[Radio Wrangler]

The industry standard test fixture was made by Cathodeon and looked like a set of three piano keys. Two opened clamp bars for the crystal pins, and the third acted as a shorting switch for a 30pf series capacitor.

Electrically it was a pair of minimum loss pads converting 50Ohms of the outside world to about 12 Ohms applied to the crystal/capacitor.

The rig was used in a 2-port transfer function measurement with a synthesiser and a vector voltmeter, or with a VNA.

Someone in the network analyser division published a paper on using simpler 1-port measurements with s11 on a VNA. Their aim was to flog VNAs to people using old vector voltmeters. As a result of this many Cathodeon fixtures were stripped out and rewired to become posh crystal sockets. Another paper by Neil Wotherspoon in the standards labs analysed sources of errors in the 1-port versus two port approaches. It remained internal, never published outside. It compared results with national standards labs and those of crystal manufacturers. The conclusion was that you were better sticking with the 2-port approach. We bought a new Cathodeon fixture to replace the one which had been gutted.

Quartz crystals are extremely high Q and can resolve and PM-to-AM the phase noise of the source in your VNA or synthesiser. The best unit we found was the 3577A 200MHz VNA or the 8663 family of low noise synths.

This was all pushing things to state-of-the-art limits, but you can learn surprising things while doing so, and learn a few things which can be avoided in day-to-day work without adding to the work.

David