Hi,
The receiver that I am building, has a 455KHz second IF. This consists of a
common gate amp followed by a collins mech filter 6KHz BW, and then two
AD631 gain controlled amps, with a max gain of 100dB.

The first IF is at 45MHz, and consists of a 20KHz BW monolithic crystal
filter with dual gate mosfet amps either side.

The second LO is at 44.545MHz, and the mixers are SBL-1's.

I have for the time being disonnected the first mixer, and am able to feed
in a 45MHz signal straight into the firts IF amp if I wish.

Now if I look at the output of the second IF with a spectrum analyser, with
the power off to the first IF, but with the 2nd LO running, I can wind up
the second IF's gain, and the noise floor goes up, which I guess is
expected, but there are no peaks at 455KHz.

If I connect the power to the first IF, and then wind the gain of the
second IF up, I get a noisy peak at 455KHz, that varies in aplitude with
the gain. The input to the first IF has a dummy load.

The second LO is a crystal oscillator, see my previous questions regarding
this.

The question is, should I expect to see a peak, I dont think so.
What could be causing this? The first IF is in a double sided PCB
enclosure.

I hope someone can help as this is driving me nuts....

Thanks,
John