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

Sat Dec 24 17:14:42 EST 2016

Hi Les:

***** I'll comment below.
For general information about "frequency meters" and why they were needed see:
http://www.prc68.com/I/USM159.html

-- 
Have Fun,

Brooke Clarke, N6GCE
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
The lesser of evils is still evil.

-------- Original Message --------
>
> >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?"
At this time the Earth's rotation was the definition of time, i.e. 86400 seconds per mean solar day.  This was measured 
by the Naval Observatory using either an astronomical transit telescope or a photographic zenith tube.  The accuracy was 
on the order of a few milliseconds, or parts in ten to the 8 power (0.3 Hz in 10 MHz).
Crystal oscillators were evolving and could maintain their frequency much better than the above time measurement.
>
> This raises the question of how did they use their standard; what was
> their method.

Time interval counting is the most accurate way of making comparisons between frequency standards.  For example HP had 
instruments or you could use an oscilloscope in X-Y mode and use Lissajous curves.
https://en.wikipedia.org/wiki/Lissajous_curve
http://www.prc68.com/I/PTR.shtml - Phase Tracking Receivers
http://www.prc68.com/I/TF_rack.html - the HP 1214 used the audio from WWV as an input for time comparisions
http://www.prc68.com/I/117A.shtml - WWVB frequency comparison receiver uses strip chart recorder to compare phase of 
local standard.  No provision for time recovery other than the operator.
http://www.prc68.com/I/PST1020.shtml - WWV time code receiver

> 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.
There are a number of sources of accurate time & frequency.  WWV is not used today because it's not very good.
At that time WWVB or the VLF Navy RTTY stations were much better and LORAN-C (when it was active).
Here's a list of time-frequency stations and roughly their accuracy:
http://www.prc68.com/I/timefreq.shtml#TT
Note that will all comparison methods the accuracy improves as the square root of the number of observations, so the 
ultimate accuracy cited in the above table can be improved upon by making many measurements over a long time (days or more).
>    
>
> 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.
Currently the best setup is a GPS Disciplined Oscillator (GSPDO). Unlike the lab quality quartz crystal oscillators of 
old that needed annual calibrations the GPSDO is constantly being adjusted so it's frequency is very accurate as long as 
the GPS receiver is locked. When GPS is lost then the long term stability comes into play and so a GPSDO with a Rb 
oscillator fares better.
http://www.prc68.com/I/Z3805A.html
http://www.prc68.com/I/ThunderBolt.shtml
http://www.prc68.com/I/KS-24361.html
http://www.prc68.com/I/O1814.shtml - time standard for GRC-206 for UHF AM Have Quick crypto sync

http://www.prc68.com/I/LTE-LiteGPSDO.html
In 1967 the Earth was abandoned as the definition  of the second and a Cesium clock was the basis of the new 
definition.  But, while there is no drift of a Cesium clock there is a frequency offset that needs to be either adjusted 
out or determined.
http://www.prc68.com/I/5060A.html - early HP Cs
http://www.prc68.com/I/FTS4060.shtml
>
> 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.
You might want to join the Time Nuts mailing list.
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>
> Best Christmas greetings to you over there in cold old Moscow!
>
>     73 de Les Smith
>     vk2bcu at operamail.com
>