AM IF Filter bandwidth

[blkfrd]

I am building a non-regenerative repeater for several waveforms, one of which is AM voice. I'd like to reduce the IF filter bandwidth from 25 KHz to 10 KHz or less to reduce amplified noise power and ACI.

I've seen filters as narrow as 6 KHz used for AM voice HAM radios. Since voice tends to be with 300 to 3000 Hz, this seems like a reasonable bandwidth and i'm wondering if it has the added benefit of improving distortion since the distortion sidelobes for voice freqs above 2000 Hz or so would be getting some rejection thru the filter.

With digital modulation, filtered sidelobes tend to get regenerated when transmitted, but with AM i'm wondering if filtered distortion sidelobes don't get regenerated. Thanks

Tracy Blackford
Comm and GPS Systems Engineer

 

[IRstuff]

Standard land lines are similiarly bandlimited, and their audio quality is barely adequate.

TTFN

FAQ731-376
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[VEBill]

The 300 to 3000 Hz bandwidth is absolutely standard for voice comms (not for broadcasting, obviously). Single Sideband (SSB) can fit into 3kHz total bandwidth; AM obviously needs twice that. FM/PM bandwidth gets a bit complicated as you probably already know.

By way of real world example (one of the few services still using AM for voice comms), VHF-AM air traffic radios in European airspace are using 8.33kHz channel spacing.