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, reports 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 to 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] ---- Bob Albert via time-nuts <[email protected]> wrote: > 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 <[email protected]> > 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 <[email protected]> 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? > > _______________________________________________ > > time-nuts mailing list -- [email protected] > > To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
