Interfacing Repeater Builder STM32-DVM with Motorola GM300 for DMR

Help with other MMDVM Repeater Boards (with NO onboard RF)
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Posts: 30
Joined: Thu Jan 09, 2020 1:41 am

Interfacing Repeater Builder STM32-DVM with Motorola GM300 for DMR

Post by N6PL »

I'm trying to get a wideband GM300 working with a Repeater Builder STM32-DVM v3 on DMR and am having one perplexing problem. I am using the flat audio in and discriminator out connections on the 16-pin connector and am having no problems with transmit, but am seeing a high BER on receive. I have the internal jumpers on the logic board configured for unmuted flat discriminator output on pin 11 of the connector but no matter how I set the receive level I see a BER of 12-15% as measured by the pistar-mmdvmcal utility. Receive will work ok when close to the radio and swamped with signal, but fails at anything less than an extremely strong rx signal level. Oddly the Pi-Star dashboard status display indicates low BER (under 1%) but the mmdvmcal utility indicates otherwise (consistently around 12-15% BER as mentioned above) and the poor performance of the system tends to make be believe the reports from the cal program over the dashboard display.

In addition to using the recommended rx level set procedure on the STM32 web page I have also tested over a wide range of rx signal level input into the STM32 with no improvement at all, and I have netted my test radio (an Anytone 878) and the GM300 to be exactly on frequency with respect to one another so I can't think of what else to try. It's possible that there is some inherent distortion or twist in the audio path using the stock discriminator output, yet I see many online reports of people getting good results using this connection point with the GM300 so I'm hesitant to go into the radio and start hacking if that is unnecessary.

Any ideas?

Thanks and 73
Posts: 3
Joined: Thu Feb 06, 2020 1:36 am

Re: Interfacing Repeater Builder STM32-DVM with Motorola GM300 for DMR

Post by KC5MN »

1. I suggest that you check the receiver sensitivity and it should break squelch or have a noisy but discernible signal at .5 microvolts or so. If you have identical receiver and transmitters, you might switch radios to see if the issue remains.
2. You might put an amplified speaker on the discriminator output of the receiver and see if you are getting adequate receive audio noise in the absence of an input signal and full quieting with a very strong signal. A signal generator with a calibrated output would help determining the full quieting point of the receiver in microvolts. By 3 microvolts or so, the receiver should be full quieting.
3. Another point of concern. Based on what I have read, you should not connect the RSSI input to the PIHat. Without a connection to the RSSI input, you might try adjusting the RSSI input just to be sure that it does not have an effect. This is a long shot but worth a try. You might email Scott N3XCC at and ask him about the RSSI adjustment in the absence of a connection to the receiver.
4. A document from Scott's website says to ensure JU551 is in position "A" in motorola Maxtrac style radios. I think the GM300s may be in the Maxtrac family but am not sure. I was going to attach the document but could not. Email at my qrz address and I will send you the document.

Good Luck and 73s
Bill KC5MN
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