On Wed, 28 Jun 2006 07:43:20 -0500, tim gorman
wrote:

john wilkinson wrote:



Hi,
If I feed my second mixer into a spectrum analyser, and inject a 44.545MHz
LO, with no power to the first IF stage, I see spurs at about 48-50KHz
intervals on the mixer output, from 0 to the 455KHz normal output freq.


The mixer is an SBL-1.

Any ideas as to where these are comming from?

Best regards,
John

A couple of observations.

If you simply disconnected the second mixer from the 1st mixer then it
sounds like you do not have the output of the 1st mixer properly
terminated. Unless, that is, the spectrum analyzer provides a good 50ohm
termination in the probe. The SBL-1 is a double-balanced mixer but in order
to get the "balance" to work right you must properly terminate *all* ports.
Otherwise all kinds of impedances can be "thrown" back to the other ports -
causing all kinds of problems with attached LO's and RF stages.

Regularly spaced spurs from DC to 455khz does not sound like spurious mixing
products. It sounds more like an oscillator being driven into saturation
causing square waves to be produced which are providing regular harmonic
outputs. This could be a product of poor terminations on the mixer ports.

Tim, the LO is 45.455 (or 44.545) either way there should be no LF
outputs from the mixer below the LO injection unless it's a spur on
the LO.

Directly connecting the output of a SBL-1 to the input of another mixer,
even another SBL-1, is not recommended either. The SBL-1 does not have
built-in terminations. A good, symmetric, 50ohm 3db pad between the stages
would provide a much better setup and would not significantly impact your
overall noise factor. Be sure and connect your spectrum analyzer to the far
side of the pad.

Thats true though his testing is with a 20db IF after it and one would
hope that does present a 50ohm match to the IF port. Usually if the
poarts are badly matched the mixing spurs are greater and the port
to port isolation go to pot. If the only thing going in is the LO
then likely the LO is the source but if there is gain flollowing then
the gain stage is also suspect.

I've worked with enough SBL1 and MD108s and all their similar DBMS
to know the SBL1 in this case is not the offendor Even if
misterminated. Likely causes are the 45mhz filter is looking reactive
to the IF amp and the IF is unstable (makes a fine OSC) or there is
some other source of RF that is unaccounted for (spurs).


Allison
Kb!GMX