Actually the three beat method is a variation on the AGC surging method. The
latter, rather than using a pure tone, uses the background noise that comes up
as the AGC makes the receiver gain increase. What you do is listen for the
slow surging and adjust to make it barely stop surging. You can guess the
error by counting the number of surges while watching the clock. What you
can't do so easily is decide which side of zero beat you have, as the
frequencies can cross over and give you the same surge rate.
When calibrating my transceiver I use the three tone method. I listen to WWV
(SSB mode) when it's transmitting a tone, then switch sidebands. If the tone
changes pitch, I readjust until it doesn't. That way the oscillator is
calibrated without concern for dial reading increments. It helps to have a
musical ear, as some people have difficulty in recognizing pitch change.
Further, some older radios retune the radio to the opposite side of the filter,
thus making accurate calibration impossible.
Bob
On Sunday, February 12, 2017 2:00 PM, Bob Stewart <[email protected]> wrote:
In fact, I used a similar method for adjusting the AOR DDS-2A synthesizer on
my Collins rig. I tune an AM radio to WWV. And I tun the DDS-2A / Collins to
the same signal only using SSB. Then when WWV is sending the tone modulated
signal, I tune the DDS-2A calibration control to eliminate the beat note
between the two tones. Then flip to the other sideband and repeat. It's much
better than trying to null two carriers. Even better if you use stereo
headphones with the left on one radio and the right on the other.
Bob
From: "[email protected]" <[email protected]>
To: Discussion of precise time and frequency measurement <[email protected]>;
Bob Albert <[email protected]>
Sent: Sunday, February 12, 2017 2:49 PM
Subject: Re: [time-nuts] Vintage Frequency Measurement
A more accurate way to adjust for zero beat is to tone modulate one of the
signals. The waxing and waning of the tone is easier to discern than for the
background noise.
"Accurate Zero Beating, another perspective.
When trimming an oscillator so it or one of its harmonics zero beats with WWV
or other standard frequency transmission, much comment has been made over the
ability to approach true zero beat. When the harmonic is directly zero beat,
the stated accuracy is generally in the 1-5Hz range. There is a technique that
allows one to repeatedly zero beat to a much higher accuracy. The method is
called the “Three-Oscillator Method” and dates back to the 1930’s, or earlier.
The earliest discussion I have found was on page 47 of Bulletin 10, “Frequency
Measurements at Radio Frequencies,” published by the General Radio Company in
February 1933. The bulletin states that the “method has been in use for a
number of years…” The technique is also presented in sections II and XII of
the 1956 Technical Manual (TM11-2665) for the AN/URM-18 Frequency Calibrator
Set, the military version of the General Radio Type 1100-A Frequency Standard.
More recently, Alan Melia, G3NYK, repo
rts an
accuracy of 0.1 Hertz using the same technique,
http://www.alan.melia.btinternet.co.uk/freqmeas.htm .
The three oscillators are the standard, the unknown, and either another lesser
accuracy oscillator or a receiver BFO. The AN/URM-18 and the General Radio
1100-A frequency standards utilize regenerative receivers. Using reception of
WWV as an example; in normal practice the unknown or a harmonic of the unknown
is adjusted to zero beat with WWV by injecting a sample of the unknown source
into the antenna of an AM receiver tuned to one of the WWV transmissions. As
the unknown is trimmed or adjusted to match WWV, a beat frequency will be heard
that approaches 0 Hz or zero beat with the WWV transmission. Unfortunately,
the audio bandpass of the receiver and the observer’s ear limit hearing a beat
frequency much below ten Hz. It is possible to reach closer beat frequencies
by listening to the background noise wax and wane, but the results are not
readily repeatable. Now, a third source is introduced when the receiver BFO is
turned on or the regenerative receiver is adjusted t
o osci
llate. With the unknown source temporarily disconnected, the receiver is
tuned to give a nominal 1 kHz beat frequency while receiving the WWV
transmission. When the unknown source is once again added, the 1 kHz beat will
wax and wane at a rate equal to the beat between the unknown source and the WWV
transmission. Changing the BFO or receiver tuning only changes the frequency
of the tone that waxes and wanes. The waxing and waning rate is determined
solely by the beat between the WWV transmission and the unknown source. It is
now easy to reliably adjust the unknown, or its harmonic, to within a fraction
of a Hertz of the WWV transmission.
John M. Franke WA4WDL
4500 Ibis Ct.
Portsmouth, VA 23703
[email protected]
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