On Jan 4, 6:54*pm, "Joel Koltner" wrote:
Hi Tom,

"K7ITM" wrote in message

...

?? *So, "triplexor" to me means something with three bands, which you
obviously won't do with just a high pass and a low pass.

What I meant was... output of mixer to parallel connectors of (1) the crystal
filter (bandpass) , (2) series inductor/resistor (low-pass), and (3) series
capacitor/resistor (high-pass), hence, a triplexor.

But I realize now that a duplexor is just as viable, consisting of a parallel
connection of (1) the crystal filter (bandpass) and (2) a parallel LC
resonsator in series with a resistor (bandstop).

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.

Yeah, I see what you mean. *I should do some actual measurements to see just
how much improvement is possible with a proper wideband termination...

Well, as the second paragraph of my previous posting hints, a 50 ohm
termination at all frequencies may actually not be optimal. There's
no law of physics, as far as I know, that says the mixer's lowest
distortion (especially distortion products that fall in the following
filter's passband) happens when it's loaded by 50 ohms. If you set up
to make some measurements, you may find that you actually get lower
distortion at some other load impedance. Perhaps optimal for the
mixer's performance would be a 50 ohm load in the filter passband, to
maximize signal output, and, say, a short at all other frequencies.
I'm not saying it IS that way for any given mixer, and perhaps not for
ANY mixer, but it's a question worth pondering if you're looking for
the best possible distortion performance.

But then you'll find the next problem: the input impedance of the
crystal filter will change dramatically, very quickly, in the region
of its passband -- and likely will be capacitive on one side of the
passband and inductive on the other side. Assuming it's a reflective
design with low internal dissipation, by definition it must reflect
out-of-band energy (and pass in-band energy to the load at the other
end of the filter). Given that situation, how do you design a circuit
that will maintain the load impedance you want for your mixer, for
frequencies a little below the filter passband, in the filter
passband, and a little above the filter passband? If you try to do it
with inductors and capacitors, where do you get parts with high enough
Q to allow the required extremely rapid change of impedance near the
filter's center frequency? Or do you just accept that the distortion
won't be optimized? Or do you throw away some of the signal and put
in a bit of attenuation (a 50 ohm pad) between mixer and filter? Or
do you go looking for the "Holy Grail": a buffer amplifier that runs
on low power and offers good input and output return loss, a third
order intercept that doesn't degrade further what the mixer has
already done, and a good enough noise figure? The buffer amplifier
can solve the matching problems (at least if you're happy with a
constant load, e.g. 50 ohms, on the mixer), but it's not trivial to
find an amplifier that will do what you need without introducing
problems worse than the cure.

As you think about all this stuff, it becomes easy to see why nobody
has yet built the perfect receiver. ;-)

Cheers,
Tom

Thanks for the help!

---Joel