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Re: Subject: [Discuss-gnuradio] APCO 25

From: Dave Emery
Subject: Re: Subject: [Discuss-gnuradio] APCO 25
Date: Mon, 20 Jan 2003 20:27:35 -0500
User-agent: Mutt/1.4i

On Mon, Jan 20, 2003 at 11:12:39PM +0000, Ian Wraith wrote:
> Hello
> >Not familiar with the RF2711.  Who makes it?
>  It was made by RF Micro Devices but doesn't seem to be in production
> anymore. I was given a few samples last year. Basically you give it an
> IF and LO which is twice the IF and out comes an amplified I & Q.
> >Have you considered just an appropriate bandbass filter centered at
> >10.7 MHz (if not already suitably band limited), followed by mixing
> >with 10.7 MHz (or there about) to convert to (or near) baseband,
> >followed by low pass, then soundcard?  This setup could then be used
> >to demod pretty much anything that will fit within the bandwidth limits
> >of the sound card.
> >
> >We've already got a quadrature demod block:  VrQuadratureDemod
>  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.

        Tapping off the wfm IF might be possible as you suggest, however,
if you want to modify the radio.   Most radios have a 150-250 khz
wide ceramic or xtal filter for wfm (but I think the AOR5000 may have
more than one bw filter selectable here).

        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.

        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.   I know of 96 khz capable
soundcards, and have heard of 192 khz cards maybe being introduced
but twice that has little use for digital audio purposes so I rather
doubt they are available.

        I personally think the original I and Q hardware quadrature
approach is better for your purposes - and generating accurate
quadrature LOs (or BFOs if you will) is pretty easy at 10.7 mhz using
modern 200-600+ mhz IC families and a 42.8 mhz clock.   Plus of course
as you point out, there are chips to do this already.

        As for 10.7 mhz gain, just use a MMIC gain block.  There are
lots of parts that will give you a stable 15-25 db 10.7 mhz gain at 50 ohms
in and 50 ohms out.

> Ian
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        Dave Emery N1PRE,  address@hidden  DIE Consulting, Weston, Mass. 
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