Jack Smith wrote:
In contrast, the K3, as I understand it, take an IF sample ahead of the IF
filter, with a source follower. Hence, a signal injected into the source
follower's output will leak back into the main IF path and be heard in the
receiver, reduced by the reverse gain of the source follower. It would be
unusual, in my experience, to find a source follower to have much more
than 40 or 50 dB reverse isolation at 8 MHz. If we assume 50 dB reverse
isolation, then the -40 dBm signal leaking out of the Softrock would be
fed back into the K3's IF chain at around -90 dBm, which is clearly a
problem. One solution would be a second amplifier between the K3's IF
output port and the Softrock. This amplifier should be designed to have
high isolation and about 0 dB gain. Perhaps a grounded gate amplifier
would be a good starting point.
------------------------------------------------------------------------
PA3AKE was using two grounded gate J310 FETs in cascade after each generator
for improved isolation while running IMD tests on H-Mode mixers. He reported
a reverse isolation of 75db and about 4db forward gain for each pair of
J310s at HF and 6m.
73,
Geoff
GM4ESD
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