Measure Crystal Characteristics Automatically Using a NanoVNA

Introduction

Use this script to automatically characterize your crystals using a test fixture and a Nanovna. The script will search for the series resonant point of the crystal and from there make measurements. It will also search for the parallel resonant point of the crystal, if the --stray option is given, to find the crystal's holder capacitance.

The script operates by drilling the Nanovna down to the series resonance point of the crystal and measuring its Cm, Lm, and Rm values using the phase shift method. When the measurement is finished the Nanovna's sweep frequency range will be restored to what it was before the script was run.

How to use

First set the start and stop "stimulus" values of the Nanovna to encompass the range of frequencies you expect to see from your batch of crystals. The frequency span should be large enough to capture both the series and parallel resonant points of your crystals. I have been using a span of 100KHz. (Or use options --start and --stop to force this initial sweep frequency range.)

Next calibrate the thru port of the Nanovna using the "calib" menu. Now you can run the script to make the measurement. Pass the --stray option if you want to measure the holder capacitance.

The arguments to the python script follow:

$ crystalweb.py --help
usage: crystalweb.py [-h] [--fixture] [--loss] [--theta THETA] [--stray STRAY]
                     [--repeat REPEAT] [--load LOAD] [--title TITLE]
                     [--device DEVICE] [--start START] [--stop STOP]
                     [--capture]

optional arguments:
  -h, --help       show this help message and exit
  --fixture        measure test fixture stray capacitance (default: False)
  --loss           measure test fixture loss (default: False)
  --theta THETA    phase angle for measuring bandwidth (default: 45)
  --stray STRAY    test fixture stray capacitance in pF, affects Co (default:
                   None)
  --repeat REPEAT  number of times to repeat measurements (default: 10)
  --load LOAD      test fixture source and load resistance (default: 50)
  --title TITLE    title of measurement (default: )
  --device DEVICE  name of serial port device (default: None)
  --start START    starting frequency of initial sweep (default: None)
  --stop STOP      stopping frequency of initial sweep (default: None)
  --capture        capture screenshots of the measurements as movie_xx.png
                   (default: False)

Besides measuring crystals, the script can also measure the stray capacitance of a test fixture using the "--fixture" measurement option. The stray capacitance is used to measure the holder capacitance of the crystal. To find the stray capacitance, first calibrate the thru of the NanoVNA. Next leave the fixture open and run "crystalweb.py --fixture". The "--loss" measurement is like the "--fixture" measurement except it returns the loss through the fixture instead of the capacitance.

Example

The script, as it measures the crystal, writes the results to stderr. At the conclusion of the measurement, the script writes a final comma separated formatted line, listing all the measurements previously found, to stdout. The CSV header for this line is 'XTAL,FS,CM,LM,RM,QU,CO'.

For example, I executed the following to measure a 7.03 Mhz crystal:

$ crystalweb.py --stray 1.1 --title X1
TITLE: X1
RL    = 50.0 ohm
fs    = 7027674 Hz
Rm    = 18.39 ohm
Cm    = 0.0149 pF
Lm    = 34.3163 mH
Qu    = 82401
stray = 1.10 pF
fp    = 7039743 Hz
Co    = 3.25138 pF
X1,7027674,1.4946e-14,0.034316,18.39,82401,3.2514e-12

My test fixture