Is there an app note on how to measure xtal series resistance? Does anybody have experience with evaluating xtals?

The xtal is specified to have < 10 ohm series resistance.

I connected it in series and did a S21 sweep around its freq. of operation (24MHz). I hoped to see a purely resistive point on that trace (xtal resonance) which hopefully would give me series resistance but no luck, S21 values are always complex. My next idea is to do curve fitting on this trace to the xtal model and extract the Rs this way. However, before I spend extra time doing this, I wanted to check if someone else has tackled this problem.

I understand that there are specialized devices for impedance measurements but I have only access to HP8753 network analyzer.

Any ideas?

Thanks,

The xtal is specified to have < 10 ohm series resistance.

I connected it in series and did a S21 sweep around its freq. of operation (24MHz). I hoped to see a purely resistive point on that trace (xtal resonance) which hopefully would give me series resistance but no luck, S21 values are always complex. My next idea is to do curve fitting on this trace to the xtal model and extract the Rs this way. However, before I spend extra time doing this, I wanted to check if someone else has tackled this problem.

I understand that there are specialized devices for impedance measurements but I have only access to HP8753 network analyzer.

Any ideas?

Thanks,

I still can't figure out where that inductance is coming from? Any ideas how to extract that information from those plots?

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Sorry, for some reason I can see your tiffs, maybe you can convert them to png or jpg?

If the the xtal is mounted in a series connection, you should be looking at S21. In the 8753, there is a function called S-parameter conversion, which will show you R (if the format is real) and jX (if the format is imaginary) of the equivalent seriers impedance that gives the same S21 as measured. Have you tried the Z-converstion function yet? This is in the form of a series R+jX.

1. Connect crystal between center pin and ground of the VNA's port 1 in series with high precision 50 ohm resistor (or measure your resistor first).

2. Set VNA to measure S11 (calibrate)

3. Switch to imaginary plot and find the point where Zim=0

4. Switch to Smith chart and read out impedance.

xtal's impedance = VNA' number - 50 ohm

Note that depending on the crystal cut, you will have a number of resonances some of them might be pretty close to the fundamental.

In my case, I get 58.3 so 8.3 ohms. The crystal was characterized by the vendor to have 8.5 ohms.

The reason S21 will not work in this case is that S21 does not really measure insertion loss of the xtal and hence I cannot use the transmission coefficient (2Zl/(Zl+Z0)) formula to extract the resistance with Zl = Z_xtal. It is also why the imaginary plot does not show correctly impedance vs freq. of a xtal.