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Re: Subject: [Discuss-gnuradio] APCO 25

From: Matt Ettus
Subject: Re: Subject: [Discuss-gnuradio] APCO 25
Date: Tue, 21 Jan 2003 02:59:28 -0000

> >  This sounds like a better route. I can take my IF from the output
> > of the WFM ceramic filter rather than its input so I will then only
> > have a bandwidth of 200 KHz or so. I have 10.8 MHz xtal somewhere and
> > have some SBL-1 mixers in my junkbox. Now you have set me thinking ..
> >
> > Regards
>       You might have some aliases here I'm afraid.   Most receivers
> (R-7000, R-7100, R8500, AOR5000-3) that have 10.7 mhz outputs have very
> wideband 6 mhz or so wide outputs designed to even handle NTSC/PAL video
> (ICOM eveb makes/made a video demod for this output).   So you obviously
> need serious added selectivity if you are going to digitize the IF
> directly and aviod terrible aliases.

Why?  If you are digitizing with a soundcard, it has antialiasing filters
already.  Besides, you can get narrow 10.7 MHz filters that are the size of
small tantalum caps for less than a dollar from digikey.

>       But these wfm filters aren't all that sharp (probably with a
> LO at 10.8 mhz the close in part of the other sideband (the image)
> would be down only 15-25 db or so).  I guess you could use an
> image rejecting mixer, but these are less common in junk boxes.

see above

>       The only problem with doing this with a stock FM broadcast bw
> filter (~150 khz) is that you have to be able to digitize say 200 khz
> of bandwidth with the center at around 100 khz and this rather
> implies 400 ksamples per second sampling.

Not true.  Soundcards have their own antialiasing.  Very good antialiasing, I
might add, since they use delta-sigma converters with very high oversampling


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