I just posted _WA5BDU 1r7 to the site.  In this revision, I added a function that will automatically scan your crystal (or similar resonator) , find the peak and loss and -3dB points.  It then uses that information to calculate and display crystal parameters Rs, Lm, Cm and Q.


You will need to input limits for the scan.  If you don't know the crystal's resonant frequency within a few hundred hertz, you may want to do a "regular" scan (Option 1) of plus / minus a few Khz of its marked frequency with a step of 25 Hz or so to get a ballpark idea.

When in the new Option 7, you are first given the opportunity to read the signal level through your crystal fixture with a short across the socket.  This is used to figure the crystal's loss at resonance.  You can save the value determined to eeprom and it will be recalled each time you run.  (Might be a good idea to re-verify periodically though.)

Next you are asked for the low and high limits of your scan.  Since you know the approximate resonant frequency, input values  of a few hundred hertz below and above it. It depends on the Q and frequency of the crystal.  If you get weird results, broaden the limits.  But I default to 1 Hz steps here, so don't go too wide.  I currently have the software bomb out and issue a complaint if going > 2,500 steps in one direction without finding a half power point.

The software then scans the crystal and finds the frequency and signal level at resonance.  By subtracting the signal level from that measured with the short across the socket, the crystal's loss at resonance is calculated.  The software takes the found peak frequency and signal level and scans down, then up and finds the -3 dB points. It will display the frequencies and the bandwidth.

The user is asked for the termination resistance(s) of the test fixture.  Typical values are 50 ohms and 12.5 ohms.  Now it has enough information to calculate all typical parameters:  Rs (series resistance), Cm (motional capacitance) and Lm (motional inductance). Q is also calculated and displayed.  

Note that parallel (holder) capacitance isn't calculated with this method -- if it is needed, an estimate of 3 pF to 5 pF can be used, or just measure across the crystal's electrodes with an AADE meter or equal.

After you measure a crystal, you are given the opportunity to re-do the measurement or do another crystal with the same parameters, saving you a lot of data entry.  If you do need to change some parameters, just exit this sub-menu and re-enter it again from the main menu.  You will be asked to enter the parameters again.

Repeatability looks pretty good so far.  Occasionally a reading changes and I wonder if  the crystal is loose in the holder, etc.  Once Q started degrading and I eventually found that the 9V battery in my AD8307 box was down to 5.7 V.  We'll get more idea of it's accuracy with use.  I need to check against some crystals I've previously measured.

Note that when entering scan limits, even though you enter values in kHz, you can use decimals to enter down to the Hz if necessary, as in 7999.750.

I thought it would be simple -- it took the better part of two days.  I'm now returning to my normal pre-Christmas activities.  

(Will be happy to look into bug reports though.)

BTW, this revision is for the "terminal" mode of the software.  Incorporating it into the PLX-DAQ (Excel) version is down the road.  Could be automated in Excel using VBA too, I suppose.
