I am surprised no-one mention the 3-beat method, which was fairly common for
Hams with comms receivers. You put the BFO on and adjusted so the main slow
beat modulated the level of the output tone. You can judge zero beat to much
better than 0.1Hz that way probably near as low as 0.01Hz. (1E-9 for 10MHz
!!)
Lissajous figures are not ideal either but don't require a comms receiver,
(but do require a 'scope with X-Y facilities :-)) ) a better technique
certainly for fixed frequencies (stand comparison) uses Z-mod on the scope
or a running "toothed wheel" display see Radio Laboratory Handbook by
Scroggie.
Ah and the BC-221......
Alan
G3NYK
----- Original Message -----
From: "Bob Albert via time-nuts" <time-nuts@febo.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts@febo.com>
Sent: Sunday, February 12, 2017 5:00 PM
Subject: Re: [time-nuts] Vintage Frequency Measurement
First you need a standard, a crystal oscillator. If you want serious
precision, you'd have one in an oven. Zero beat that with WWV. Then make a
very stable VFO and calibrate the harmonics against the crystal. Assume
linear calibration on the VFO between check points.
The military LM and BC-221 were very good units. I had one. The check
points in the calibration book were too far apart but there were others
that weren't documented that would make for more precise calibration.
I also built a frequency meter that was amazingly accurate, from a GE Ham
News project printed back in the early 1950s. It used a VFO that went
between 100 kHz and 101 kHz for its full range, adjusted by a micrometer
dial (military surplus). Its harmonics would be zero beat with the
unknown. Using a successive number of harmonics would identify the
harmonic number and the scale could be interpolated to within much less
than 1 kHz over the HF range.
Of course, zero beat was hard to identify so you could use an oscilloscope
lissajous pattern (if you had an oscilloscope, which I didn't). What I did
was turn up the volume and listen to the beat. When it got down near zero
the sound of the AGC surging would tell me the frequency of the beat and I
could adjust to make it stop surging.
When I got my hands on a Beckman counter I was in heaven.
Bob
On Sunday, February 12, 2017 4:01 AM, Neville Michie
<namic...@gmail.com> wrote:
Back in the early sixties I worked in a lab adjusting filters for line
transmission.
We had numerous oscillators, built to be boat anchors, and CROs set up for
X-Y display.
The lab had 100hz, 1kHz, 10kHz standards wired in.
We were expert at recognising lisajou figures. We might have several
oscillators running together,
and we could establish almost any frequency with precision.
Calibting an oscillator would not have been difficult.
Cheers, Neville Michie
On 12 Feb 2017, at 5:08 PM, Scott Stobbe <scott.j.sto...@gmail.com>
wrote:
I was inspired recently coming across a Lampkin 105 frequency meter, as
to
how frequency measurement was done before counters.
Certainly zero-beating a dial calibrated oscillator, would be one
approach.
Is there a standout methodology or instrument predating counters?
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