"Ashhar Farhan" wrote in message
om...
here is a spectrum analyser design that i would like the group to
comment upon.
1) we take the input via a low pass filter, up convert it to an IF of
100Mhz or so, and follow it up with a direct conversion receiver at
100 Mhz with 20 khz bandwidth.
2) the upconverting local oscillator is a VCO that is controlled by a
sweep generator. the sweep is controlled by a PWM modulated signal in
the audio range.
3) the sweep generator input is connected to the output of a PC sound
card. the output of the direct conversion receiver is connected to the
input of the PC sound card.

Now, by clever programming of the sound card on the PC, we can make
the VCO sweep our passband of interest. The sound is often digitsed at
16 bit levels (in the better systems at 32-bit level). This will
effectively give us 90db range. the lograithmic scale can be
implemented in software. DSP can be used to set the bandwidth to any
particular size.

the most important benefit of this design will be that even hams
without expensive oscilloscopes will be able to easily make a PC based
analyser that is easy to assemble and use.

if there isn't any glaring problem with this design, i would like to
pull out my soldering iron and take a go. is anyone here with spectrum
analyser experience willing to share knowledge?

I have recently completed a spectrum analyser, see
http://www.hanssummers.com/electroni...yser/index.htm.
It is
awaiting possible magasine publication so there are not yet any circuit or
construction details on the page above. If you want the full details, email
me privately and I'll show them to you.

I also tried a direct conversion receiver initially. It doesn't work on in
analogue (i.e. non-PC) analyser, because there are all sorts of heterodynes
of the sweep frequency against the directly converted incoming signal. Of
course I kicked myself afterwards for not thinking of it in advance to save
myself the time of the experiment.

I think broadly speaking the final IF should be substantially higher than
the frequency of the sweep waveform, so that the final filtering works
faster than the sweep. There are lots of people in this forum far more
advanced than me who will probably be able to explain it better in terms of
filter response times or group delays or something.

Though there might be a way of untangling everything in software so it may
work. Being direct conversion you'll also have both sidebands present, which
will create further complications. Again, clever software might untangle it
but I think it's far from straightforward.

Another problem is the narrow bandwidth. 20KHz is a nice bandwidth to have
but I think in a spectrum analyser you also want wider bandwidths available.
In particular, if you are digitally generating your sweep voltage, and
trying to cover the whole 100MHz, you need of the order of 100,000 / 20 =
5,000 discrete measurement intervals. You can't display that many horizontal
pixels on screen. You could average them in software, but at the low 20KHz
bandwidth, you're going to need quite a slow sweep rate. 5,000 measurements
are a lot and will take a long time.

It's a nice idea but I don't think it will work as it stands.

My recommendation would be to add a 2nd IF to your design, 2nd IF amp and
logarithmic detector. In my design I used a 145MHz 1st IF, so the VCO sweeps
145 - 290MHz. The 2nd local oscillator is at 153MHz for an 8MHz 2nd IF,
amplified then passed into an AD8307 logarithmic amplifier. Anything similar
would work well. I used an SA602 front end for simplicity, but a diode ring
mixer would give potentially better performance than the 65-70dB dynamic
range I achieved.

You can still use the PC for a nice display, rather than an oscilloscope.
Just feed the log output into your PC sound card, and have the PC sound card
control the sweep as you suggest. I think you'll solve a lot of problems by
adding these few extra modules to the analogue front end before introducing
the PC. Incidentally, this is exactly what I'm doing with my Mk2 analyser,
see
http://www.hanssummers.com/electroni...ser2/index.htm.

Now I'm hoping for someone to put it all more clearly and professionally
than I have here ;-)

73
Hans G0UPL
http://www.HansSummers.com