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

Sat Dec 24 17:27:22 EST 2016

Hi Les,

Peoples' hearing response varies all over the place. You don't see me 
pooh-poohing the audiophools because I cannot prove they don't hear the 
difference. I do have a lot of doubts. I play music and I have seen 
numbers as low as the 20s in cps and as high as 100 or more. When we 
hear '60 cycle' hum we are supposedly hearing a harmonic of 60. Maybe 
some of us can hear dog whistles, too. I used to interpolate between the 
two points where I lost the note on the low side and on the high side. 
Then the bobbling, wobbling S-meter. Then I got a scope and thought I 
was in hog heaven. Most recently I have been using the soundcard 
software as I described. I am also planning to look into the gps 
signals. I suspect that if I have a gps reference signal and a signal 
"in the wild" I need some means to measure the relationship between the 
wild signal and the reference signal (gps) if I am to tame the wild signal.

The LMs (and BC-221s) can be used to receive the signal to be measured 
or they can transmit a signal the 'unkown' signal can compare. We 
usually have some notion about the frequency to be measured but if we 
really knew we wouldn't have to measure it would we - evil grin. I have 
that RAK and first crack at making the tuning charts I tried to listen 
for the oscillating detector with the LM. Ain't gonna happen. That pair 
of RF amps and all that shielding prevent it. However the RAK can hear 
the LM (pleased grin). Try that with your digital counter! Huh? Doesn't 
compute? My most recent set of graphs were measured with a Heath signal 
generator and a digital counter to capture the data points for the 
spread sheet. That was just to expedite things. I did check the clock in 
the digital counter beforehand. Do I have it down to a couple of 
milliHertz? Bah! Humbug! I don't know but I doubt it.

I suspect the accuracy of WWV - like *everything* - varies and the 
amount of variation varies and that variation varies too. Down the 
rabbit hole we go. Or did somebody flush?

73,

Bill   KU8H

On 12/24/2016 04:52 PM, Leslie Smith wrote:
> Hello Bill,
> As I understand it, a C.R.O. or meter is a useful aid to determining
> zero-beat.  That was the method used in the LR-1 heterodyne frequency
> meter.  (Now there is a REAL frequency meter!)
>
> But a technician must deal with the problem of losing the beats at the
> lowest frequency and interpolate from above and below.  Alternately one
> can get on the output from the detector and use a center reading meter
> or a CRO.
>
>   I read some-where that a tech. type measured the frequency of an
>   "unknown" oscillator (using a heterodyne frequency meter) more
>   accurately than an early HP digital counter.   It was a contest.   I
>   don't have any facts, and I don't know about the supposed accuracy of
>   the HP counter.  I'm certain the suggestion/claim was made, because I
>   thought it was strange that an analog system (such as a BC-221) could
>   be more accurate than a counter and this some-time in the early 70s.)
>
> Maybe connection the HP to the oscillator "pulled" the crystal.  The
> LM/BC-221 don't have that problem.
>
> Anyway, how close can you get to zero-beat before you lose the
> low-frequency beat?
> Another question:  What degree of accuracy is possible with WWV?
> When does the shifting ionosphere (and Mr. Doppler) become a problem.
> These are all interesting aspects of measuring frequency accurately
> using WWV, and I don't know the answers!
>
> 73 de Les Smith
>     vk2bcu at operamail.com
>
> On Sun, Dec 25, 2016, at 04:37, Bill Cromwell 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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-- 
bark less - wag more