The MSA as a
Crystal Analyzer

Feb 9, 2010
Page released.

The MSA as a Crystal Analyzer
    Two versions of the MSA will function as a Crystal Analyzer.  The MSA with Tracking Generator (build level 2, also referred to as the SNA), and the MSA with Tracking Generator and VNA extension (build level 3, also referred to as the VNA).  Both will peform with the same accuracy but the procedures are a little different.

Crystal Basics
     A crystal is an electro-mechanical device that can be modeled like the circuit below:

    In reality, Cm, Lm, and Rm do not exist within a crystal.  However, a crystal "acts" like it has these components because of its electro-mechanical (piezo) nature.  These are "motional" (m) characteristics.  You may see these modeled components called Cs, Ls, and Rs (series).  The component, Cp is quite real.  It is the parallel interelectrode capacitance of the mechanical connections to the crystal blank within the crystal case.  In most crystals, Cp is less than 10 pfd, and can be as low as 1 pfd, or less.

These motional characteristics are important when using crystals as filters.  Obtaining crystal characteristics using manual or semi-automatic software techniques have been written and published for many years.  The MSA/TG (SNA) or MSA/TG/VNA (VNA) can derive these characteristics with very good accuracy.  The only required accessory for the MSA is a Series Test Fixture (and in some cases, even that is not necessary).

Serial Test Fixtures
    The external coaxial attenuators are assumed to be 50 ohms.  The internally integrated attenuators can be 50 ohms or they can be a impedance transformation circuits (
R0 to 50 ohms) to match to the 50 ohms impedance of the MSA.  Some builders prefer to use an R0 of 12.5 ohms when testing crystals.  The DUT is the crystal under test.
    If you are testing crystals with very long leads, the Series Test Fixture could be eliminated.  The crystal is simply placed between the MSA Tracking Generator output and the MSA Input.  Although the results will be less than "great", this method can be used for matching sets of crystals.

msapictures/serfixturecryst.gif  msapictures/serfixtureclip.gif
Serial Test Fixture with integrated attenuators.     Serial Test Fixture requiring external coaxial attenuators.

Testing Crystals with an SNA or VNA
If a crystal is placed in a Series Fixture, a VNA (or SNA) sweep will show a peak at its series resonant frequency (Fs), and a dip at its parallel resonant frequency (Fp). These frequencies are marked in the following scan by the P+ and P- markers.  For meaningful results, a line calibration must have been performed.  The MSA has been "line calibrated" with the Series Test Fixture to display the following results.

Plot 1

This scan of a series crystal (Plot 1) contains all the information needed to calculate motional parameters.  Data collected are series resonant frequency (Fs, displayed as P+), parallel resonant frequency (Fp, displayed as P-), and the 3 dB points of the response (although not displayed). Fs is the series resonant frequency of the crystal, for which accuracy is most important to derive motional characteristics of a crystal. It would seem that the resolution of 100 Hz/step is not enough to pin down Fs very precisely.  However, the VNA uses phase measurements, and are used to locate Fs quite precisely.  Phase is so linear near Fs that the interpolation works very well.  Since the SNA does not use Phase information, we need better resolution to abtain results that are equivalent to the VNA.
    The scan could be re-run with higher resolution, which would make it very slow, or we could simply record the value of Fp (P-) and zoom in on the area around Fs (P+), as shown below:

Plot 2
The above graph shows the area around Fs with enough resolution to determine its series resonant frequency fairly well, though a narrower scan would be better.  For extreme accuracy, the scan should provide a resolution of 5 Hz/Step or less.  Markers can be automatically placed by clicking the menu item, Functions / Filter Analysis.  Once this scan in place we click the menu item, Functions / Crystal Analysis, to access the Crystal Analysis dialog window.
The Crystal Analysis dialog window allows us to indicate whether, or not, the scan includes the dip at Fp. Plot 2 does not. If not, Fp is specified, which we obtained from the broad scan (Plot 1, P-). It also allows us to specify the R0 of the fixture. Most fixtures associated with the MSA will have R0 of 50 ohms, but in the case of crystals, some users may prefer using a fixture with R0 of 12.5 ohms. Here, though, we use a 50-ohm fixture.
Clicking the "Analyze" button causes the MSA to place the markers (shown in Plot 2), calculate the crystal parameters, and display the parameters in the long box at the bottom of the dialog. If testing just one crystal, we can write these values down. However, if we are testing a batch of crystals we can click Add To List, and the MSA will display a new window with the test results for this crystal and every other one we have added to the list. The crystals will be numbered sequentially in the list. In case we want to delete some entries from the list and back up the numbering a bit, we can click Set ID Num to specify the ID number for the current crystal.  To repeat a sweep, click Rescan.
    The Zoom button will automatically command the MSA to sweep between the 3 dB points of Fs (P+).
One feature of "Zoom" is that you can zoom twice. If the current resolution is less than 100 Hz, then Zoom aims for 15 Hz resolution. If Zoom starts with resolution of 100 Hz or better, it aims for 5 Hz resolution. But it never uses fewer than 50 or more than 400 steps.  Note: If using Zoom and then Rescan, the checkboxes will get set to "sweep contains Fs and Fp" during Rescan. To repeat the Zoom feature, the "Zoom" button must be clicked.

    Before clicking the "Analyze" button, to calculate the crystal parameters, you should verify that optimum conditions are met.  You will want the previous scan to have as "fine" a resolution as possible (Hz per step) and that the MSA line calibration was current for that scan.
(a) You can create a fine resolution scan extending from below Fs to above Fp, with enough resolution to identify Fs with the desired accuracy. For example, if you want +/-20 Hz accuracy, you could select a number of steps that makes the step size 20 Hz or less. The required number of steps for this method is typically large, which can make the scan extremely slow.  Using the 40 KHz sweep example of Plot 1, the number of steps required for a resolution of 20 Hz per step would be 2000 steps.  One sweep would take well over 1 hour to complete.  Using an SNA, this is the only way to have high resolution for a wide band sweep.  However, if your MSA measures phase (Build Level 3), Crystal Analysis will use the phase information to locate Fs by interpolation between adjacent points, which means good accuracy can be obtained with a coarse (fast) scan. In the same example, a 40 KHz sweep with 400 steps would result in a resolution of 100 Hz per step.  The calculations would use the phase interpolation of the VNA, and would be sufficient for obtaining Fs with good accuracy.
(b) You may do a coarse sweep that includes Fs and Fp (as in Plot 1), and rely on the Zoom to Fs feature of Crystal Analysis to get a more precise scan of Fs. As an example, you could perform the coarse scan with 400 steps per sweep and then the "Zoom" would perform an automatic scan between the 3 dB points of Fs.
(c) You may record Fp from the scan (P-, in Plot 1) and then change the scan for a finer resolution focused around Fs (including its -3 dB points), using a step size small enough to identify Fs precisely. This will be a narrow scan, so a small step size can probably be achieved with fewer steps.  For example, the finer resolution scan from Plot 1 could be from 10.364 MHz to 10.368 MHz. With 400 steps, the sweep would have a resolution of 10 Hz per step.  This is the method I prefer when testing a large batch of same type crystals.

Suggested Procedure for measuring the motional parameters of a crystal

1. Set-Up
a. Attach the
Serial Test Fixture between the MSA Tracking Generator output and the MSA input.
b. Install the Crystal in the Serial Test Fixture.  HINT: If the crystal is soldered into the test fixture, allow the crystal to stabilize to the ambient room temperature.  When matching crystals, it is imperative that each crystal test be at exactly the same temperature.  Fs, the Series Resonant Frequency can change several Hz per degrees C.
c. Enter the VNA Transmission Mode or the SNA Mode (for MSA/TG).
d. Set the video filter to Narrow (large capacitor) and the wait time to at least "50". (Both the hardware video filter switch and the software Video Filter combobox must be set.)

2. Coarse Sweep
a. Identify the frequency range necessary to show both Fs and Fp. You can actually do this without a line calibration, because the peaks are so strong.
Run an initial sweep and then Halt, to obtain a plot that is similar to the above screen shot, "Plot 1".  "Similar" meaning a sweep wide enough to show the Series Resonant Frequency and the Parallel Resonant Frequency (as indicated by the markers, P+ and P- in Plot 1).  Ampltude and Phase characteristics are not important.  They will be correct after the line calibration.  You can change the Magnitude Scale to display the low level of the Parallel Resonant Frequency.
c. Click Menu item, Functions/Crystal Analysis.
d. The Crystal Analysis Window will open.
e. Enter a different Test Fixture crystal load circuit if it is not 50 ohms.
f. Click the "Analyze" button.
g. The crystal parameters will be calculated and placed in the box at the bottom of the dialog.  If a line calibration has not been performed, the motional values will be in error.  However, Fs and Fp will be quite accurate.
h.  If VNA is used, skip to the next section of the procedure and perform a line calibration. If the SNA is used, or if you want ultimate performance from the VNA, continue with the next step.
i. Write down the value of P-, which is the parallel resonant frequency, Fp.
j. Scan with a narrower sweep to include only the range of frequencies just outside the 3 dB points of P+ (the crystal's Fs).  Using Plot 2 as an example, the sweep range could be from 10.365 to 10.367 MHz.  Using 400 steps, this would sweep with a resolution of 5 Hz per step.
k. Halt the sweep.

3. Line Calibration
For best performance, the MSA must be "line calibrated" before sweeping a crystal under test.  This will ensure that the Serial Test Fixture and extension cables will not have any effect on the final crystal calculations.  There are two types of line calibrations that can be performed, the Baseline Calibration and the Band Sweep Calibration.  Normally, a Baseline Calibration for the MSA will cover the total frequency range for which the MSA is in, i.e., 0 to 1000 MHz.  A
Band Sweep Calibration will cover only the frequencies that are to be used in the current sweep, which would be the frequency range that was chosen in step 2.j.  The Band Sweep Calibration will provide the best results for crystal analysis.  This is the procedure:
a. Short out the crystal with a short wire. Or, remove the crystal and replace with a short.
b. Click Menu item, Operating Cal/Perform Cal. The Perform Calibration Window will open.
c. Click Perform Band Cal.  The MSA will enter a single sweep cycle and halt.
d. Click Done.
e. This Line Calibration will be good for all subsequent sweeps as long as the sweep frequency range is not changed.  On the next sweep, the MSA will automatically select the Band Cal as its reference.
f. HINT: I always run test sweeps after performing any Line Calibration to assure that everything is repeatable. Do so by clicking "Restart". The Magnitude should indicate 0.00 dB +/- .1 db.  For VNA, the Phase should indicate 0.0 degrees +/- 1 degree. Move the Test Fixture and extension cables around. During movement, expect to see the Phase change a little.  It should return to 0.0 when you stop the Fixture/Cable movement.

4. Fine Sweep
a. Remove short from crystal, or re-insert crystal into the Test Fixture.
b. Click "Restart".  The Graph Window parameters will change from Cal = None, to Cal = Band.  The resulting sweep will look similar to "Plot 2". 
You may use a large number of steps in order to get the step size in the 5-50 Hz range, so the MSA can determine Fs precisely.
c. Click Halt At End.
d. Click Menu item, Functions/Crystal Analysis.
e. The Crystal Analysis Window will open.
f. Enter a different Test Fixture crystal load circuit if it is not 50 ohms.
g. Check "The scan includes the series resonance peak only....".
h. Enter the value of P-, that you recorded earlier.
i. Click the "Analyze" button.

j. The crystal parameters will be calculated and placed in the box at the bottom of the dialog.
5. Create a list of crystals.
If you want to create a list of tested crystals, click Add To List. The list will be in a normal text window where you can delete entries or make any other changes, and you can copy the data to the clipboard or save it to a file. Each crystal is numbered in the list; if you need to change the current number, click Set ID Num.

6. Testing multiple crystals of the same type.
a. Remove the current crystal under test and replace it with a new one.  HINT: I suggest using tweezers to handle the crystals, so as not to change their temperature by the heat in your fingers. Yes, just a few seconds of handling will change a crystal temperature several degrees (and many Hertz).
b. Click "Rescan" to obtain a new trace with new data for analysis. The current line calibration is still in effect.

c. A single sweep will be initiated and will automatically Halt at End.
d. Click the "Analyze" button.
e. The new crystal parameters will be calculated and displayed in the box at the bottom of the dialog.
f. You may add this crystal's parameters to the list or write down the data for future reference.
g. When finished with a series of crystals, click the Close Box at the upper right of the dialog. If you created a crystal list, the list window will remain open until you close it.

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