EMRFD Message Archive 2657

Message Date From Subject
2657 2009-01-29 19:53:20 Steve Crystal resistance measurements -- follow up
Hi all,

I want to follow up on the posting I made last week and again thank
those who offered advice.

My goal (which I neglected to mention in my initial posting) for this
crystal is to use it as the resolution bandwidth filter in a spectrum

I mounted the filter on a small piece of scrap FR4 double-sided circuit
board. Each port of the crystal was impedance matched with a 3uH
inductor in series and a 43pF capacitor in parallel with the crystal. I
used pins 1 and 3 (the corner pins) as input and output, grounding the
other two pins on each end of the crystal.

I made transmission mode and reflection mode measurements with a VNA and
have posted them to the groups's Files section in the "ITT filter VNA
measurements" folder.

The transmission mode measurements indicate what seems to me to be more
passband ripple than would be expected and greater loss than specified
for the crystal. I don't know what to make of the that I'm just a beginnner who is learning about crystals? The plots look
about the same going either way through the crystal.

The reflection mode measurements are made with the other port terminated
in 50 ohms, although not terminating the other port didn't make a lot of
difference in the input impedance measurements. Measuring either port
produced about the same plots. The input impedance in the passband
wanders around a good bit.

So, having said all that, any thoughts about my crystal? Is this typical
data or does it look suspect? Crystal OK for the intended application?
Errors on my part?


Steve K8JQ

-------- Original Message --------
Subject: Crystal resistance measurements
Date: Tue, 20 Jan 2009 21:02:10 -0500
2661 2009-01-30 07:52:01 joop_l Re: Crystal resistance measurements -- follow up
It looks as if you did not properly terminate the crystal filter (note
it is not a crystal as you keep calling it).

For an example of the effect of not terminating it at the designed
impedance look here (scroll fully down to last picture):

The blue line is probably how it was measured (50 Ohm). The black line
is a recalculation of the response in a proper termination.