XTO-1 Crystal Test Oscillator

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The intrepidity of the amateur-radio homebrewer knows few bounds. Once the province only of major manufacturers, the making of a crystal bandpass filter for single-sideband generation and reception can be done by a dedicated Ham. The flood of inexpensive “xtals” produced for computers and other digital gadgets makes homebrewed SSB filters quite practical. In addition to a little advanced knowledge–and a good calculator–a few pieces of (also homebrewed) test gear are needed.

Some of the now-ubiquitous nanoVNA instruments can be used, especially for measuring and visualizing the pass-band of a completed filter. For the measuring the characteristics of individual xtals, though, the 101-point scan resolution of a hand-held nanoVNA makes it’s use problematic. If you use the nanoVNA with available computer software, the effective scan resolution can be greatly increased, but still not necessarily to the point of the precision afforded by other means.

So enter, then, the XTO-1. It’ based on the classic xtal test oscillator devised by G3UUR to find the resonant frequency and other important parameters of a batch of crystals. It is used in conjunction with a good frequency counter (with a resolution to 1Hz). With the help of computer software (several pieces of which are free), you’d then use this data to design a filter of a given 3dB bandwidth, shape factor, pass-band ripple, and other important characteristics. For more on the G3UUR method, watch M0NTV’s video, “Crystal Filters for the Fearful.”

The XTO-1 is also useful to determine the series resonance of individual crystals to sort them into narrow batches (with a frequency spread of less than 50Hz) for use in filter designs that do not rely on the motional parameters calculated using the G3UUR method. These alternative designs, such as the quasi-equiripple (QER) variant of the Cohn-type filter, can be built using the resonant frequency (for close matching of xtals) and experimentation with different values of shunt capacitors.

The XTO-1 is available as a kit of PCB and components (including SMA connector) only. Assembly time (including getting a cup of coffee and heating up the soldering iron) is less than one hour.

Schematic Diagram