On Dec 31 2010, 6:36*pm, "Joel Koltner"
wrote:
I'm designing an IF filter for a receiver that's placed at the output of a
passive (diode-type) mixer. *While the mixer's RF input is being fed with a
(nominally) 50ohm source, is it correct to think that "looking" back into the
IF port the filter is going to "see" a 50ohm impedance as well? *(Normally I'd
have a buffer stage right after the mixer to re-establish impedance levels,
but in this case I'm trying to keep things low-power and hence directly feed
the mixer's output into the filter.)

Thanks,
---Joel

Normally the LO port is driven hard enough that one pair or the other
of the diodes looks like a low resistance almost all the time. Thus
the source impedance feeding the RF port is echoed, plus a little, to
the IF port, assuming 1:1 transformers in the mixer. As I recall, I
measured something close to 60 ohms for one MiniCircuits mixer
(probably a +7dBm type), under conditions I don't recall now other
than that they were nominally as you're describing. As Ken Scharf
says, it's good to use a diplexer to maintain a constant load at all
frequencies.

Cheers,
Tom