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:
  • 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
Desense is more prevalent in Capacity Plus and Linked Capacity Plus because:
  • 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.
The solution is quite simple but costs a little more in terms of hardware: always install proper RF filtering. In this customers case: a single high gain antenna (with some downtilt maybe) and a multicoupler combiner system.
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.
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