[ARC5] The BC-221 low frequency tank circuit puzzle.

[Brian]

Hello Les,
By my calculations 170 pF and 10.4 mH resonate at 119,695 Hz.
As the BC-221 and its clones all rely on harmonic generation, then 200 kHz 
is only the first harmonic. If the tank were truly resonant, there would be 
no distortion and hence, no harmonics.
But wait, there's more. I thought the BC-221 started at 125 kHz?
73 de Brian, VK2GCE.

On Sunday, February 14, 2016 9:50 PM , you said:


               BC-221 Frequency Meter.

A number of web pages provide information about the BC-221 hetrodyne
frequency meter.  Some provide circuit diagrams - well drawn & with a
readable parts list.  I have one of these diagrams.

Be warned!  Item 15 - the low frequency coil - is shown as 10.4uH - yes
ten point four micro-Henries.  This is an obvious mistake - the coil in
a tank circuit operating at 200 kHz MUST be larger than 10.4uH

Tracking back to an original manual, item 15 - the low frequency tank
coil - is given as 10.4mH - and that value solves the mystery.  Well,
not quite.  175pF and 10,400uH (according to calulation) resonate well
below 200kHz.

Beyond this - the 10.4 uH vs 10.4mH difference nicely illustrates one of
my pet peeves.  This is  a failure to distinguish corretly between unit
designators.   By this I refer to those pesky prefixes - such as "u"
(properly mu, not "u") or 10e-6 and "m" (milli, one one-thousandth),
Mega (x10e6)  and so on.  On the web we often see MegaHertz abbreviated
at mH.  Please!  milliHertz (mHz) and MegaHertz (MHz) differ by a factor
of 1,000,000,000!  I'm certain ARC-5 list-readers would NEVER make the
mistake of confusing MHz with mHz!

For the benefit of non-list readers who are inclined to argue "it
doesn't matter" - it's obvious that mHz means MegaHertz - let me remind
you it DOES matter.  NASA lost a Mars orbiter as a result of confusion
about the correct understanding of units.  Millions of dollars!  Ouch!
Closer to home the same problem arose with Air Canada flight 143, - The
Glimli Glider.  Confusion of units.  I rest my case.   Units of
measurement MUST be expressed accurately.

Back to the BC-221!

Can anyone solve the mystery of how a 170pF and 10,400 microHenry
coil-capacitor combination resonates at 200kHz.
Freq equ sqrt(25330.29/LC) What's going on here?

Les