On Jan 3, 7:09*pm, "Joel Koltner" wrote:
Thanks guys, I appreciate all the help.

The filter is a bandpass filter (the IF is 45MHz), though, so don't I really
need a triplexor if I really want to wideband terminate the mixer "nicely?"
(Low-, band-, and high-pass outputs.) *Although my understanding is that
"triplexor" in this context can consist of a single inductor for the low-pass
port and a single capacitor for the high-pass port, which certainly is quite
doable.

---Joel

?? So, "triplexor" to me means something with three bands, which you
obviously won't do with just a high pass and a low pass. In any
event, the goal is to keep a reasonably constant load on the mixer at
all frequencies where it may have significant output. The distortion
generated by a mixer depends on the load it sees.

If you go straight into a crystal filter from the mixer, it can be
problematic to get a constant load impedance, since the impedance
looking into the filter changes so rapidly in the sharp transition
between passband and stopband. For example, if your passband is 44.99
to 45.01 and the mixer output has strong signals at 44.96 and 44.98,
how can you, with a practical LC circuit, make sure those signals are
terminated in an impedance which maintains low distortion, while still
passing the signals in that range to the sharp filter? Note that 3rd
order intermod between those two out-of-band signals I suggested lands
right in the middle of the crystal filter bandpass. That's why you
want to be sure to terminate the mixer in an impedance that doesn't
lead to excessive distortion (in particular, distortion products that
can land in the passband of the crystal filter). I think a reasonable
answer is to learn what range of termination impedance causes
problems, and what range is OK. For example, it may be bad if the
mixer is terminated in an "open", but of relatively little consequence
if it's terminated in a "short." That gives you a handle on how to
achieve the lowest practical distortion--which is generally the whole
point of controlling the load impedance seen by the mixer over a wide
frequency range.

Cheers,
Tom