On Wed, 28 Jun 2006 19:48:20 +0100, john wilkinson
wrote:

You have spurs on one or both of the inputs to the SBL1. Passive
doide mixers will not generate spurs like that though they may pass
them through if the sources are dirty.

Also the IF could also be oscillating and your see ing that
contribution. If its the 100db IF duscussed before, thats a lot of
gain and I'd hope its bypassed and shielded very well.

Allison
Hi Allison,

I have disconected the second mixer from the output of the first IF amp.
That amp is powered down, and disconnected from any supply.
The LO input to the mixer is from a crystal oscillator, at 7-8dBm.
The output from the mixer is into the spectrum analyser only, not the
second IF.

This morning I did an experiment. If I left the RF port unterminated, the
output was quite clean on the analyser.
If I put a 50 Ohm term to gnd, the spurs appear.

Either the LO is dirty and your not seeing it due to overdriving the
analyser or the stages after SBL1 are dirty.

What I'd expect with the input of the SBL1 terminated and the LO
applied is the LO (40db or so down) some second and third harmonic
also way down and if you go far enough down some noise.

Maybe it is on the LO itself, but looking at the LO on the analyser it
looks OK to me.

Ok whats at the output of the SBL1? Or more specifically whats
between the SBL1 and the Analyser?


Allison