MMDVM Calibration

For TX the best way is to use a spectrum analyzer to observe the TX RF directly. The MMDVCal program provides a 1.2kHz tone which deviation on the RF signal should be adjusted on a level where the "Bessel zero" is clearly visible: Set the spectrum analyzer to 1kHz RBW and tune to the RF TX frequency. Use MMDVMCal to switch the repeater to TX and emit the DMR cal signal. While slowly turning the TX deviation pot on the ZUM board observe the sidebands of the RF signal on the analyzer. When you've hit the right deviation level, you will notice that the carrier in the center of the RF spectrum shows a clearly visible dip. This is the "Bessel zero" where the deviation of both tone sidebands "zero out" the carrier. Adjust the pot in a way that the dip shows maximum depth.

For the RX I've simply used a scope to check the incoming RX signal delivered from the ZUM board to the ADC pin of the Arduino Due: The ZUM board should create a 1,65V DC offset at the SAM ADC input. Key you radio (on TG9 or similar...) and observe the audio on the ADC pin overlaying the DC offset. (I've used the rising edge of the receiver's RSSI signal for triggering the scope onto the data bursts of my portable radio) The RX pot on the ZUM board must be adjusted in a way that the peak amplitude of the audio signal does neither reach the zero line nor the 3.3V maximum. Keep a margin of 0.3 to 0.4V to both limits to be safe! It must be assured that the ADC on SAM3X is never overdriven by the audio signal.

With these HW adjustments you should have a suitable basic setting for operation. For a fine tuning you may play with the TX / RX settings in the MMDVM.ini, but I found this was not necessary when the HW tuning has been performed properly as above.

73 de Bert, DD5XL