Author Topic: FM Receiver bandwidth.  (Read 847 times)

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Online CJay

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FM Receiver bandwidth.
« on: January 06, 2017, 12:40:05 am »
Feeding single tone frequency modulated output from my sig gen into a 2M converted VHF PMR radio I get acceptable audio quality but off air speech is distorted, almost as if the IF filters are too narrow (seems unlikely as it was designed for 12.5KHz channel spacing)

How do I measure the IF bandwidth?

Do I sweep the sig gen to span the centre RX frequency and measure level at 1st IF or, as it's FM, do I sweep the modulating tone on a static carrier?

What's the accepted definition of bandwidth, is it -3dB perhaps?

Offline Yansi

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Re: FM Receiver bandwidth.
« Reply #1 on: January 06, 2017, 12:48:29 am »
I think the modulation index is very low for these, like 1.  I.e about 3kHz maximum.

So the IF bandwidth of the 2m / 145MHz FM radio is like few kHz. (Compared to the commercial broadcast that uses something like 200kHz)

Offline zl2wrw

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Re: FM Receiver bandwidth.
« Reply #2 on: January 07, 2017, 12:37:28 am »
Generally, amateur 2m NBFM uses ~ 3 kHz audio bandwidth, +/- 5 kHz deviation, 15 kHz BW IF filtering and 25 kHz channel spacing.
Because PMR spectrum is crowded in some areas, the "new" NBFM standard used by a lot of PMR gear is +/- 2.5 kHz deviation, 7.5 kHz BW IF filtering and 12.5 kHz channel spacing.

So, off-air receive probably sounds distorted because the 7.5 kHz BW IF filter(s) in your PMR radio will be clipping the deviation peaks of a signal which is 15 kHz wide. You probably won't hear this distortion between two radios "across a room" because at 7.5 kHz away from centre frequency, your 7.5 kHz BW IF filter is probably only something like -40 dB down, so enough IF energy will get through on the skirts of filter to make the FM limter clip and the demodulator produce undistorted audio. You should be able to fix this by replacing the IF filter(s) with 15 kHz ones and re-aligning the IF strip/demodulator.

Your transmit audio will probably also sound a bit quiet (-6 dB down) to anyone receiving you on 2m until you increase the TX deviation to +/- 5 kHz.

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