[ARC5] Zero Beat Question

[D C _Mac_ Macdonald]

Attempting to "true" zero beat with a modern receiver 
or transmitter can be frustrating, for sure. 
 
This new gear has either crystal or digital filtering which 
effectively removes low pitched "sounds" below around 200 or so Hz.  
 
However, a receiver/transceiver with passband tuning is 
capable of restoring sound down to below 60 Hz.  The audio 
circuits in communications gear probably won't go much 
below 25 Hz, though. 
 
As others have noted, as two signals get closer and closer 
to equal, you can hear an increasingly slower wah-wah-wah 
sound. The slower that sound gets, the closer you are to 
true zero beat. 
 
You can frequently hear this at night on an AM broadcast 
radio when two fairly weak distant stations are within just 
a very few Hz of each other. 
 
It's a phenomenon with which we old farts and the primitive 
gear which we used "way back then" are more familiar! 
 
As some others have noted, using skill and stable VFOs, we 
could frequently work the SSB folks (using full carrier AM) 
and not be discovered until an SSBer tuned off frequency 
for some reason or another!  'Twas great fun for sure! 
 
* * * * * * * * * * * 
* 73 - Mac, K2GKK/5 * 
* (Since 30 Nov 53) * 
* Oklahoma City, OK * 
* USAF, Ret'd 61-81 * 
** FAA, Ret'd 94-10 * 
* * * * * * * * * * * 
 
 

 
> To: arc5 at mailman.qth.net
> From: 1oldlens1 at ix.netcom.com
> Date: Sun, 10 Jul 2016 20:36:43 -0700
> Subject: Re: [ARC5] Zero Beat Question
> 
>      I have posted quite a bit on zero beating so this may be too much.
>      I learned the "three oscillator method" from a General Radio 
> instruction book for a heterodyne frequency meter which was part of a 
> frequency standard.  The idea was to beat the unknown signal with a 
> known marker and use a BFO such as that in a communication receiver or a 
> regenerative detector set to just oscillate to give an audio beat that 
> would wax and wane as the unknown beat against the signal from the 
> interpolation oscillator.  It is possible to match two frequencies to 
> within a few Hz per _minute_ if a meter is used to watch the relative 
> strength of the beat. This can be used to set the calibration oscillator 
> in a communications receiver to WWV or some other high accuracy 
> standard. The idea suggested by someone (should look it up) of using 
> background noise is essentially the same thing, the noise taking the 
> place of the third oscillator although it may be harder to hear. I just 
> tried this on a Collins 51J-3 and it works very well.
>      There are probably other tricks.
> 
> -- 
> Richard Knoppow
> 1oldlens1 at ix.netcom.com
> WB6KBL
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