Zero beat

[Kenneth G. Gordon]

On 16 Nov 2007 at 15:48, Dick KF4NS wrote:

> Ok guys, I am going to show my ignorance here. I understand the theory
> of which you speak but the "V" thingy is leaving me blank. Exactly
> what is this representation?

OK. If your receiver is set up THIS way:

1) BFO CENTERED in the IF passband.

2) Selectivity kinda broad.

THEN, if you start to tune to a steady carrier, as you approach it off 
frequency from one side, the audio pitch of the beat note ( from 
BEAT Frequency Oscillator, or BFO) will be quite high, and as you 
tune closer and closer to the center frequency of the carrier, the 
pitch of the beat-note will get lower and lower until you can no 
longer hear it since at exact "zero beat" (!) there will be no beat 
note, or zero beat. Get it? :-)

Then as you keep tuning in the same direction as you started 
doing, the pitch of the beat note will start out very low and rise and 
keep rising until it gets too high pitched to hear and you will have 
"tuned through" the signal and are now off frequency on the 
OTHER side of "zero beat".

Therefore, the high-pitched ends of the beat-note are the "tops" of 
the "V", and the low (or no) pitched sound is the exact bottom point 
of the "V".

The terminology is quite old, but still accurate.

In a modern receiver with both 1) adequate filters at the IF, and 2) 
the BFO properly set to one side of the filter passband, you will 
only hear ONE SIDE of the signall. As you tune it the beat-note will 
start out very high-pitched, and fall lower and lower in pitch as you 
tune to the center of the signal until it finally gets too low to hear.

But the pitch will not rise on the "other side" as you keep tuning in 
the same direction since the filter cuts it off.

Does my description adequately explain the process to you? If not, 
I can perhaps point you to some graphics which explain it better.

Ken Gordon W7EKB

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