Although I hadn't noticed it before, I would say that design in the K2
is more about stability and DC offsets than anything to do with pick up
of the BFO. The balanced demodulator and last two crystals deal with that.
The old, basically AM sets, used high level, unbalanced detectors. That
required more total gain from aerial to detector, although that might be
split between RF and two IF frequencies.
Balanced detectors can work with much lower level signals, which is
particularly helpful for a single conversion design, like the K2.
However, the AGC detector doesn't want to be balanced, and with a low
level signal, would have a large DC offset, that might exceed the wanted
rectified AGC, therefore additional gain is needed before the AGC
detector. If this were done at the IF frequency, the gain from input to
output at one frequency would be much greater, and proportionately less
stray coupling would be needed to make the whole system oscillate. By
shifting the frequency, which can be done at low signal levels, the
design reduces the gain at any one frequency.
The risk with doing all at one frequency is not pickup of the BFO at the
input of the IF amplifier, but pickup of its own output.
If there were leakage of the BFO it would get amplified just as much as
if additional gain was tapped off at the same point and amplified at the
original frequency. The design seems to rely on the balance of the
detector and the two pole crystal filter, to stop the BFO getting into
the AGC path, and not the difference in frequency.
--
David Woolley
Owner K2 06123
On 28/10/15 03:15, Ron D'Eau Claire wrote:
The most successful solutions (before SDR technology took over) were to
split the I.F. with a second mixer that was far removed from the main I.F.
the second I.F. was used to generate the AGC voltage and, since it was on a
wholly different frequency from the main I.F. the BFO didn't trigger the
AGC. In fact, the Elecraft K2 uses that technique quite successfully.
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