|Following are general comments Duncan made over several emails. I didn't use quotations as it would be too redundant. Hope this helps others who get really serious about having a very good SWL Receiver.|
I was reading the latest ARRL handbook, looking at the state-of-the-art in receiver design. Not much difference between the basic design of the Electroluminescent and the top receivers in block form.
Just the top receiver designs us DDS VFO's with clean-up phase-locked loop, a mixer using quad FET switches, and synchronous demodulation!
I haven't done much on the SW bands, but there is plenty to listen to. Did I mention that I also covered medium wave, as that was one of my interests from way back (though sunspot activity doesn't seem to be good for MW at the moment)?
Anyway, I wound a humungously large toroid for MW (180 uH) using a 1" blue core I had in my box, and it took ages! Over half an hour to wind each one! And it makes the additional RF stage quite cumbersome. But works fine.
I've been tuning across the band comparing the signals I receive to my Sangean 808. There are a few spurious, but not as many as before (I've done some shielding), except up at high frequency (16+ MHz) where the VFO frequency is 25+ MHz.
But there isn't much to listen to up there. What I have found is that there is a lot of noise in my area, which varies from night to night. I am not far from large transmission lines which carry the power up and down the country (about 1 km as the crow flies). These may be the cause. Worse on the lower bands (2 MHz - 7 MHz).
Following similar circuitry to yourself, added two more RF bandpass filters covering 500 - 1600 kHz and 1600 - 4 MHz. So now 4 switchable bandpass filters.
2. Sensitivity seems to be lower in the 17 - 18 MHz region. I'm using a 9 MHz IF, so oscillator injection is around 26 - 28 MHz. Is this nearing the limit for the 1st mixer stage? There isn't much to listen to here above about 22 MHz anyway.
3. Spurios and intermods. There are more than I would have expected. I think this would be reduced through some screening, and when I get some more PC board (Dan's Small Parts is really slow to deliver to NZ!) I may screen off the oscillator stage.
4. The AGC stage doesn't quite cope with local AM stations on medium wave. I am looking at an RF input attenuator which would be switched in at high AGC levels. I expected this to be the case.
However, the AGC does cope with a wide range of SW stations better than my little Sangean 808, which overloads easily with many stations (Radio Netherlands really blasts in here from somewhere) - it has a little switchable attenuator which I have to use regularly.
5. BFO works really well on SSB signals. I switch the BFO and AM detector in and out and this works well.
Another thing I have noticed which you may want to address in future designs, is that the RF tune control has two peaks. The two coils are obviously not tracking exactly. There is little that can be done about this.
As the coils are pretty similar (I checked with an inductance meter), I assume this is variation in the varactor diodes. Not something this is much control over. Any ideas how to address this?
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