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

[Leslie Smith]

Well done Brian!
You solved the mystery - that is the "low" range covers 125 to 250kHz;
the figure in my mind (200 to 400kHz) was simply wrong.
(I will write 100 times:  Always read the manual carefully, do not rely
on memory)

 
   73 de Les Smith
   vk2bcu at operamail.com

On Sun, Feb 14, 2016, at 22:44, Brian wrote:
> 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 
> 

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