[Icom] Re:Zero Beating CW - a bit more info

[Gary Pearce KN4AQ]

So now that we've figured out what to do, where did the name "zero beating" 
come from?

It's based on the fundamental principle of mixing - when you mix two 
signals together, the output of the mixer contains the original signals, 
plus two new ones that are the sum and difference of the originals (and you 
use filters to select which of those results you want to use).  This works 
for all signals, whether they're in audio or radio frequencies (or light, I 
suppose).  And the difference between two radio frequencies can end up as 
an audio frequency.  Two cw radio signals that are 1000 Hz apart make a 
mix, or "beat," at the audio frequency 1000 Hz, and you hear that in your 
speaker.  (Another term for this is heterodyne, but I'll stick to "beat" - 
there are always two or three terms for everything in radio).

So you use that principle to align a transmitter to the same frequency as 
something else - either another signal, or something generated in the 
receiver (a BFO, or "Beat Frequency Oscillator").  As the transmitter and 
the "other" signal come closer together in frequency, the "beat" between 
them gets lower and lower in frequency until it's finally as close as 
practically possible.  If your receiver audio circuit is flat enough and 
your speaker has enough low frequency response, you can hear that beat down 
to maybe 20 Hz.  With a scope or other tuning aid, you can get a lot 
closer.  Right down to "Zero Beat", or close.

Most ham radios roll off low frequencies a lot higher than that, so 
listening for the zero beat in a speaker isn't practical, and most of us 
don't have a scope.  If you're goal is to come to exactly the same 
frequency as a received signal, it's easier to match the pitch of your 
signal to that of the incoming signal.  If you're really careful, you can 
probably come within 10 or 20 Hz just by listening to the difference in 
pitch.  And that's certainly close enough.

But you can come closer if the signal you're listening to stays steadily on 
the air, and you've got your own continuous signal to listen to (the Icom 
radio with the "break-in" turned off will work).  Once again, the mixing 
principle.  Keep turning your VFO, and you're getting closer and closer to 
two signals making their own 600 or so Hz signals in your speaker.  Those 
two audio signals mix right there in the speaker, and as you get really 
close you begin to hear the sum and difference signals.  Keep coming 
closer, and when you get within 10 Hz or so you begin to hear a flutter 
between the signals.  You can't hear the pitch of the beat - it's below the 
response of the speaker.  But you can hear an effect - the signals going in 
and out of phase and getting softer and louder at the rate of the 
difference in frequency.  Keep tuning carefully, and you can hear this 
phenomenon to well below a difference of one Hz, if your circuitry is 
stable enough to maintain it.  You can even count the "beats" or flutters 
in one second to see how far you are off frequency - just a few Hz now or 
it's too fast to count.  Not bad for radios operating at tens or hundreds 
of MHz!

73,
Gary KN4AQ


>At 09:52 AM 7/28/2002 -0700, you wrote:
> >I am just getting started in CW, so I know next to
> >nothing about it.  I am struggling to understand the
> >concept of zero-beating a station and how it works
> >with my 706 MKIIG.  The instructions in the operators
> >manual say nothing about it, so I don't know if I have
> >to do it manually or if the transceiver does it
> >automatically.  All I know is that the transmit
> >frequency is shifted 600Hz from the received
> >frequency, and I am not sure how to perform the
> >zero-beating properly.  Can anyone help me, please?
> >
> >Thanks!
> >Gary
> >WA1TJB
>
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