EMRFD Message Archive 14368

Message Date From Subject
14368 2017-10-01 16:22:00 nothdurftm Input/Output Impedance of Crystal Filter

Can someone give me guidance on how to determine the input/output impedance of a crystal filter (unknown internals but center freq. known).  I assume you can't just measure it with an arbitrary termination on the output.




14369 2017-10-01 17:13:42 Nick Kennedy Re: Input/Output Impedance of Crystal Filter
There may be ways of doing it that are more direct, but a method I've heard of and used is to terminate the thing with various resistances and sweep or plot the response, then when it "looks good" (sounds kind of subjective,I know),  you've found the right resistance.  I guess you'd use either transformers or resistive pads between generator and filter and filter and detector.

There may be a few standard values often found in commercial (or even homebuilt) filters that you could try first.


Nick, WA5BDU

14370 2017-10-01 18:40:28 Phil Sittner Re: Input/Output Impedance of Crystal Filter

A very useful tool is on found within the DG8SAQ VNWA.  With this tool you perform a sweep and then use your mouse scroll wheel to establish the actual characteristic of the filter and a corresponding matching network is displayed on your screen. A free download of the help file and software is available from their site. I have no affiliation with the vendor, only being a very satisfied customer.

Phil. KD6RM

14371 2017-10-01 21:23:44 Ashhar Farhan Re: Input/Output Impedance of Crystal Filter
What kind of test equipment do you have? The answer depends upon that.
- f

14373 2017-10-02 07:15:22 Clint Re: Input/Output Impedance of Crystal Filter
For testing Atlas 210X crystal filters (5645 Khz IF), I used the variable pot method between my Rigol sweep generator and the filter.  At the design impedance (around 800 ohms), the filter had the best curve, least amount of ripple, and lowest insertion loss.  If you do not have the tracking generator function on your spectrum analyzer, you can use a wide-band noise generator for the signal source.
14376 2017-10-02 13:51:02 John Lawson Re: Input/Output Impedance of Crystal Filter
Clint, I have some Atlas crystal filters in the junk box....just wondered if yours has the manufacture  stamped on the crystal filter's case....if I recall correctly mine have Network Sciences Phoenix, Az on them. BTW your method of sweeping the filter is similar to what I have done in the past.. Thanks, John K5IRK 

Sent from my iPad

14388 2017-10-14 10:26:10 Jim Amos Re: Input/Output Impedance of Crystal Filter
Here is a link to the KVG website where they show they how to measure filter response using the resistive matching technique being discussed by everyone.  

I've also used this and it gives you good confidence what the match needs to be.


On Monday, October 2, 2017 4:51 PM, "John Laws