I have been experimenting with three mixing devices: an RF
double-balanced mixer (MCL SRA-1B), a DFF (74AHCT74), and an EXOR
(74ACT86). The mixer and EXOR give similar results - the sum and
difference frequencies, and a slew of various other products that
need to be filtered out. What was appealing about the DFF is that I
thought it should give only the difference, so the upper sum
sidebands should not be possible (except for the harmonics of the
difference frequency).
When I tried the DFF, this appeared to be the case, but it also
contained some of the lower frequency spurs that the other mixers had
too, but at much higher levels. I'm not sure if it's due to that
metastability thing, or the asymmetry of the input frequency sources,
due to their waveforms (one is nearly triangular) or the logic
levels. In the time domain, maybe the edges (one or the other)
correctly represent the difference, but it looks nasty in the
frequency domain, and turned out very difficult to filter out, being
below the desired output - I was hoping to only need low-pass
filtering to clean it up.
The best bet so far seems to be the EXOR, since it provides some
conversion gain, compared to the RF mixer, and a reasonable amount of
spurious content. I'll be trying various tricks with the DC bias,
levels, and symmetry to see if it will clean up a little easier. This
section has to provide two outputs - one digital for the PLL, which
can be ugly as long as the edges are right to land at the right
frequency, and one very clean sinewave to run the PLO. It looks like
this will need lots of filtering regardless of the mixing method.
Ed
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