[ARC5] The not-so humble BC-221/LM-xx frequency meters.

[Bruce Long]

Bill
The frequency off WWV and other off the air frequency standards is fundamentally limited by Doppler shift as the height of the ionosphere shifts up and down over the course of the day 

This does not matter for ham radio frequency compliance but it does matter for other applications

Sent from my iPhone

> On Dec 24, 2016, at 12:37 PM, Bill Cromwell <wrcromwell at gmail.com> wrote:
> 
> Hi Les,
> 
> I don't have rubidium or other "atomic" clocks here. I use WWV transmissions as my standard. I'm a radio amateur and thos signals are sufficiently accurate for my purposes and legal obligations (licensing requirements). Why would I want to have two standards? A man who has a clock knows what time it is. A man with two clocks can never be sure - evil grin-.
> 
> The VFO may drift a little over time but we keep going back to the xtal "secondary standard" to keep up with that. It should not take long enough to make your measurement for that VFO to drift 2 kc! If it does then just get in a little more practice <wink>. I check my xtal references against WWV from time to time and I almost never find them even a little off. It takes *years*. If WWV is my standard then my markers and references are "secondary standards". After all these years using WWV as my standard - no pink slips have arrived in my mail box. I use digital frequency counters, LMs, and xtal markers - all traceable back to WWV. These days I see some other people using the "gps" time signals but I think those are controlled by the same people who operate WWV. So where does that leave us except at a much lower frequency?
> 
> In recent years I have made my comparisons to WWV using sound card software. We can identify the WWV carrier on the spectrum display and we can watch the secondary standard as we tweak it creep up one side of the WWV pip, reaching the top at *zero beat*. We can't hear zero Hertz but we can see that display quite easily.
> 
> I am not sure how the ancients did it but zero beating to references or standards is *NOT* new.
> 
> 73,
> 
> Bill  KU8H
> 
>> On 12/24/2016 09:22 AM, Leslie Smith wrote:
>> 
>> >From Ken Gordon:  "I am curious, Les. Why is it that you feel the need
>> to dig into the BC-221? Is something wrong with it, or is it simply your
>> innate curiosity?
>> 
>> Hello Ken,
>> Both and more.  I also have an LM-something that has white "powder" on
>> the tag-terminals and a sad-looking green liquid staining the nice
>> white-ceramic crystal socket.  I want to make it "go".   Perhaps I
>> should say I want to stop the rot.
>> 
>> More generally I'm interested in the specification, accuracy and
>> long-term stability of this type of instrument.
>> 
>> Beyond the set itself, the manual is a cornucopia of useful info for the
>> person willing to read the well set-out manual.  (The same can be said
>> for the ARC-5 document.)  I leaned more about how a good manual should
>> be set out by studying the ARC-5 manaul than by reading a book on how to
>> write a good manual.
>> 
>> The temperature stability for the crystal oscillator (holder, crystal &
>> circuit) in the LM series is given as 0.0001 percent per degree C, with
>> respect to +10C.  In my book that's 1Hz per degree C.  The first (and
>> most obvious question) is "how could "they" measure this, with the gear
>> available in the time before octal tubes became available?"  What was
>> their primary standard?  I have a very common Fluke frequency counter -
>> but can I trust it?   (I have access to a rubidium standard, in case
>> any-one want's to suggest that might be a useful point of reference.)
>> But rubidium standards have only been available for the past few
>> decades.  So one obvious question that the LM/BC-221 sets raise is "What
>> was their primary standard?"
>> 
>> This raises the question of how did they use their standard; what was
>> their method.  For my Fluke, I probe the circuit and read the dial - but
>> it isn't so easy without digital electronics.  Then there is the problem
>> raised by connecting my Fluke to any circuit.  Unless the connection is
>> well buffered the Fluke will "pull" the oscillator.  That isn't a
>> problem with a LM-xx meter.
>> 
>> According to the specs the worst-case drift for the VFO is 0.02 percent
>> (on the higher band).  At 10,000kHz that's an in-accuracy of 2kHz - not
>> too spiffy by todays's standards - but useful of you're making contact
>> with a distant station that must spend minimum time actually on the air.
>> 
>> I have never used WWV to check the accuracy of either the Fluke or the
>> military heterodyne frequency meters.  I want to learn how to do that.
>> 
>> The frequency of the 1000kHz crystals were specified to a close
>> tolerance.  Now they are 70 years old (at a minimum).  How much have
>> they drifted?  If I want to KNOW the REAL long-term drift I must
>> calibrate my Fluke frequency count against some primary stanard - and
>> I've never done that before.
>> 
>> So - all in all Ken, my humble heterodyne frequency meters are
>> instruments of some capability (even today).  That's why I'm interested
>> in them.
>> 
>> Best Christmas greetings to you over there in cold old Moscow!
>> 
>>    73 de Les Smith
>>    vk2bcu at operamail.com
> 
> -- 
> bark less - wag more
> 
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