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To The Editor, TRE Journal
Sir,
I should like to draw attention to a point made in Mr. Wootton's article on Crystal Valves in the July TRE Journal.
Under the heading "Crystals for use as low level detectors", he states that they have a square law characteristic up to 10 to 20 microamps. This is perhaps true for S and X band crystals to which of course the article mainly refers, but it is not true for K band crystals. The deviation may begin at 2 microamps and at 4 microamps it may be 25% in R.F. field voltage. Moreover the K band crystal is extremely sensitive to electrical and mechanical shock which may cause large variations in the crystal law.
Thus for accurate measurements it is essential to calibrate the crystal, and thereafter treat it with profound respect. In this way consistent results of attenuation have been obtained (to 3% or 4%) by the writer over longish periods. The method of calibration and other precautions are similar to those of Mr. Birks' report T.1495. ("Crystals as 9 cm. Standing Wave Detectors".)
P.H.T. Brook