Remote speaker-microphones and DMR portables discussed RF ingress to Speaker Mics (RSM) used with DMR radios in digital mode.
I purchased a RSM branded Kenwood but obviously a Chinese fake for an MD-390 for about $5 posted, but it turned out to be lousy with RF interference in the form of the ‘motorboat noise' on transmit audio.
Dismantling the RSM I found there is precious little RF filtering, just a single SMD cap near the electret capsule.
Above is the modified RSM.
I cut the track between the existing bypass cap (which they did not fit… these things cost tenths of a cent each) and the electret and installed a 10µH SMD choke, and soldered a 1000pF SMD cap across the electret terminals.
It is the best of the RSM I have now (ie least RFI… can't hear any).
Works well on my MD-390.