Can you make a MOTOTRBO repeater by connecting two mobile radios together?

I've been asked this question a few times by different people but in different ways: Can you make a MOTOTRBO repeater by connecting two mobile radios together? One radio to transmit and one radio to receive.

The short answer is no.

The longer answer is that you can make a repeater of sorts but only for analogue operation. For DMR to work, the repeater needs to pass a bunch of control and signaling information that is not presented on the accessory connector. For DMR operation you need to use a repeater.

At one time, Motorola produced hardware that allowed two mobile radios to be connected together to form a repeater. Production of these items was discontinued many years ago.

None the less, nearly all Motorola mobile radios have an accessory connector on the back with pins that have received (discriminator) audio; carrier detect (COR); transmit audio and PTT on them. Using these inputs and outputs, one can achieve the above. I have written a few posts about different ways this can be used but for an analogue repeater, one would need to make a custom cable that looks like this. The Potentiometer and resistor are there to handle the impedance and level difference (a T-match/attenuator).

You can find the pin numbering for the DM4000 series here. CSQ Detect and PTT need to be assigned to a GPIO pin using CPS. Remember that the active level for both of these must be the same. As is the case with a repeater, you still need a duplexer. 

I was also asked about the possibility of making a DMR repeater using mobile radios. This is possible but there are some limitations.

CC BY SA 3.0 DB9MAT DF2ET DO7EN

Many radio amateurs have made DMR repeaters using two mobile radios; a Raspberry Pi and a MMDVM board. While this seems to work quite well, there are two caveats: the homebrew repeater will not be able to work as part of a MOTOTRBO network and the setup requires analogue mobile radios.

The MMDVM (and any equivalents I've missed) do not support the session layer protocols used by MOTOTRBO (this may also be true for many other manufacturers). The MMDVM is intended to be interfaced with services like Brandmeister and is therefore intended for use by radio amateurs only.

The MMDVM requires an analogue radio because it will generate the FFSK data itself. Most analogue two-way radios produced after the 1970s use a voltage controlled oscillator (VCO) and phase locked loop (PLL) to generate the frequencies needed for transmission and reception. In these radios, FM (or ΦM) modulation is achieved by injecting audio into the VCO.

Although nearly all MOTOTRBO radios support analogue operation, none of them are truly analogue: When transmitting in analogue mode, the microphone audio is fed into the Motorola RFIC which digitises it and generates the required FM modulated signal. Trying to inject FFSK audio into a MOTOTRBO radio running in analogue mode will result in unusual behavior. This is because the RFIC can only really process voice.

Also, mobile radios typically have around 45dB adjacent channel selectivity (at 12,5kHz) and an intermodulation rejection of 70dB. A repeater will have more than 55dB adjacent channel selectivity  and an intermodulation rejection of more than 80dB. Mobile radios are also designed to have a TX duty cycle of less than ~30% whereas a repeater can handle 100% TX duty cycle at full power with no degradation. This means that a mobile radio is not really suitable for repeater applications as it will not be able to filter out interference and cannot handle large amounts of traffic.