Each BC221 frequency meter was supplied with its own custom prepared, calibration book. The method used to generate the book is most surprising because it was done by computer.

An automatic calibration computer using 126 tubes was developed by engineers at the Philco Corporation, Research Division, Engineering Department to handle this task.  The BC-221 was calibrated by noting the dial reading for internal heterodyne beats, and calculating how many cycles per dial division from the previous calibration point. The computer consisted of an automatic calibrator combined with an adding machine (semi-automatic) which recorded the calibration data at 327 points, interpolated between those points, and automatically printed the 3252 different frequency value numbers in each individual calibration book.

The time to do all this was 6.5 hours, the actual printing of the values in the book taking just 16 minutes.  The time for an experienced human to do the work manually, in contrast, was averaged at 16 hours per frequency meter. A mechanical hand automatically turned the dial of the BC-221 to various pre-determined settings while measurements were underway.  The calculations were carried out by automated complex adding machines of the type then used by banks and finance companies, and the results printed in the calibration book and also saved on paper tape.  The adding machines were fitted with solenoids to depress the keys; the tubes the "computer" used were mainly 0A4G's.

Presumably Philco shared this technology with Zenith and the other makers of the frequency meter.  It is believed that the original production runs of the BC-221 were calibrated manually, and that the computer system was a mid-war development, in operation by at least mid-1942.  The production of model LM naval frequency meters also incorporated this computer system.

An in-depth article about this system including photos, partial schematics, integration formulae and much more can be found in the magazine 'Electronics'  for May, 1944 at page 98.  It was surprising that such information would have been made public during the War, but evidently it was decided that either the enemy already knew about this process by 1944 or else it was too late for them to make beneficial use of it.  To the best recollections,  no other nation at the time produced a micrometer frequency meter with the accuracy or quality of the BC-221, a wonderful piece of equipment which seldom receives much attention from collectors when compared radios. It must have certainly impressed the Russians because they copied the design and mass produced it for their own needs.

 In the 1960's, frequency meters without the calibration book and internal crystal were sold for as little as $5.00, versus $50 to $125 for complete units, because they were thought useless.  The QST article "Calibrating the LM Frequency Meter," shows how calibration is done, and generating calibration for the ham bands is not difficult or too time-consuming.  Generating the whole book would take some time, and be pointless for most people.


1.  "Notes on the BC-221," Herbert W. Gordon, W1KWB/W1IBY, CQ Magazine, August 1962, pages 52-56.
2.  "Calibrating the LM Frequency Meter," Gilbert L. Countryman, W4JA, QST, April 1965, pages 18-20.
3.  "Frequency Measurement with the LM/BC-221," Kenneth N. Sapp, W4AWY, QST, September 1965, pages 28-31.
4.  "Solid-State BC-221 Frequency Meter," R.S.N. Rau, VU2CX, QST, February 1977, pages 35,36.

My thanks are extended to  Geoff Fors WB6NVH,  Monterey California
and  Richard Brunner, AA1P, Foxboro, Massachusetts for providing the material for this story.

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