[ARC5] Zero Beat Question

[Brian]

Hello folks,

We all seem to be glued to the audio realm.
Our eyes are much better detectors of slow moving artefacts. So, connect a 
CRO across the AF output, but before any iron, such as an AF OPT - the OPT 
will have virtually zero audio transfer at low frequencies, and absolutely 
zero at 0 Hz; for some mil radio equipment, the lower AF cut-off is set 
around 300 Hz. So, connect the CRO to the hot end of the AF volume control 
or the output of the 2nd detector.

If you must use your ears, then set up a stable AF oscillator at a known 
frequency that you can reliably detect, say 800 Hz. Then tune your device to 
be aligned to beat with the 800 Hz AF oscillator. If you don't have a stable 
AF oscillator or a CRO, connect a d'Arsonval meter across the audio output 
at the top of the AF pot or 2nd detector; you are at zero-beat when the 
meter needle stops moving. A DVM is less useful because of internal noise 
artefacts.
Piano tuners, wind instrument and stringed instrument players know this 
technique for accurate tuning. You don't need absolute pitch, just good 
discrimination of relative pitch. I can easily hear within less than 1 Hz 
when tuning my trumpet to 440 Hz, and I can hear within less than 0.1 Hz 
when tuning my piano - I listen for a beat with my tuning device, eg, a 
tuning fork, for at least 13 seconds. A really good piano technician will 
listen for longer. Why the difference between trumpet and piano? I can alter 
the pitch of my trumpet while playing. As luck would have it, presbycusis 
(loss of hearing - mostly with age) affects our hearing of higher 
frequencies, but seldom affects lower frequencies.

There is a problem with using WWV or WWVH: the actually received frequency 
varies because of subtle Earth-bound EM effects - some people call it 
'libration', although this is usually applied to astronomical observations.

As radio amateurs and mil radio buffs, we really need to ask ourselves, "Why 
do we need such high accuracy?" Art Collins used to specify an accuracy of 1 
Hz in 10 MHz for SSB mil equipment.

73 de Brian, VK2GCE.

On Friday, July 08, 2016 12:09 PM , Ken said:

Yes. That is exactly what he meant. I use this method very often to 
zero-beat certain crystal
oscillators with 10 MHz WWV. One or the other of my BC-221s, and a Heathkit 
frequency
counter with a heater in it, for instance. I can fairly easily adjust that 
"waxing and waning" to
very near zero: to much less than 1 CPS in fact....MUCH less.

Of course it doesn't hold too long, but it holds "long enough".

Ken W7EKB