[vna] measurement of quartz crystal

[Koji Terai]

Hi Tom,

Thank you for your res and suggestions.

So far, I had no chance to use VNAs, but I read some books before that 
described importance
of defining measurement planes and consistency of a testfixture to be used.
Any way I have just started to use the VNA and I have to learn more about 
measurement
by the VNA.

The way I tried was that I measured crystals in S12 mode and tried to read 
out f(o) and f(a)
from curves of magnitude and phase, but as for f(o), I had to expand 
frequency pitch
to read the frequency very precisely then the peak was displayed very flat, 
almost it turned a line
and I could not find where the peak was. So I gave up reading our f(o) from 
a magnitude and I
read f(o) from a curve of phase instead, but as for f(a), a curve of phase 
changed in every sweep
so that I could not determine f(a). I think that a crystal exhibits great 
attenuation at f(a) and the VNA
is not able to determine phase change at around f(a) due to the limitation 
of the phase detector of
the VNA. I am not sure the way I tried was correct or not, or there are some 
better way to measure
a crystal at this moment.

Koji/ JA3XNO


----- Original Message ----- 
From: "Tom McDermott" <tom.mcdermott4 at verizon.net>
To: "'TAPR/Ten-Tec VNA Users List'" <vna at lists.tapr.org>
Sent: Sunday, December 04, 2005 8:08 AM
Subject: RE: [vna] measurement of quartz crystal


Hi Koji:

The TAPR VNA can be calibrated against an external reference
oscillator for good frequency accuracy. However in order to determine
accurately the 4 motional parameters (Ls, Rs, Cs, Cp) of the crystal
requires extreme precision of the fixture stray reactances. I am not an
expert in crystal filters, but I think the 4-parameter model is used pretty
extensively in filter modeling. 100 Hz frequency accuracy is not a problem,
but stray reactances are a problem.

I think Agilent has a software package to extract the motional
parameters from swept S parameters. The TAPR unit does not have this
software, but it perhaps could be an interesting exercise; certainly for
those who enjoy working on software.

Key to measurement is that the fixture parallel measurement will be
contaminated by fixture parallel capacitance. This will rotate the series
resonance frequency away from R + j Zero X and must be compensated. The
parallel anti-resonance will have the same issue. S-parameters will show the
crystal resistance as compared to 50 ohms but the fixture will rotate the
phase angle. The fixture perhaps can be calibrated out of the measurement if
the crystal is replaced with a 50 ohm resistor, but I have never tried it.

-- Tom, N5EG






> -----Original Message-----
> From: vna-bounces at lists.tapr.org
> [mailto:vna-bounces at lists.tapr.org] On Behalf Of Koji Terai
> Sent: Saturday, December 03, 2005 9:14 AM
> To: vna at lists.tapr.org
> Subject: [vna] measurement of quartz crystal
>
>
> Hi, everyone! I would like to know if the VNA is able to
> determine resonant frq. and antiresonant frq. of a quartz
> crystal in accuracy of 100 Hz. I plan to construct Cohn type
> crystal filters, and I tried to measure crystals but I was
> not able to measure these two frequencies definitely,
> especially ant frequency measurement was very critical. I am
> looking forward to a good solution for this.
>
>  Koji
>
>
>
> _______________________________________________
> vna mailing list
> vna at lists.tapr.org https://lists.tapr.org/cgi-bin/mailman/listinfo/vna
>


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