"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