Re: Matching a monolithic xtal filter



Tony Williams wrote:
In article <1189405394.912612.186830@xxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Tom Bruhns <k7itm@xxxxxxx> wrote:

On Sep 9, 1:37 am, Tony Williams wrote:

What is the disadvantage of using a 1:5 stepup to get
the 50 Ohm up to 1250, plus a parallel 1.8pF, in order
to get a direct (wideband) output Z of 1200//1.8pF?


That would be just fine, too, 'cept that in my quick look, it
looked like the 4:1 turns ratio from MiniCircuits was much
cheaper than any suitable 5:1 in their catalog.


Thanks Tom. As you know rf is not my bag so I'm an
interested lurker on these threads, trying to follow
the sums..... and where the sums produce something
that may not be practical to implement. That was the
reason for the question.


M/A-com has some similar small transformers and may have
something in a 5:1 turns ratio that would be appropriate.


A 1:4 transformer could be used as a 1+4 auto of course.


I'm not sure how sensitive these particular filters are to load
impedance. With many filters, if you're not trying for the
absolute best conformance to the specified filter shape, a modest
mismatch from the recommended load and source impedances doesn't
really matter that much. It's one of those "YMMV" things--test
to be sure you get what you want.


AFAIR from the data ***, that filter is about 75KHz
bandwidth, but they also specify a guaranteed attenuation
out at +/- 1MHz from the centre frequency. So presumably
the source/load impedance has to be somewhere near the
req'd 1200//1.8pF over that range.


It's like all the other crystal parameters, that impedance derives from the motional parameters near resonance and in most applications designers use bandpass impedance matching to avoid exciting spurious responses and non-linear mixing in the crystal.

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