Calculation and Manufacturing of a Crystal Filter
Building Crystal Filters at Home: A Complete Practical Guide
1. Introduction and Objectives
A crystal filter (CF) is a key component of any modern superheterodyne transceiver or receiver, determining the adjacent-channel selectivity in the intermediate frequency (IF) stage. In this article, we will break down the methodology for creating a high-quality CF, allowing even a beginner radio amateur to achieve professional results with minimal costs.
The main focus is on practical implementation: from laboratory measurements of quartz crystal parameters to the final tuning of the finished product using modern, accessible instruments.
2. Required Equipment and Materials
To implement this project using an "engineering approach", you will need the following tools:
Measuring Instruments:
- Frequency Counter: with an 8-digit display. The most crucial requirement is a resolution of ±1 Hz at frequencies up to 10–15 MHz. This is strictly necessary to minimize cumulative error when calculating the dynamic inductance (Lm).
- Sweep Analyzer / VNA (NanoVNA): required for verifying the filter's characteristics. If using long coaxial cables, OSL (Open-Short-Load) calibration is mandatory to neutralize the capacitance of the measurement circuit.
- Capacitance Meter: to determine the static holder capacitance (Co) of the crystal.
Software and Tools:
- DISHAL Program: specialized software for crystal filter calculation (use the standard English version).
- Data Processing: Use the calculator at the bottom of the page to process the data array and calculate the average parameter values.
Materials:
- A batch of crystals: 50 to 100 pieces to ensure a high-quality selection.
- Frequency choice: It is optimal to use crystals above 7 MHz (for example, the popular 8.867 MHz).
Technical limitation: Low-frequency crystals often have a small resonance spacing (the difference between parallel and series resonance), which physically limits the maximum bandwidth and makes them completely unsuitable for SSB filters.
3. Building and Tuning the Test Oscillator
To measure the crystal parameters, you need to build a Colpitts oscillator according to the schematic recommended in the DISHAL software.
- Feedback divider: Two 470 pF capacitors. They ensure stable oscillation in the circuit.
- Measurement load: A high-quality 45–47 pF capacitor is connected in series with the crystal. It is this specific component that creates the frequency shift required by the software to calculate the dynamic parameters.
- Assembly: It is recommended to assemble the circuit on a prototyping board, providing a high-quality socket for quick crystal swapping and a push-button to switch measurement modes.
Measurement Modes:
- OPEN: Measuring the oscillator frequency without the additional load.
- CLOSE: Measuring the frequency with the series load capacitor (45–47 pF) connected.
4. Crystal Selection and Sorting Process
Selection accuracy is the foundation of a flat passband with minimal ripple.
Action Algorithm:
- Insert the crystal into the socket, measure the frequency in the OPEN mode, then in the CLOSE mode.
- Record the data (Crystal No., F_open, F_close).
- Repeat this process for the entire batch.
Selection criteria for a group of 8 crystals:
- Ideal: Exact frequency match.
- Excellent: Spread within 50–60 Hz.
- Acceptable (for QER topology): Spread up to 100 Hz.
Exceeding the 100 Hz threshold will lead to a sharp increase in passband ripple and distortion of the filter skirts.
5. Determining Crystal Parameters in DISHAL
After the initial selection, proceed to calculate the equivalent parameters in the "Xtals Parameter Calculation" window.
- Input Co: Measure the holder capacitance (static capacitance). Usually, it is 3.5–3.8 pF. This is the least critical parameter; a value of 3.0 pF is also acceptable.
- Input data: For each of the 8 selected crystals, enter the measured F_open and F_close.
- Result: The program will calculate the series resonance frequency (Fs) and dynamic inductance (Lm).
Using the calculator: Use the calculator at the bottom of the page. Transfer the Fs and Lm of all 8 crystals into the calculator to compute the arithmetic mean values. You can also use it to calculate the maximum bandwidth (BWmax ≈ Δf/2) and determine the compensating inductance needed to neutralize the capacitance of long measurement cables.
6. Topology Selection: QER (G3VXO) vs. Chebyshev Filter
For home manufacturing, we highly recommend choosing the QER (Quasi-Equi-Ripple) filter.
| Characteristic | QER Filter (G3VXO) | Chebyshev Filter |
|---|---|---|
| Calculation Complexity | Minimal | Medium / High |
| Component Identity | All Ck capacitors are identical | Different values required |
| Passband Ripple | Minimal (typically < 0.5 dB) | Can reach 3–6 dB |
| Crystal Match Sensitivity | Low (forgiving to spread) | Critically high |
| Skirt Steepness | Medium | High |
QER Advantages: Using identical capacitors makes it incredibly manufacturer-friendly. Even when working with budget crystals, the QER topology provides a "clean" frequency response without the deep dips typical of poorly calculated or mismatched Chebyshev filters.
7. Filter Calculation and Capacitor Matching
In the QER filter calculation window (DISHAL), enter the averaged Fs and Lm, the Co value, the number of crystals (8), and the target bandwidth (e.g., 2.8 kHz).
Engineering "Compromise" Method: Often, the program outputs a calculated capacitance Ck (e.g., 84.9 pF) that does not exist in standard E-series ranges. To avoid using trimmer capacitors:
- Iteratively change the "Desired Bandwidth" value (for example, set it to 2.95 kHz instead of 2.8 kHz).
- Monitor the change in Ck until it gets as close as possible to a standard value (e.g., 80 pF or 82 pF).
This approach allows you to use standard components by only slightly shifting the bandwidth while maintaining the calculated shape of the frequency response.
Component Requirements: Capacitors must be SMD type with NP0 (C0G) dielectric. This guarantees thermal stability and completely eliminates frequency drift when the equipment heats up.
8. Final Assembly and NanoVNA Testing
Once the filter is assembled on a printed circuit board, you must verify that it matches the calculated data.
- Connection: Use the NanoVNA in S21 measurement mode (LogMag).
- Analysis in NanoVNA Saver: Set markers at the -3 dB level relative to the peak of the frequency response.
- Verification: The distance between the markers must correspond to the calculated bandwidth (in our example, 2.8–2.9 kHz).
- Visual check: The top of the passband should be flat, without sharp peaks or dips. Clean, steep skirts indicate that the crystals were selected correctly.
9. Conclusion and Recommendations
Building a high-quality crystal filter is not magic, but the result of accurate measurements. The main conditions for success are:
- Careful crystal sorting (frequency spread under 60 Hz).
- Using the QER topology to simplify assembly.
- Using NP0 standard capacitors.
Important Note:
The DISHAL software also calculates the input and output impedance (Z) of the filter. When installing the CF into a transceiver circuit, it is critically important to match this impedance with the amplification stages or mixers. A mismatched filter, even if perfectly assembled, will show an unsatisfactory frequency response.
Additional materials, firmware for frequency counters, and links to verified crystal suppliers are available in the appendix to this guide.
Calculating the Average Fs and Lm Values of Crystal Resonators
Basic Schematic Diagrams of Crystal Filters
Component designators correspond to the calculated values in the dDishal software.
QER Filter Schematic Diagram
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