Winter is easing its grip, but it is not yet time for antenna work, so the
anti-parallel dipoles mentioned in the previous post will have to wait.
While the weather is warming up, the 85 watts dissipated by the IC-761
during WSPR-reception is a bit of a waste; soon the shack will need
additional cooling with all equipment running. Therefore a "hundredfold"
reduction in rx-power is very welcome. Presently 15 volts 40 mA is fed
to a 7812 regulator followed by a 7808 to provide 12 and 8 volts bias to
four 8-pin chips, namely a SA612A - NE592 - SA612A - LM318H string.
SA612A mixers have very low bias, so they are prone to overload and
IMD when fed with wide input spectra. In this prototype receiver some
risks are taken, but narrow pre-filtering is expected to minimize the IMD-
problem. Due to the availability of standard crystals for certain frequencies
the 10140.2 kHz band-center seemed within reach with a 4.000 MHz and
a 6.144 MHz crystal. Both had to be pulled down a little off their nominal
frequencies for WSPR action. So we are talking about a superhet.
The first SA612A has the 4 MHz crystal in its oscillator section so
6144 kHz is serving as the intermediate frequency. Actually another
6144 kHz crystal is used as the IF-filter in the NE592 stage between
pins 2/7. It is in fact not a straightforward filter: it kind of "shorts"
two emitters of the NE592 at X-tal series resonance, which peaks
the amplifier gain enormously at around 6140 kHz. Bandwidth is not
much more than the 200 Hz WSPR-band, so it is quite a remarkable
response for a single high-Q component in that particular circuit.
The second SA612A makes a 6144 kHz crystal oscillate round about its
nominal frequency, so both the 4 MHz and 6144 kHz crystal have to be
pulled down with a series coil to arrive at band-center 10140.2 kHz with
1500 Hz BFO offset, actually in LSB mode. As standard WSPR offers