Crystal Parameter Checker
Introduction
This web page is a supplement to JavaScript Applet G on this web page. This software does the math using the simple method to determine motional inductance and capacitance developed by David, G3UUR. This is a very basic tutorial meant as an introductory guide for novice builders.
Shown in the photo above is my crystal parameter checker. The schematic may be found in many places including EMRFD Figure 3.35 (See Errata) and on this pdf by Nick, WA5BDU. A power indicator LED has been added, but the circuit is the standard design. In this breadboard, the crystal being measured is tack soldered in.
Many builders just copy other builder's I.F. filter schematics, however, your crystal filters will perform better if your design is based upon the exact parameters of the crystals you have. For the simple design or optimization of a crystal filter, it is necessary to measure crystal parallel capacitance plus take other measurements to calculate motional inductance and capacitance. Determining your crystal parameters is not difficult if you have a capacitance meter, a frequency counter and some math skills. It is easiest to use a program to crunch the math; hence the JavaScript applet was written. Designing filters is another story; it takes knowledge, practice and good software for this. Filter design theory has been extensively covered by Anatoly Zverev, Wes Hayward and others. The work of Nick, WA5BDU is also greatly appreciated. His presentations and references are excellent for those keen on learning more about filter design.
A four crystal 5.00 MHz SSB I.F. filter was desired. 20 crystals were on hand. They were from the same batch. The crystals were all placed in the above oscillator and their frequency was measured. The 4 crystals closest matching in frequency were set aside. The crystal parameters of these 4 were then determined. Typically these values are averaged and this average is used to design or tweak the filter using software.
1. Measure Capacitance
The procedure for determining the parameters of 1 crystal is described. The first step is to measure the crystal capacitance (called parallel capacitance) using a capacitance meter. The parallel capacitance of the open switch is then measured. This is performed with the switch soldered in place, but before you solder any other components into the circuit containing the switch. Record these values carefully.
Measuring crystal parallel capacitance
Capacitance of the 5.0 MHz crystal in an AADE LC meter
Capacitance of the open switch wired in-situ
2. Measure Frequencies
A crystal is put in the oscillator with the switch open. Record the frequency. Your counter must have resolution down to 1 Hertz. After recording this value, throw the switch and measure and record this frequency. You now have all the measured values required to calculate motional parameters and adjust or design a filter. Motional parameters are calculated in Applet G.
Frequency measured with the switch open = 4.999274 MHz
Frequency measured with the switch thrown = 4.998317 MHz
3. Do Math by Hand or with Software
The applet G calculation of the crystal parameters using the above measured values
4. Example Filter Adjustment
It is assumed that most builders will use software to design or tweak their crystal filters. The only 2 programs tried include AADE Filter Design and the Ladpac software collection that supplements EMRFD. I am more familiar with the Ladpac programs written by Wes, W7ZOI. Only these programs are demonstrated. Please read the instructional file Ladpac2008 Manual.pdf to understand these programs.
The purpose of this tutorial is not to teach crystal filter design, but to describe a relatively simple method to tweak an existing design using your measured crystal parallel capacitance and its calculated motional inductance.
The first step is to digitally format your filter into a file that can be analyzed in GPLA. In my opinion, the easiest way to do this is to use the ladder circuit editor ladbuild02.exe or better yet, its update - ladbuild08.exe. The model filter follows:
The model 5 MHz SSB filter
Clear any existing components and enter the termination R, C-par and Lm values. Qx is set at 100000
Build your filter within the editor. Save your work. Start up GPLA and load your newly saved filter.
Set a sweep and x axis increment (-7000, 1000 and 7000 in this example). Push the Plot button
Lets say you wanted to use this filter design and have determined the average parameters of the 5.00 MHz crystals in your parts collection. Lets us assume that for your crystals, C-par = 3.1 pF, and Lm = 0.098H. Input these values in GPLA.
Look what happened to the crystal filter's bandwidth. Our - 3dB bandwidth is now somewhere around 1464 Hertz. This simple experiment illustrates how important it is to use the parameters of your crystals to obtain a desired filter response.
Experiment with the various functions in GPLA to learn how to use it. Set whatever reasonable sweep you want. This program is best learned by using it repeatedly.
In the above screen capture, the above filter was tweaked to "re-establish" a -3dB bandwidth of ~2.172 KHz. All adjustment was performed entirely in GPLA by swapping capacitor values and observing the resultant waveform. When you get an overall pleasing bandwidth plus shape, but there is too much ripple at the top, generally you must increase the terminating R values. This is the brute-force, manual way to tune filters. For this method, you need not understand terminology such as as series resistance, MESH, K or Q values, Butterworth response, or Chebyshev with 0.1 dB of ripple. Admittedly, at first, this method can be quite time consuming and tedious, however, with practice, you may be able to tweak a filter in only a few minutes. Clearly, the more you dig into understanding crystal filter design, the better your filters can be, however, getting overly complex can scare off builders who are new to this hobby. Note these filters use standard value capacitors and resistors; perfect for popcorn I.F. filters.
The original 5 MHz Model filter with updated C and R values using Lm = 0.098 and C-par = 3.1
GPLA zoom of the Y axis showing the first 20 dB of attenuation.
5. The Model 5 MHz SSB filter Design
Although this page is not about crystal filter design, an example follows for reference purposes.
For designing filters, the application xlad08.exe is a good choice. The following 3 screen captures show the raw design process and GPLA analysis of the model 5 MHz Lower Sideband filter shown earlier with C-par = 4.65 pF and Lm = .0578. There are some great articles in print and on the Web to study if you want to learn about filter design. The Ladpac software from EMRFD is excellent. My special thanks to Wes, W7ZOI for answering my questions about his software. From this information, I was able to make this web page.
6. Conclusion
This web page presents a brief method to calculate crystal motional parameters and as required, to adjust crystal filter circuits to function optimally. This approach is simplistic. Listening tests are also valuable for assessing crystal filter function. Is the bandwidth as expected? Does the filter ring excessively? Does it sound tinny? In the recent past, the crystal filters in 2 kit receivers/transceivers were tweaked as a favor to friends. Please note, I have total respect for people who sell kits and appreciate the contribution they make to our hobby. The crystals of these Cohn type filters were removed and analyzed and the bandwidth was not as specified. In 1 case, the filter shape looked terrible in GPLA. Clearly the kit sellers provided crystals which had markedly different parameters from those used by the original circuit designer. The I.F. filter capacitors were replaced with appropriate values and the R values were adjusted via either resistors and/or transformer ratios. The improved filters were pleasant. It is a real treat to listen to a receiver with a well designed crystal filter.
In my opinion, the best sounding CW crystal filter design is the Gaussian to 6 dB. Some operators would never use such a filter in a contest-grade receiver as the filter skirts are not steep enough for them. There are tricks to make the stop band better (more like a Chebyshev response, but without the ringing), however, this topic is out of scope.
I sincerely ask for your feedback on the G. JavaScript Applet. Does it work correctly? How could it be improved? Can you contribute better code? Thanks and 73.
