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

[Leslie Smith]

               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

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