[ARC5] Zero Beat Question

[Kenneth G. Gordon]

On 8 Jul 2016 at 12:57, Brian wrote:

> 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.

That is why we use the rise and fall of the antenna or "band" noise: eliminates any issues 
with the audio bandpass.

> So, connect the CRO to the hot end of the AF volume control 
> or the output of the 2nd detector.

Yes. That works.

> 
> 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

and bagpipe 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?"

We don't NEED it, Brian: we simply enjoy having it or hearing it.

> Art Collins used to specify an accuracy of 1 
> Hz in 10 MHz for SSB mil equipment.

Yup. Well, Art was pretty good, but he wasn't God either. ;-)

Ken W7EKB