[Radio Wrangler]

There are so many different parameter sets to choose from. Each set has its own group of blind spots wher physical conditions are hard to arrange when attempts are made to measure them directly, or where measurement errors are exaggerated.
s-parameters are closely related to the concept of a Zo vector network analyser so their suitability is no surpris. One linking figure was Dick Anderson.

HP did an application note on using a VNA to measure quartz crystal parameters. The real aim was to flog VNAs and blow their trumpet. The industry standard way up to that point was to use a 2-port pi-network with switched loading capacitor. Cathodeon made a beautiful one.

This new 1-port measurement had a fatal flaw. Accuracy of phase measurement around series resonance was crucial. In this region the crystal looks like its ESR. 50 Ohms is well within the bounds for many parts, so you want to know the phase of the nulled reflection....oops.

Unfortunately the folk in our standards labs had attacked our nice, gold-plated, Cathodeon mounts and converted them to mere sockets for the network analyser s11 method. The innards had been junked and were probably in the Craigie landfill. Then we started getting parts failing acceptance tests in incoming inspection, then swapping of test parts to-and-from suppliers showed there was a discrepancy and we were the odd ones out. The lead time on a new Cathodeon mount was painful, but necessary.

Theoretically the network analyser 1-port method is all that's needed. But that theoretically over-simplified and glosses over the finite directivity of the coupler involved. The VNA has multiple receivers, so dump the coupler, use a 2-port mount and the isolation of sepatate paths will easily beat the directivity of any realisable coupler.

The blind spot of the new method was centred right on where measurements were going to come out, and the VNA division had the glimmer of sales twinkling before their eyes when they wrote that app note.

We did replace our old sig gen and vector voltmeter with a shiny new 3577A, but strictly in s21 mode!

Oops!

David