Help - Zero Beat

[Al Waschka]

You are beating two signals.  They are the incoming signal, reduced to some IF, and the VFO.  Their product is typicall another frequency which is at another IF.  When you tune two audio frequencies close to each other, you hear the sum of the two.  When they are very close to each other, they add and subtract.  If you could adjust them so they are exactly the same frequency and level but 180 deg out of phase, they
would cancel each other and you would hear nothing.

But instead of two frequencies which are both in the same general range, you are beating the incoming signal with a VFO/BFO in a mixer and the product comes out at an IF (which is typically multiplied by a BFO in the product (get it? PRODUCT) detector).  As that product comes into the audio range, you hear a high pitched tone.  As you continue to tune, that tone decreases in frequency until it passes through zero and
goes back up.  This is true in all cases. The thing that makes it sound different is the IF filter.  Set your receiver to USB and tune through the signal, then do the same thing for LSB.  Notice that you are hearing different sides of the same process.  Now, listen VERY carefully on the other side of "zero beat".  You can still hear the signal, it is reduced in volume by the selectivity of the filter.  In receivers with
wider filters you can hear both sides.

Hope this helps.





James E Temple wrote:

> Hello,
>
> Adding to Bob's question, I have had the same thoughts.
>
> Some signals will go from a tone, to silence (zero beat), to the tone
> again as the dial is moved in one direction.  With a cal tone, the
> sequence seems to be silence, to a tone, to silence as the dial is
> moved.  The last example is where I am uncertain where zero beat is, since
> there is no silence (null) in the tone between tones.
>
> Thanks for any help and advice.
>
> Sincerely,
>
> Jim
> 73, KF4ICZ
>
> On Fri, 24 Mar 2000, Bob Ahrens wrote:
>
> > Group,
> >
> >     Can someone please explain the idea of zero beating a receiver to a known signal?
> >
> >     I know what zero beat means when it refers to two audio signals... simply meaning that two signals are at exactly the same freq., eliminating the beating effect. However, I am trying to align an HW-101 and there are constant references to zero beating the receiver to either the calibration signal or a known frequency. Since there is only one signal, say the 3.7 MHz cal signal, how do I zero beat the receiver??
> >
> > Any and all help is greatly appreciated.
> >
> > 73
> > KC2FYG
> > Bob
> >
> >
>
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