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?

"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

"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

"Hans Summers" wrote in message news:bmgam6
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.


a) i am not proposing a direct direct conversion. i am proposing an up
conversion exactly like han's MK1. for the second conversion, i am
suggesting a direct conversion to base-band from the high IF. as you
rightly point out harmonic mixing is a problem with broad-band direct
conversion receivers, therefore, you need to have a low-pass or a
band-pass ahead of a diode mixer working as a product detector.
b) my second suggestion is to do the logarithmic calculations
digitally on the PC. that will simplify the design.
c) as for the granuality of the sweep, for finer resolution the sweep
range will be decreased. that is one way to get higher resolution, the
other is to slow down the sweep.

what kind of a VCO are you using? and how are you ensuring that the
vco output remains constant throughout the sweep? i expect that the
VCO's amplitude will also effect the first mixer gain.

- farhan

"Hans Summers" wrote in message news:bmgam6
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.


a) i am not proposing a direct direct conversion. i am proposing an up
conversion exactly like han's MK1. for the second conversion, i am
suggesting a direct conversion to base-band from the high IF. as you
rightly point out harmonic mixing is a problem with broad-band direct
conversion receivers, therefore, you need to have a low-pass or a
band-pass ahead of a diode mixer working as a product detector.
b) my second suggestion is to do the logarithmic calculations
digitally on the PC. that will simplify the design.
c) as for the granuality of the sweep, for finer resolution the sweep
range will be decreased. that is one way to get higher resolution, the
other is to slow down the sweep.

what kind of a VCO are you using? and how are you ensuring that the
vco output remains constant throughout the sweep? i expect that the
VCO's amplitude will also effect the first mixer gain.

- farhan

"Ashhar Farhan" wrote in message
om...
"Hans Summers" wrote in message news:bmgam6
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.


a) i am not proposing a direct direct conversion. i am proposing an up
conversion exactly like han's MK1. for the second conversion, i am
suggesting a direct conversion to base-band from the high IF. as you
rightly point out harmonic mixing is a problem with broad-band direct
conversion receivers, therefore, you need to have a low-pass or a
band-pass ahead of a diode mixer working as a product detector.

That wasn't the effect I was worrying about... after all your up mixing and
direct conversion of the VHF IF down to baseband, and low pass filtered it,
you'll end up with say 20KHz of audio baseband to feed into your soundcard.
Unfortunately your sweep rate is also down in the same range. Which is a
problem because whatever method you use for envelope detection of the
baseband will have to react faster than the sweep rate. Though as I say, if
you do it all in software you might be able to untangle it.

b) my second suggestion is to do the logarithmic calculations
digitally on the PC. that will simplify the design.

It simplifies the analogue design and complicates the software. 16 bits of
resolution should give adequate dynamic range. Depends what you want to
spend time on... The AD8307 chip is very easy to use, if a little on the
expensive side.

If you have to use the PC to unwrap the envelope detection mess as mentioned
above then you have to do the logarithmic bit in PC anyway.

c) as for the granuality of the sweep, for finer resolution the sweep
range will be decreased. that is one way to get higher resolution, the
other is to slow down the sweep.

Agreed, but if the widest resolution is only 20KHz, then you have to do
5,000 samples to cover 0-100MHz input bandwidth. This fine resolution is
imposed by the relatively narrow bandwidth, otherwise you will suffer
significant loss of accuracy on your displayed result. The way I think of it
is in terms of frequency spikes falling into the holes between samples. In
the extreme imagine taking a VCO going in 100 steps, so making an amplitude
measurement at each 1MHz of the range 1-100MHz. Your bandwidth 20KHz.
Signals exactly on the MHz will be no problem. But what about a signal at
say 12.5MHz... what does it look like on the analyser? It's 500KHz away from
the centre of the 20KHz passband for both adjacent measurement points 12MHz
and 13MHz. It's fallen into the hole. What you'd see on your display depends
on the skirt selectivity of your filters. If you're using a 20KHz soundcard
as the filter, the skirts will be quite sharp, so it's likely you'd see
almost nothing at 500KHz baseband. This would mean the majority of
frequencies in your input spectrum would be absent from your display or at
entirely the wrong amplitude.


what kind of a VCO are you using? and how are you ensuring that the
vco output remains constant throughout the sweep? i expect that the
VCO's amplitude will also effect the first mixer gain.

I'm just using the internal oscillator of the SA602 mixer/oscillator chip.
As you suggest, probably not at all constant ;-)

Hans
http://www.HansSummers.com

"Ashhar Farhan" wrote in message
om...
"Hans Summers" wrote in message news:bmgam6
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.


a) i am not proposing a direct direct conversion. i am proposing an up
conversion exactly like han's MK1. for the second conversion, i am
suggesting a direct conversion to base-band from the high IF. as you
rightly point out harmonic mixing is a problem with broad-band direct
conversion receivers, therefore, you need to have a low-pass or a
band-pass ahead of a diode mixer working as a product detector.

That wasn't the effect I was worrying about... after all your up mixing and
direct conversion of the VHF IF down to baseband, and low pass filtered it,
you'll end up with say 20KHz of audio baseband to feed into your soundcard.
Unfortunately your sweep rate is also down in the same range. Which is a
problem because whatever method you use for envelope detection of the
baseband will have to react faster than the sweep rate. Though as I say, if
you do it all in software you might be able to untangle it.

b) my second suggestion is to do the logarithmic calculations
digitally on the PC. that will simplify the design.

It simplifies the analogue design and complicates the software. 16 bits of
resolution should give adequate dynamic range. Depends what you want to
spend time on... The AD8307 chip is very easy to use, if a little on the
expensive side.

If you have to use the PC to unwrap the envelope detection mess as mentioned
above then you have to do the logarithmic bit in PC anyway.

c) as for the granuality of the sweep, for finer resolution the sweep
range will be decreased. that is one way to get higher resolution, the
other is to slow down the sweep.

Agreed, but if the widest resolution is only 20KHz, then you have to do
5,000 samples to cover 0-100MHz input bandwidth. This fine resolution is
imposed by the relatively narrow bandwidth, otherwise you will suffer
significant loss of accuracy on your displayed result. The way I think of it
is in terms of frequency spikes falling into the holes between samples. In
the extreme imagine taking a VCO going in 100 steps, so making an amplitude
measurement at each 1MHz of the range 1-100MHz. Your bandwidth 20KHz.
Signals exactly on the MHz will be no problem. But what about a signal at
say 12.5MHz... what does it look like on the analyser? It's 500KHz away from
the centre of the 20KHz passband for both adjacent measurement points 12MHz
and 13MHz. It's fallen into the hole. What you'd see on your display depends
on the skirt selectivity of your filters. If you're using a 20KHz soundcard
as the filter, the skirts will be quite sharp, so it's likely you'd see
almost nothing at 500KHz baseband. This would mean the majority of
frequencies in your input spectrum would be absent from your display or at
entirely the wrong amplitude.


what kind of a VCO are you using? and how are you ensuring that the
vco output remains constant throughout the sweep? i expect that the
VCO's amplitude will also effect the first mixer gain.

I'm just using the internal oscillator of the SA602 mixer/oscillator chip.
As you suggest, probably not at all constant ;-)

Hans
http://www.HansSummers.com

I say go for it !

Ignore the doubters and those that don't like the idea. just go for it I say.

It's a good idea and a fully workable one, I've been a hardware/software
designer for years and theirs nothing that's so difficult in the idea.

Though what I'd do is for the unit to have its own fast ADC - at least 18-bit.
and it's own little cpu - ATmega's are nice and easy to use. Then send the
current freq and log level down the RS232 port to a PC.

Happy experimenting !

Clive

I say go for it !

Ignore the doubters and those that don't like the idea. just go for it I say.

It's a good idea and a fully workable one, I've been a hardware/software
designer for years and theirs nothing that's so difficult in the idea.

Though what I'd do is for the unit to have its own fast ADC - at least 18-bit.
and it's own little cpu - ATmega's are nice and easy to use. Then send the
current freq and log level down the RS232 port to a PC.

Happy experimenting !

Clive

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