Noise suppressor improves reception, why?

[sasilverbulle]

Newbie here, not an EE, just an old Air Force electronic tech from the 70's...please be gentle. :)

I recently installed a noise suppressor inline to the 12v on my 70 Lincoln AM/FM radio. It cleaned up the "whining" noise just fine - but one interesting side note is that my FM stations come in a LOT stronger, not just clearer. Before I would loose my favorite FM station on the way to work in the same spots each time. After the noise suppressor I get the station strong the whole way.

This is creating a bit of a problem as the grey matter in my head can't seem to come to grips with why this could be.

The only thing I can think of is that the power is totally unfiltered in the radio and the "noise" on the line affected the tuning circuit. I don't have access to a schematic for this radio.

What do you'all think? Why do the stations come in much stronger now?

Thanks in advance!

 

[DragonPetter]

Someone can answer this better than I can, but the noise on your power supply can make its way into all the circuits of your device and interfere with the signal it is trying to output.

Power supply noise can be worse than noise in the actual signal, because usually filters are designed to remove the signal noise, but the noise they use to power them can make it to the output regardless of filters.

I'm sure there's lots of other reasons, but noise on power supplies is a problem not just for radio receivers.

P.S. Ya also you're right, the noise can get in the PLL or whatever tuning filter they're using and cause the tuning frequency to fluctuate with the noise

 

[berkeman]

Newbie here, not an EE, just an old Air Force electronic tech from the 70's...please be gentle. :)

I recently installed a noise suppressor inline to the 12v on my 70 Lincoln AM/FM radio. It cleaned up the "whining" noise just fine - but one interesting side note is that my FM stations come in a LOT stronger, not just clearer. Before I would loose my favorite FM station on the way to work in the same spots each time. After the noise suppressor I get the station strong the whole way.

This is creating a bit of a problem as the grey matter in my head can't seem to come to grips with why this could be.

The only thing I can think of is that the power is totally unfiltered in the radio and the "noise" on the line affected the tuning circuit. I don't have access to a schematic for this radio.

What do you'all think? Why do the stations come in much stronger now?

Thanks in advance!

Welcome to the PF.

One big reason for the better "quiet line" performance is the Automatic Gain Control (AGC) circuit can now increase the gain of the input amplifiers more. That allows you to get better range and sound quality.

When there is noise in the circuit, the AGC will only increase the gain to the point where the noise is coming through strongly. Once the noise is reduced, the AGC will increase the input gain more to try to amplify weaker signals.

 

[Averagesupernova]

Does FM actually have AGC? I don't think so. A huge advantage of FM is to be able to run it wide open gain and limit it as to reduce AM type noise from lightning and etc.

 

[berkeman]

Does FM actually have AGC? I don't think so. A huge advantage of FM is to be able to run it wide open gain and limit it as to reduce AM type noise from lightning and etc.

I think it would be pretty hard to have 70dB of dynamic range without AGC. You could probably do a simpler FM receiver with reduced dynamic range and some distortion of high RSSI signals, but I don't have any quantitavie numbers on a receiver like that.

Here's a typical WiFi-type receiver chip that shows the AGC feedback (block diagram on page 2):

http://www.analog.com/library/analogdialogue/archives/40-03/wireless_srd.pdf

.

 

[waht]

In most FM receivers, a limiter is often applied after the IF stage which functions similarly to an AGC, and many designs have feedback loops to the IF stage, or the local oscillator in order to lock on the strongest frequency.

 

[sasilverbulle]

Thanks for the answers! That validated that I wasn't completely off-base on my deductions.