Batboard

I have a European MCS2000 (low/mid band) where the carrier squelch is set high ie. you need a stronger signal to open the SQ). I also have a GTX (actually its an GM350 but looks similar to a GTX) where the SQ is set low.

When connected to a dummy load, everything works as expected - you need to inject a stronger signal to open the SQ on the MCS2000 vs the GTX/GM350.

But when I connect an external antenna, the MCS2000 SQ immediately opens just on band noise (even though the SQ is set high). BUT the GTX/GM350 SQ stays closed using the same antenna, even though the SQ is set low. You would expect the opposite to happen!!!!

So what's happening??

I think that the antenna is subject to some general low level wide band noise coming from switch mode power supplies etc BUT its not excessive. Using an AM receiver, you can hear the increase in band noise when you connect the antenna but its low level with no S meter indiction and the SQ on the AM radio stays closed at all times.

One possibility is that the MCS2000 has a much wider band pass filter so that the noise level seen by the receiver is much higher. So when you connect the antenna, the greater noise level (including the extra noise from switch mode power supplies etc) causes the SQ to open whereas the much tighter band pass filtering in the GTX results in a much lower noise floor which does NOT open the SQ even though the SQ is set lower?? Both radio are set to 12.5KHz narrow band so I don't think its an IF filter issue. The effect occurs on other MCS2000's so its not a problem with my particular radio.

Any other ideas?

Any thoughts?

Why does wideband QRM from switch mode PSU's cause squelch to open on a low/mid band MCS2000 where squelch is set HIGH but the same QRM (and same antenna) does not cause SQ to open on a GTX where the SQ is set low?

Any ideas?

Switch mode power supplies are great harmonic generators. If you can get your hands on a spectrum Analyzer, it might show you all the spurs and spikes that the power supply is generating. These type of power supplies are low cost and small. But they do cause problems around radio receivers.

The best solution is to find an analog power supply and enjoy the radio.

Another example of this is computers. They work nice as long as you don't have a communications receiver near by. I have one high speed clocked computer that generates a very strong signal smack dab on a UHF channel I want to listen to.

Jim

All FM receiver squelch circuitry operates in essentially the same manner: highpass filter the raw discriminator baseband output in the 4 - 10 khz region, then detect the average noise level converting it to a DC level and use that DC level(in some cases AND it with a DC level indicating an on channel carrier) to open or close a squelch gate(with or without hysteresis designed into it). You might be seeing a difference due to differences in the highpass filtering(bandwidth and/or cutoff frequencies) and any hysteresis designed into the squelch gate circuitry. Without knowing specifics about your particular receivers, it's hard to guess exactly where the differences lie.
The squelch operates because with an onchannel carrier the OBN(out of band noise) in the 4-10 khz region of the baseband decreases approximately in proportion to the strength of the carrier.

Jim - I am using a linear PSU - its switch mode PSU's belonging to neighbors!!

Al - thanks - i will do some more reading on the subject.