Zero Beat

[Kenneth G. Gordon]

On 14 Nov 2007 at 14:43, Tom Buchanan wrote:

> Thanks for all the responses on my inquiry regarding the SB101 & Zero
> Beat. 

> The Heathkit HW-100 seems to be the rig that is not working.

Well, at least not as well as the SB-101.

> I also have a Yaesu FT-101E and its calibration crystal does the 
same thing as the SB-101 with the one side on each sideband.

Yup. I have one of those too. They are actually a nice little rig.

>  I am glad I asked first because I could have been working on the 
two rigs that didn't need it.

Yup. If it ain't broke, don't fix it is my motto. :-)

  > I have been doing some experimenting with my vintage rigs and 
the
> calibration markers.  I can hear one side on USB then go to LSB and
> hear the other. I have a 1961 ARRL Handbook and it showed an
> illustration of a V discussing zero beat which brings me to a few more
> questions.  
> 
> I have heard two things on zero beating, one being find the bottom of
> the V to get the rig on the same frequency as the dial. Using the
> HW-100 I found that I was off at least 6 KHz with this approach.

Hmmmmm...sounds to me as though your calibration crystal is not 
zero beat with WWV. There should be a variable capacitor near the 
crystal which will help you adjust the 100 Khz oscillator to be exactly 
on WWV. You may have to wrap a few turns of wire around the 
calibration oscillator tube and connect the end to the antenna end of a 
receiver which will tune to WWV.

> I have also heard that while in CW mode, that to match the CW sidetone
> to the crystal frequency to calibrate the dial frequency?

Well, that is true with the Heathkits, but only if your side-tone 
oscillator is at the correct pitch (the components drift high), and only if 
your BFO/Carrier Oscillator crystals are properly placed in the IF filter 
pass-band. My HW-101's sidetone oscillator is quite high, so I have to 
tune lower.

> Hearing only one side of the V in the SB-101 or FT-101E, do I need to
> listen on LSB then go to USB and find the frequency in between or do I
> listen to the one side of SSB or CW only?

Well, no, the reason being that when you switch from USB to LSB or 
back, there is a bias voltage which causes the LMO to shift slightly. If 
the adjustment for that shift is not exactly on, zero beat on both sides 
will not exactly coincide. In fact, that is the method you use to adjust 
the bias so that both ARE on.

What I do with my HW-101 to calibrate is to turn the volume up a bit 
and listen carefully for the slow "swish-swish-swish" when I get very 
close to the "bottom". Once you hear it, it is easy to find the next time 
you need it. To explain it a bit better, when you get very near to the 
"bottom", the background noise will "mix" with the calibration oscillator 
and alternately get a little louder, then softer. Howvever, you have to 
be very near zero beat, like a few cycles, and you have to tune VERY 
slowly to hear it.

> It would seem that listening
> to one side only and depending on which end of the filter I started
> on, I could essentially be off frequency 2 to 3 KHz.  

Oh, no. Not EVER that far. Your ear can hear sounds down to 100 
CPS or so. If you tune for a lower and lower pitch, and tune for the 
lowest pitch you can hear, you will never be off more than about 100 
CPS. I can get within a few cycles by using the method I told you 
about above.

Besides, the dials in the Heathkits are only really accurate to about 1 
Khz anyway, although it is pretty easy to tune to 1/2 way between 
each mark. That would give you 500 Hz calibration accuracy, if 
everything else is correct.

If you are really off 2 or 3 Khz by doing the above, then the calibrator 
itself is not zero-beat with WWV.

Ken Gordon W7EKB

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