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High frequency filters for s.s.b.

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Technical Editor, QST:

My good friend Herman Shall, W3BWK, has pointed out an error in my October, 1960, QST article.(1) The statement "BT-cut crystals have Co/C ratios around 4000 minimum" is incorrect. Herman tells me that the high ratios I measured on FT-243 surplus crystals result from the nature of their mounting. Actually, BT-cut crystals can exhibit ratios as low as 400, and some of the FT-243s on the surplus market may be AT cut. AT-cut FT-243s may have ratios around 2000, and AT-cut crystals which are plated can have ratios as low as 250. If the ratio is other than indicated in the article (around 4000) then the termination tuning will be slightly different. However, the range of tuning available with the specified coils should be adequate for any surplus crystals available.

The impedance formula on page 39 should read

eq 1

Footnote 9 on that page outlines one way to find the motional capacitance, C. A somewhat better method which I have used consists of connecting a small capacitance, C1, in series with the crystal (see accompanying figure), and

Fig 1

measuring the shift in resonant frequency. The resonant frequency of the crystal alone is

Eq 2

If fr1 is the resonant frequency of the crystal and C1 combined, then

eq 3

Since Eq 4 is much leas than 1,

eq 5

or

Eq 6

where Δf is the difference (fr1 - fr) between the resonant frequencies.

Finally, that long equation from Herzog should read as footnoted below.(2)

The objective of this article was to provide the ham with a design for a good high-frequency s.s.b. filter using surplus crystals. Other designs would be employed if plated crystals were available. The difference is that the filter described can be built at a cost of less than $5.00 and an interesting expenditure of time, if one is an old-time-type ham. Modern types who have not operated with tank coils wound of bell wire on shellacked Mother's Oats boxes might be better advised to purchase a crystal filter from one of the several companies making good ones!

  1. Healey, "High-Frequency Filters For S.S.B." QST, October 1960, p. 35.
  2. Attenuation, dB = Eq 7

D.J. Healey, W3HEC.