I have been following this discussion with casual interest, and I
observe that we have come "full circle" once again with receiver front ends.
In the beginning we had 'receivers' that could hear much of the RF
specturm - a detector connected to an antenna, later selectivity was
used ahead of the detector to restrict the signals reaching the detector
and we had TRF designs with lots of tuning knobs
Then we had superhetrodyne receivers that were known to do better if one
placed some selectivity prior to the front end, but the selectivity was
achieved at some intermediate frequency.
Then there were mixers and stable oscillators developed which could work
well into the VHF region, so receivers were designed with wide open
front ends and a 1st IF in the low VHF spectrum and was then
downconverted to achieve selectivity - while some of these were very
good receivers, they suffered from front end overload and other
deficiencies.
Then the K2 came along substantiating that a single conversion receiver
could be a top performer with bandpass selectivity placed prior to the
mixer to reduce the amount of undesired signals in the receiver front
end and the ultimate selectivity placed as close to the front end as
possible. The K3 followed by still using front end bandpass filtering
and a roofing filter at the 1st IF followed by an ADC to get into the
digital processing arena.
Now we have ADCs available that will work into the upper HF and lower
VHF at a reasonable price and we are once again creating receivers with
wide open front ends. Yes, they are SDR receivers with the loads of
features that digital processing can provide - and it will get better as
more processing power is packed into smaller and smaller spaces. We
seem to be fascinated with the panoramic display of the spectrum and the
ease of picking out signals visually and simply clicking on them to tune
- the current technology makes that easy.
I wonder what is next - It seems to me that we reach a practical limit
for front end sensitivity when a receiver can hear below the band noise
- and then BDR, IMD and other parameters become important. It will be
interesting to see what the next major generation of receiver concepts
provides. Ain't technology wonderful?
73,
Don W3FPR
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