very clear, but in my opinion I've a problem with the modulation or the input level in the radio.

Could you please help me ? I've a spectrum analyser but not familiar at all with metering modulation.

I thank you in advance.

Oli

[wavetar]

You are very likely over modulating the radio and 'rounding off' the pocsag waveform. It may still 'sound' fine on another radio, but that's because the limiter circuit in the GM350 keeps it at 5KHz, but the waveform is distorted. It's difficult to say for sure, without a communications analyzer capable of showing the transmitted waveform and measured deviation. I'm not familiar with the pocsag software, or the GM350 RSS, but if there are adjustments for audio level, I would try turning them down.

The other thing I've seen when feeding pocsag through most Motorola radios is it forces the center frequency off by about 2.5KHz when transmitting, as measured by a service monitor. This may be causing you some issues as well.

[Jim202]

You are very likely over modulating the radio and 'rounding off' the pocsag waveform. It may still 'sound' fine on another radio, but that's because the limiter circuit in the GM350 keeps it at 5KHz, but the waveform is distorted. It's difficult to say for sure, without a communications analyzer capable of showing the transmitted waveform and measured deviation. I'm not familiar with the pocsag software, or the GM350 RSS, but if there are adjustments for audio level, I would try turning them down.

The other thing I've seen when feeding pocsag through most Motorola radios is it forces the center frequency off by about 2.5KHz when transmitting, as measured by a service monitor. This may be causing you some issues as well.

Pay attention to what Wavetar has said. Low deviation is much better that high deviation in paging.

Another issue might be in the way your connecting to the radio. My bet is that you didn't cap couple the audio from the computer to the radio. DC bias on that line that the sound card is just not making the radio happy with. You could start with a 1.0 uf cap and see if things don't get better. You might have to go larger.

Jim

[oli2bo]

Hi Wavetar, hi Jim,


Thank you for your answers.

I've just found a little interface who whas used with the GM350, to do paging in Pocsag on this radio. Do you think it would help if I show it to you ? I'm not good enough in electronics to understand the schematics, but perhaps you coul see something interesting...

I'll try to take some pictures of this interface.

Oli

[oli2bo]

Hi all,


Here are the pictures of the interface I found, who was placed between an encoder (that did'nt exists anymore) and the GM-350 (via the accessory pin) :

https://www.dropbox.com/s/sjwrh5ah3bnemi0/IMG_2373.JPG
https://www.dropbox.com/s/2o6o1mjf9u7uek9/IMG_2375.JPG
https://www.dropbox.com/s/z8cyw65k8w6c616/IMG_2376.JPG

I thank you for your help.

Oli

[wavetar]

Hard to say for sure, but it certainly appears to be some sort of level setting circuit.