Testing most passive components is relatively straightforward, but assessing the functionality of a quartz crystal proves challenging with standard measuring instruments. Essentially, a quartz crystal consists of a precisely cut slice of quartz sandwiched between two metal electrodes or coated with metallic contacts to serve the same purpose.
However, due to this construction, traditional instruments like an ohmmeter or capacitance meter are ineffective for measuring a crystal’s properties. A crystal typically exhibits several megohms (MΩ) of resistance and a stray capacitance of a few picofarads (pF), regardless of its operational status. Thus, the only viable method is to incorporate the crystal into a circuit, specifically an oscillator, to observe its oscillation behavior. This is precisely what our tester accomplishes, and remarkably, at an exceptionally low cost.
Since the crystals we handle often span a broad frequency spectrum, typically falling between 1 MHz and 50 MHz, it becomes essential to construct an oscillator capable of operating across this wide range. Transistor T1 takes on this responsibility, functioning as an aperiodic oscillator, meaning it is not specifically tuned to any particular frequency. Those familiar with this type of oscillator circuit may observe the unusually high value of feedback capacitor C1, designed to accommodate crystals within the frequency range of 1 to 50 MHz. Consequently, if the crystal meets the necessary criteria, a pseudosinewave signal corresponding to the crystal’s fundamental frequency will appear at the emitter of T1.
The generated signal is rectified by D2, charging capacitor C4 via D1. When the voltage across C4 reaches a specific threshold, transistor T2 conducts, illuminating the LED in its collector circuit, indicating the crystal’s usability. It’s important to note that due to its operational principle, this circuit doesn’t enable the measurement of the crystal’s exact frequency. However, practical experience indicates that faulty crystals fail to oscillate altogether. When they do oscillate, it’s usually at their designated frequency or one of their harmonics.
For precise frequency measurement, a frequency meter or oscilloscope can be connected across resistor R2. The circuit is straightforward and can be assembled on a dedicated PCB provided in our design  or on prototyping boards like perfboard or Veroboard. Using fiberglass as the base material is crucial due to the high frequencies involved. To connect the crystal for testing, parallel soldering of two HC6/U and HC18/U sockets is recommended, accommodating crystals with these pin-out formats.
Crystals with wire leadouts can be easily connected to either of these sockets. A 9V power source, such as a standard 9V PP3 battery, is suitable, given the circuit’s low power consumption and limited usage duration. As previously explained, this circuit is compatible with crystals ranging from 1 to 50 MHz, covering virtually all market-available crystals. It’s vital to understand that while crystals might be labeled with frequencies above 50 MHz, they rarely operate directly at this frequency. Instead, they typically operate at a harmonic frequency, tuned by the oscillator they are integrated into.
This unique approach is influenced by the manufacturing technology of these devices, necessitating increasingly finer quartz slices as the fundamental frequency rises. Attempting direct oscillation at the fundamental frequency could render the slice too fragile, leading to unintended breakage.