YU1LM/ SM wide range tunable BP filter.

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clifford.c. wright:
Has anyone else tried this circuit. Mine is based on the
YU1 design, but I have seen a very similar circuit from a
Swedish ham.
It claims to be tunable from 1.8 to 30 MHZ with 3 toggle switches
and 1x 2 gang capacitor. Certainly it tunes over the range.
But it sure isn't always looking like 50 Ohms!
Basically the input impedance is all over the place apparently  dropping as low as
10 Ohms or even less on certain frequencies especially as the filter is tuned
 through resonance. It is being run from a known 50 Ohm source  and loaded
 with a 50 Ohm termination. Very hard to interface with other stages and antennas.
So I just got a new HF L/C meter good to 0.01uH and checked out all the coils
which are frequency appropriate 15mm toroids.
But NO CHANGE!!!
It is beginning to look as though my whole front end is going to have to be
redesigned around a more conventional circuit.
BTW many thanks for the high level mixer tips. I put them into practice
with 2 layer of shielding around the mixer and isolating the LO pin.
It is greatly improved and now 1 uV signals are loud and clear.
I can even pick up the leakage from my generator which is on the order of
0.1 uV clearly

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