A PICAXE-based Equipment to Measure Quartz Crystals


Low cost quartz crystals are often used by radio amateurs to make selective filters for transmitters and receivers. Quartz crystals can be thought of as having an equivalent electrical circuit consisting of a high-Q series tuned circuit in parallel with a capacitor. These equivalent component values do not form part of the published data for low cost crystals. The aim of this project was to design and build a piece of test equipment that automatically measures the four components of the equivalent circuit, displays the values and enters them into a spreadsheet on a PC. Each quartz crystal to be measured is plugged into a Colpitts oscillator. Capacitors are switched in series with the crystal and three frequency measurements are made. From these measurements the series inductance, capacitance and parallel capacitance can be calculated. The amplitude of oscillation is maintained at a low level (1 mV rms) so that the oscillator operates as a predictable Class A amplifier. The emitter current is a measure of the series resistance of the crystal. Each crystal is numbered for traceability and a batch of 100 crystals can be measured in about 45 minutes. The equipment is controlled by a PICAXE 40X2. The PICAXE also sends the measured frequencies and oscillator emitter current to a uM-FPU floating point co-processor, which calculates the component values and returns the results to the PICAXE for display on a 20 x 4 OLED. The PICAXE also send the results to a PC running PLX-DAQ software from Parallax Inc and this program puts the values into an EXCEL spread sheet. After a batch of crystals has been measured, the spreadsheet can be sorted to group those crystals with similar equivalent component values. The software occupies the four slots of the 40X2 and also includes routines for measuring the values of the series capacitors (and the stray capacitance of the associated wiring) that are switched in series with the quartz crystals being tested. The project was published in the November/December 2013 edition of the ARRL’s QEX magazine. The rationale behind the equipment and a more comprehensive description of the operation of the equipment can be found in the article, which may be downloaded from the QEX web site.
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