"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