The importance of adequate RF filtering
I recently had a call from a customer complaining about garbled audio on his MOTOTRBO Capacity Plus system. The root cause: desensing.
This particular system has three DR3000 repeaters, each with their own duplexer and antenna - so three antennas in total mounted a few meters from each other. The three repeaters had a TX-TX frequency separate of around 500kHz. All in all, not a good combination.
In traditional analogue radio systems, desense is in most cases, invisible - the repeater is just deaf to varying degrees. In DMR, the symptoms are different: instead of a lousier signal to noise ratio (i.e. more hiss), we get bit errors. These bit errors can present several symptoms:
The customer called back to report he has just installed the multicoupler combiner system and the problem is solved. It so happens that installing this filtering also improved the coverage.
- Call fails
- System busy - even though there are idle channels
- Transmit Interrupt does not work
- Garbled audio
- Call terminates unexpectedly
- Missed calls
- Extended late entry into calls
- GPS not updating
- Text messages failed
- Poor coverage
- There is more than one repeater at the site and in the same band (and frequency range).
- In an idle state, the rest channel is transmitting a beacon.
- In high traffic conditions, more than one repeater (or all the repeaters) may be keyed up.