Re: Soundcard spectrum analyzer through regenerative frequency division

Alas, on the wretched hour of Sat, 24 Feb 2007 00:10:07 +0000 John
Barrett thusly proclaimed:

"john jardine" <john@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:45df6643$0$16268$88260bb3@xxxxxxxxxxxxxxxxxxxx

"Ceriel Nosforit" <ceriel@xxxxxxxxxxxxxx> wrote in message
news:lRhDh.241$dY5.67@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi,

I'm trying to build a spectrum analyzer for the shortwave spectrum that
uses a simple computer soundcard for data acquisition.

Specifically what I'd like to do is compress the mean signal levels from
DC
to 30MHz to DC to 30 kHz, which my sound card should be able to sample. I
have no need to be able to rebuild the signals into an intelligible
format. What I hope to do is merely to see at what frequencies
transmissions are taking place. With 30Mhz compressed to 30kHz it should
be a simple matter of reading the frequency and mentally multiplying it
with 1000.

Extensive googling suggested regenerative dividers might do the trick,
but
if it is too broad a range I'd also be very happy with steps of 3MHz
sparated over a 10-way switch. However, I don't quite know how to go
about
designing something like this. I'm much in need of guidance here, which
I'm hoping you will provide me with. I don't even have a proper, specific
question I could ask... Help?

--
Nos

Not unreasonable to build a block of say 30 (or more) bandpass filters.
Centre frequencies staggered as felt fit.
Add 30 ways of signal scanning, analogue switch I.Cs connecting each
channel in turn to Chuck's AD8307. Take the audio output from that.
Could even add a single 1V audio tone as channel 0 to provide a zero
frequency, spectral reference line.
john


Hook that array of filters up to a PIC or AVR to do the grunt work and stuff
the data back to the PC over serial -- its got the A/D converter built in,
so you just need the filters, analog switches, and an integrator/peak
detector (i.e. you listen to each band for a short period of time, looking
for a peak or summing the levels during that time, then you sample). The A/D
is a lot slower than any stand alone A/D chip, but you are just sampling the
levels, not the entire spectrum. At max resolution the Atmega168 can do 15k
samples per second.. for 30 bands, thats 5000 scans per second max -- more
than adequate.

If you really wanted to go all out -- put a peak detector on each filter,
and have the controller clear the detector after it samples -- that will get
you continuous monitoring of each band during the scan -- you wont miss
anything while checking other bands.

Run the uplink to the PC at 2 bytes per sample + a few bytes overhead for
framing = 64 bytes.. on 115kbps serial (11 kbytes/sec) will get you some
where around 170 scans per second -- again more than adequate :) (or drop
the sample resolution to 8 bits and double your scan rate)

Oh my, I thought the conversation had died. Glad to be proven wrong!

I've studied the HPSDR project, but it seems their hardware will be vey
expensive. As such, the comparatively dirt-cheap AD8307 looks very
interesting. I do however get the impression that it doesn't sample the
entire bandwith simultaneously, so it couldn't work as an SDR on its
lonesome. Might have to still use the LTC2208 for that, but no reason to
be in a hurry.

Rather than downsample the output of the chip I'd rather connect it to one
of the high-speed ports on my computer and then have it sort through it
all. That way I'd be dealing with a minimum of hardware and consequently a
minimum of complexity. I'll have to start studying the issue in more
detail.

Thank you all very much for your replies! Glad to see sucha response.

Sincerely,
--
Nos
.

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