Re: [IRCA] Yes, SAH!

["Bob Foxworth" <rfoxwor1@xxxxxxxxxxxxxxx>]

>I meant to mention, in the last post, that frequency measurement back
in the '60s was not quite so easy as it is >today. There were a few
DX'ers -- Nelson among them -- who made use of an electronic frequency
>measurement machine that got to the nearest cycle ... I recall its
designation (correct me, Bob, if I'm wrong) >was the BC-221.
>
>Qal R. Mann, Krumudgeon


Yes, it was indeed the BC-221  -unless my memory is totally
out of sync. This was a low frequency linear-tuning oscillator
in a case roughly the size of a soccer ball. It used a precision
vernier tuning knob with a very accurate vernier scale readout.

A legacy of the engineering effort carried forth during WW II.

This device was meant to be linear but in practice they could
never be made that precisely, so the BC221 came with a little
booklet, built into the case lid cover, that showed the slight tuning
variations and inperfections. It was used as a transfer oscillator,
you could tune the 221 to zero-beat the unknown signal, then by
reading the tuning knob scale and applying the correction
factor from the chart book, get a good idea of the real frequency.
Naturally each such booklet was custom-created for that
particular device, using a reference whose type I don't know.

I honestly don't remember how accurately they could be trusted,
but 1 Hz seems reasonable. I remember seeing these being
offered for sale w/o the chart books, no one wanted those.

Today it would not matter as you could use a digital counter to
read the freq. Again, with so few true off-freq signals on MW,
this also is technology of just historical interest. And you can
also now build a direct synthesis oscillator, with better resolution,
stability, and accuracy, with no moving parts, no hard to
replace tubes, and smaller size.

I never had one but I did use a Palomar osc/divider made of
TTL circuitry, using a 100 kHz xtal (the type that the HQ-series
rcvrs used in their calibrators). This one let you select the
divide interval as small as 5 kHz, so by passing the output
through attenuators and coupling into the RX ant input, I could
generate a local signal at any xxx5 frequency, peak the receiver
on it, then switch off the calibrator and wait for the real
station on that xxx5 freq to start fading in. Back then there were
a few dozen such "splits" to do this with, most of them in CA/LA/
Caribbean. Only 535, 555 and 895 remain.

- Bob





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