Normally the MMDVM is pretty foolproof regards D-Star and YSF, by DMR is a little more tricky.

Equipment required: – The repeater (obviously) and an RTL/SDR Dongle running on a PC with SDR#. (Or the equivalent of other Operating Systems.)

Running SDR Sharp set to FM on the transmit frequency of the repeater. The adjustments on the screen will need you to view the narrowest bandwidth viewed as wide as possible as we are going to be looking for a phenomenon called the “Besse Null”. Firstly in the DMR configuration/expert/MMDVMHost – Modem Section you must set TXInvert=1, RXLevel and TXLevel = 50 and DMRTXLevel=50 (and any other mode you use.)

Now you will need access to the potentiometers on the MMDVM device, and select the one used for transmit. It should be the one closest to the audio-out pin. On the ZumRadio Pi-Hat it’s the one nearest the purple wire, or to the right of the top of the board.

Now the operation of the MMDVMCAL is controlled from the Pi-Star software terminal screen in one of two ways. Either the terminal screen by SSH into the Raspberry Pi from a source computer, or in the SSH Screen from the Configuration/Expert menu.

The command used is “sudo pistar-mmdvmcal” which displays a menu.

Normally most MMDVM Modem boards should have TxInvert=1, so when checking the calibration, we will have to do the same by tapping “I” first. This will set the correct format of transmission.

Typing “D” or “d” sets the test mode for DMR. and we are ready to begin the calibration. Tapping the “space” toggles the transmit on the repeater. So tap it once to view the resultant wave-form on the SDR# screen. You should see 5 or more clear peaks. By tapping “T” to increase the deviation and “t” to decrease with show a decrease of increase of this NULL – the “Besse Null”. The centre peak should descend towards the bottom of the display. Stop transmitting, noting the value of “T” and press “q” to quit.

Now you need to repeat the procedure, but this time instead of using “T” or “t”, we need to make the adjustment by the transmit potentiometer, until the null is at its most profound. Hopefully this will now correspond with the settings in the mmdvmhost of DMR configuration of 50 in the DMR-TX level box. Now reset the repeater to normal operation, and check the BER rate%. The DMRTXlevel box can be used as a fine adjustment either up or down.

When adjusting the potentiometer, be aware that you are looking for a sweet spot. It has been known that some modems have had two sweet spots, that complicates this, however this is rare and has only occurred in the Arduino Dué (Dooay) mounted MMDVM Zum 1.0 device. Rotate the potentiometer at whole turns at first, then half turns, then eventually 1/4 turns. From time to time, exit MMDVMCal and return to normal repeater mode to check that you obtain the handshake tone, and that the error rate is close to or at 0%. Use TG 9 to do this, and try both slots, to see what if any the variation may be.

Quite possible there will be some trial and error and retrying this procedure to get the correct desired result. You will know when you transmit to the repeater and you get an ACK tone, and the live-logs show all the correct information of the transmitted signal.

Yes this is a repeater of an earlier article, where I specified the Arduino and the Zum board 1.0.

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