Re: Matching a monolithic xtal filter

On Sep 9, 1:37 am, Tony Williams <to...@xxxxxxxxxxxxxxxxxxx> wrote:
In article <1189210614.332653.299...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Tom Bruhns <k7...@xxxxxxx> wrote:

...............Instead, use a transformer such as MiniCircuits
TC16-1T. That and a 6.8pF series cap on the filter side gets you
from 50 ohms to about 800-j520, pretty close to the recommended
load for the filter.

Tom. Interested question..............

That 1:4 stepup transformer and 6.8pF raises the 50 Ohm
source to nearly the 874-j534 series-equivalent (at
45MHz only) of 1200//1.8pF.

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?

--
Tony Williams.

Hi Tony,

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

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.

Cheers,
Tom

.

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