[ARC5] The not-so humble BC-221/LM-xx frequency meters.
Leslie Smith
vk2bcu at operamail.com
Sat Dec 24 19:44:01 EST 2016
Hello Richard,
Following your lead I found a 16 page document, Instructions of 1100A
Frequency Standards.
Date: (for this specific document) December 1947.
The drawings in the document have been done on linen and reproduced
using an ammonia-printer!! (They are blue!! How's that for
authentic?)
The document mentioned a primary standard (1100AP) and a secondary
standard (1100 AQ).
I take it that "P" designated 'primary' and "Q" just followed "P".
I have seen a lecture by Richard Feynman, where he referred to the
actual words used by Isaac Newton discussing the nature of light.
Feynman was clearly trying to get inside Newton's head. He referred
to words that Newton "invented" (one was "fit" or "fits" - used in
conjunction with light - so "fits of light". Newton believed in the
corpuscular theory of light, (as we all know) and Feynman was trying
to understand how Newton understood these things he called "fits of
light".
The interesting thing about this is that we (modern) types can learn a
great deal by understanding the equipment and methods of the greats from
times past. For example the Greek method for demonstrating that there
are an infinite number of prime numbers is understandable, convincing
and very clever.
You observe that measurement to within a few parts in 10e7 is possible
with care. That's the interesting point.
>From time to time I work on an analytic instrument that can resolve 1/2
Hz at 90.000MHz. Very precise, but not necessarily accurate.
Understanding the entire system well enough to make a measurement with
that degree of precision is interesting.
I don't have a lot of money lying around, but that doesn't mean I
can't get a great deal of enjoyment doing "interesting stuff". (Like
figuring out what was in the mind of the fellow who designed the LM/BC
frequency meters).
Thanks for the idea that I should look to General Radio for info.
Incidently, GR made the LR-1 heterodyne frequency meter - a rather
thoughtful design that uses the same principle as the LM series of
meters - but more precise and easier to use. (And heavier and more
expensive!!!)
73 de Les Smith
vk2bcu at operamail.com
On Sun, Dec 25, 2016, at 09:34, Richard Knoppow wrote:
> To see how precision measurements were done before counters see a
> General Radio catalog from about the early to mid 1950s. GR made primary
> frequency standards at the time. Also available were frequency measuring
> systems consisting of a series of hetrodyne frequency meters which also
> functions as receivers. Indication was via a small CRO. The difference
> between a harmonic of the standard and the signal being measured was
> determined by a precision interpolation oscillator. These systems could
> measure within parts per ten million with some care. A synchronous clock
> was driven by the primary standard though a series of dividers. The
> clock had a contact closure every second which could be compared to the
> one second ticks from WWV or to a tick from the US Naval observatory
> transmissions. These systems were of very high accuracy and precision
> but were immediately outclassed when the first atomic clocks became
> available commercially. Because cesium oscillators have a lot of phase
> noise they were often coupled to a rubidium standard or even a crystal
> standard to act as a filter to get rid of the noise.
>
> On 12/24/2016 1:52 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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>
> --
> Richard Knoppow
> 1oldlens1 at ix.netcom.com
> WB6KBL
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