If you have log before ADC I think you need a superhet conversion rather
than direct conversion to baseband, for the reasons I mentioned earlier. If
you just ADC'ed the lot there'd be some chance of sorting the confusion out
in software, though it'd take a brave heart to try it.

So use 2 18-bit ADC's (sound card maybe 16-bit but you won't get 90dB range from
it, you'll find a fair bit of noise from your average sound card - well the
first 3 bits will be noisy), one with the I and the other with the Q (from the
DC output) and do it that way ?

Clive

"Hans Summers" wrote in message news:bmj291

Interesting ideas Len. I guess the idea of an all-digital spectrum analyser
is similar to that of an all-digital HF amateur radio tranceiver. It can be
done but at the current state of the art, it's a difficult proposal for the
hobbyist and certainly difficult to obtain the same level of performance as
the equivalent analogue device for the same amount of cost and/or effort.

I have a PC-based oscilloscope that does something pretty close. It
digitizes at the rate of 4ns per sample (taking 8bit samples
unfortunately) and generates an FFT display of magnitude/power
spectrum/power density).

I think it is feasible to use an analogue RF front end under computer
control, the a PC controlling the VCO and sampling the logarithmic output.
In essence just replacing the oscilloscope as the display system.

I guess, it is not necessary to PC control the VCO. Sweep generators
are easy to come by. probably, if there is a way to feed the sweep
into the PC to generate the X-axis, then that might be a better
alternative.

- farhan

"Hans Summers" wrote in message news:bmj291

Interesting ideas Len. I guess the idea of an all-digital spectrum analyser
is similar to that of an all-digital HF amateur radio tranceiver. It can be
done but at the current state of the art, it's a difficult proposal for the
hobbyist and certainly difficult to obtain the same level of performance as
the equivalent analogue device for the same amount of cost and/or effort.

I have a PC-based oscilloscope that does something pretty close. It
digitizes at the rate of 4ns per sample (taking 8bit samples
unfortunately) and generates an FFT display of magnitude/power
spectrum/power density).

I think it is feasible to use an analogue RF front end under computer
control, the a PC controlling the VCO and sampling the logarithmic output.
In essence just replacing the oscilloscope as the display system.

I guess, it is not necessary to PC control the VCO. Sweep generators
are easy to come by. probably, if there is a way to feed the sweep
into the PC to generate the X-axis, then that might be a better
alternative.

- farhan

i just ran a check with the sound card:

a) the sound card contributes noise. i shoved in a 50 ohms resistor
into the mic input, cranked up the mic level, save a 'silent' wave
file and saw it through a hex editor. There is a random noise of about
2-3 bits. That means, the effective range is not 16 bits but 12 bits.
12 bits can represent 4096-1 as the highest number. That means, the
dynamic range is 4096 squared: about 16 million or 72db. A little low
really for interesting work. there isnt enough 'roof' to resolve a
spur about 60db down without touching the grass

b)the sampling is at 44khz. some of the posts got confused between
20khz b/w and 20khz sampling rate.

hans is right about the PC being a kludge when compared to an
oscilloscope. I live in India and second-hand oscilloscopes are a
rarity. The new ones cost an engineer's whole year's salary. I have
purchased a tektronix 454 last month on ebay for $300, it is still on
its way to India. So, while i can imagine that in a number of more
developed countries oscilloscopes are not really a problem, they do
remain a problem in many parts of the world.

More often than not, an amateur already has a PC. Adding a simple
hardware to be able to do quick and dirty spectrum analysis might be
an interesting option. As Wes writes in SSD and EMRFD, the purpose of
test instruments is to help with the projects on hand, rather than be
projects themselves (then he went on to homebrewing a spectrum
analyser, hehe).

- farhan

i just ran a check with the sound card:

a) the sound card contributes noise. i shoved in a 50 ohms resistor
into the mic input, cranked up the mic level, save a 'silent' wave
file and saw it through a hex editor. There is a random noise of about
2-3 bits. That means, the effective range is not 16 bits but 12 bits.
12 bits can represent 4096-1 as the highest number. That means, the
dynamic range is 4096 squared: about 16 million or 72db. A little low
really for interesting work. there isnt enough 'roof' to resolve a
spur about 60db down without touching the grass

b)the sampling is at 44khz. some of the posts got confused between
20khz b/w and 20khz sampling rate.

hans is right about the PC being a kludge when compared to an
oscilloscope. I live in India and second-hand oscilloscopes are a
rarity. The new ones cost an engineer's whole year's salary. I have
purchased a tektronix 454 last month on ebay for $300, it is still on
its way to India. So, while i can imagine that in a number of more
developed countries oscilloscopes are not really a problem, they do
remain a problem in many parts of the world.

More often than not, an amateur already has a PC. Adding a simple
hardware to be able to do quick and dirty spectrum analysis might be
an interesting option. As Wes writes in SSD and EMRFD, the purpose of
test instruments is to help with the projects on hand, rather than be
projects themselves (then he went on to homebrewing a spectrum
analyser, hehe).

- farhan

Actually, I'd be tempted to rig a phasing type conversion to baseband from
the 300 MHz or whatever the first IF is....

At best it might get 40-50 dB of suppression of the other sideband, but it
would remove most of the undesired spurs and something like an AD8307 log
detector could be used to convert to a DC level.

After that, feed it into an A/D converter and into a PC using the printer
port.

Given a dual channel A/D converter, I'd also sample the tuning voltage for
the VCO and feed that into the PC as well.

All of this would move the programming from a possibly complex DSP problem
to a relatively simple Parallel port input and then number crunching to a
display.

There would be spur responses 40-50 dB down, but generally speaking, it
should give useful information.

===========

Granted, one problem is how fast the data transfer is through the parallel
port, that would limit how many samples/second the system could process.

There is the second issue, about maximum sweep speed Vs the IF bandwidth,
and the maximum useful bandwidth would be a function of the Phase shift
detector system. Still, I think this approach has some possibilities.

Almost forgot... WIN XP and WIN2000 make getting to the parallel port for
something like this MUCH more difficult then it was under WIN 98.

You could use RS232 and perhaps two COM ports to allow getting the AD8307
and the VCO sweep voltage in parallel, so to speak. Once again, it would be
a limit on how many samples per second the systme could provide.

If you want to be more complex, you could feed OUT commands to a
synthesizer to tune the VCO and this would allow really slow sweeps and very
precise accuracy, but it complicates the design.

I have to admit, I've been looking at the synthesizer design John Miles,
KE5FX, did which tunes from 1 GHz to 2 GHz with very fine tuning steps and
all osrts of ideas for various test gear built around it keep dancing in my
mind. I suppose I should see if I can build one and get some idea as to
the cost and so on. But it'd make a wonderful starting point for a
digitally tuned spectrum analyzer !!!

Admitted, the close in phase noise could limit dynamic range 'way below
the range pssible with an AD8307 detector, but even so the frequency
accuracy of an analyzer would be fantastic.

=========

Overall, Avery is probably correct that the result may be a lot of work an
less than perfect performance, but as a homebrewer it does seem like a fun
way to get something fairly useful and learn a lot as the project goes on.




Jim Pennell
N6BIU

Actually, I'd be tempted to rig a phasing type conversion to baseband from
the 300 MHz or whatever the first IF is....

At best it might get 40-50 dB of suppression of the other sideband, but it
would remove most of the undesired spurs and something like an AD8307 log
detector could be used to convert to a DC level.

After that, feed it into an A/D converter and into a PC using the printer
port.

Given a dual channel A/D converter, I'd also sample the tuning voltage for
the VCO and feed that into the PC as well.

All of this would move the programming from a possibly complex DSP problem
to a relatively simple Parallel port input and then number crunching to a
display.

There would be spur responses 40-50 dB down, but generally speaking, it
should give useful information.

===========

Granted, one problem is how fast the data transfer is through the parallel
port, that would limit how many samples/second the system could process.

There is the second issue, about maximum sweep speed Vs the IF bandwidth,
and the maximum useful bandwidth would be a function of the Phase shift
detector system. Still, I think this approach has some possibilities.

Almost forgot... WIN XP and WIN2000 make getting to the parallel port for
something like this MUCH more difficult then it was under WIN 98.

You could use RS232 and perhaps two COM ports to allow getting the AD8307
and the VCO sweep voltage in parallel, so to speak. Once again, it would be
a limit on how many samples per second the systme could provide.

If you want to be more complex, you could feed OUT commands to a
synthesizer to tune the VCO and this would allow really slow sweeps and very
precise accuracy, but it complicates the design.

I have to admit, I've been looking at the synthesizer design John Miles,
KE5FX, did which tunes from 1 GHz to 2 GHz with very fine tuning steps and
all osrts of ideas for various test gear built around it keep dancing in my
mind. I suppose I should see if I can build one and get some idea as to
the cost and so on. But it'd make a wonderful starting point for a
digitally tuned spectrum analyzer !!!

Admitted, the close in phase noise could limit dynamic range 'way below
the range pssible with an AD8307 detector, but even so the frequency
accuracy of an analyzer would be fantastic.

=========

Overall, Avery is probably correct that the result may be a lot of work an
less than perfect performance, but as a homebrewer it does seem like a fun
way to get something fairly useful and learn a lot as the project goes on.




Jim Pennell
N6BIU

I think it is feasible to use an analogue RF front end under computer
control, the a PC controlling the VCO and sampling the logarithmic
output.
In essence just replacing the oscilloscope as the display system.

I guess, it is not necessary to PC control the VCO. Sweep generators
are easy to come by. probably, if there is a way to feed the sweep
into the PC to generate the X-axis, then that might be a better
alternative.

Having the PC control the VCO via a digital-to-analogue converter gives the
PC a lot of control over the sweep rate etc. You could then control the
sweep rate, and zoom in on areas of your choice, via the PC rather than
analogue controls. But either way would work fine. You can feed the sweep
into the PC with a simple analogue-to-digital converter no problem.

Hans

I think it is feasible to use an analogue RF front end under computer
control, the a PC controlling the VCO and sampling the logarithmic
output.
In essence just replacing the oscilloscope as the display system.

I guess, it is not necessary to PC control the VCO. Sweep generators
are easy to come by. probably, if there is a way to feed the sweep
into the PC to generate the X-axis, then that might be a better
alternative.

Having the PC control the VCO via a digital-to-analogue converter gives the
PC a lot of control over the sweep rate etc. You could then control the
sweep rate, and zoom in on areas of your choice, via the PC rather than
analogue controls. But either way would work fine. You can feed the sweep
into the PC with a simple analogue-to-digital converter no problem.

Hans

wrote in message
...

If you have log before ADC I think you need a superhet conversion rather
than direct conversion to baseband, for the reasons I mentioned earlier.
If
you just ADC'ed the lot there'd be some chance of sorting the confusion
out
in software, though it'd take a brave heart to try it.

So use 2 18-bit ADC's (sound card maybe 16-bit but you won't get 90dB
range from
it, you'll find a fair bit of noise from your average sound card - well
the
first 3 bits will be noisy), one with the I and the other with the Q (from
the
DC output) and do it that way ?

I still think you stand a fair chance of problems, when essentially you're
doing your signal strength measurement in baseband and sweeping at rates in
the same frequency range. The aquisition time for the signal strength
measurement needs to be much shorter than the sweep, or the signal strength
will change while you're attmpting to measure it.

I guess it's a bit like the problem of designing audio-derived AGC in direct
conversion receivers. It takes too many cycles to measure the audio volume
at low frequencies to get a correction signal to control gain. Similar
problem. There might be a way of disentangling it in software, I don't know
the mathematics but I can intuitively imagine that it could work.

But I'm no expert on this, just a clown with soldering iron...

Hans G0UPL
http://www.hanssummers.com