David wrote:
Hi,

I am wanting to match the output of a SA602 Mixer to a 45MHz, 4-pole
crystal filter and then match the output of the filter to the input
of a SA605 IF receiver.

The filter datasheet specifies the termination impedance of 800 Ohms and
3pF.

I have not used crystal filters before and would appreciate some advice
for typology of a suitable match.

The SA605 has a input impedance of 4500 Ohms par 3.5pF. Apparently they
have a "sweet spot" if matched to 800R.
The SA602 has output impedance of 1500 Ohm (not sure of capacitance).

From what I can see, if I placed a 1k resistor across the SA605 input,
this would give me close to 800R//3pF but not max. transfer of signal.

Some of the confusion I have is what to do with the 3pF termination
requirement. I am not sure if this means that I could match the
resistive components using an "L" or "Tapped C match" and then add
another 3pF to each side of the filter ???


Any help much appreciated

Regards

David


Andy writes:

You've generally got it right....
A xtal filter wants to see a particular input SOURCE resistance and
a particular LOAD resistance to develop the passband it was designed
for....

If your source has an output impedance of 4500 ohms, you would
want to transform it down to the 800 ohms specified. This can be
done in a transformer, but I usually do it with a split capacitor tank
circuit..... making the inductor the tuneable part.

If the load wants to "see" a capacitance, I usually use a 2-20pf
trimmer , together with whatever else impedance transformation
is available. Then I can tune the variable cap for the desired
passband characterists... Note that both the input and the output
are tuned in my technique....

If you don't have a sweep generator, you can just sweep the
gen you have back and forth to see if there are any ripples in
the passband.... Optimum is generally with the minimum ripples...
So, first tune the input and output for max output... Then rock
the generator to check for ripples... Often a very small adjustment
in the tuning will minimize the ripples at that point...

It ain't rocket surgery, but if you try to use fixed components
it ain't gonna happen, since you don't know your circuit strays....

Andy W4OAH