View Single Post
Old 1st Oct 2017, 5:53 pm   #26
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,797
Default Re: Why is aeronautical VHF AM?

The MOPS doesn't specify a bandwidth.

It gives a spec for audio flatness, a spec for maximum variation between different offset charriers within the channel and a spec for adjacent channel rejection. It leaves you to find a way to meet all
at once.

With the halfway points to the next channels at +/- 12.5kHz with an 8-pole crystal filter you might manage 17kHz at -6dB points. With DSP selectivity, you can get a steeper shape factor and push it out a bit. There are audio distortion specs for offset carriers so you don't want to trim the outlying offsets to VSB. For the adjacent channel rejection you also run into synthesiser phase noise broadening the skirts of the effective selectivity..

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline