Zero beat

[Kenneth G. Gordon]

On 13 Nov 2007 at 23:51, Tom Buchanan wrote:

> I have a Heathkit SB-101 and can't zero beat it with the calibration
> crystal.  The crystal is oscillating.  If I understand it correctly,
> the audible signal should be a "V" with the zero beat at the bottom,
> but it is more like a " / ".

The "V" occurs if the IF filters are either not working 
properly, the receiver is simply so broad it essentially has no 
filters, or if the BFO is set to the CENTER of the passband 
of the filter.

There is a pictorial in your manual which shows where the 
BFO frequencies should be in relation to the filter passband. 

In a receiver with a properly working crystal or mechanical 
filter, AND with the BFO properly offset to one side or the 
other of the filter, you will hear only ONE SIDE of the "V", or 
the "/", if you like.

In the old days, we used to call this "single-signal 
reception".

>  I have a Heathkit HW-100 and it
> definitely has the "V" signal pattern when I use the 100 KHz
> calibration to zero the dial.

If that is the case, that is, if you are hearing BOTH sides of 
the 100 Khz calibrator signal at equal volume, then the HW-
100 is the one that is not working properly. 

What is most likely wrong with it, is that the BFO crystal(s) 
have drifted into the center of the passband of the filter, OR 
the filter crystals have drifted high or low so that essentially 
the same thing has occurred: i.e., the BFO frequency is no 
longer on the edge of the filter, but somewhere closer to 
center.

Since there are two BFO crystals in any HW/SB 
transceiver, the first question you must answer is does this 
"V" occur on both USB/CW and LSB or only on one or the 
other?

In any case, the SB-101 is working properly. The HW-100 is 
not.

I might add that if the HW-100 is acting as you describe, 
your opposite sideband suppression, both for receiving and 
transmitting, will be essentially non-existent, which is not a 
good thing.

Ken Gordon W7EKB

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