[ARC5] Zero Beat Question

[Richard Knoppow]

     I think you are forgetting the selectivity of the receiver. Lets 
take one question at a time:
     If you listen to an AM signal with the BFO exactly zero beat with 
the carrier it will sound fine and will eliminate the distortion from 
selective fading.  The technique is known as exalted carrier reception. 
Since getting the BFO exactly zeroed on the carrier is difficult (and it 
won't stay there) you can set the BFO off to one side and listen to the 
AM signal as a single sideband signal. If the IF filter has sharp enough 
skirts it will cut the carrier almost off leaving one of the sidebands 
and you will have essentially an SSB signal. Sometimes you can get good 
reception of a badly fading AM signal this way. Your Kenwood has modern 
filters so it will hear a properly tuned AM signal as though it was SSB, 
again, you can often get better reception of AM this way than with a 
normal diode detector and wide bandwidth.
     The selectivity of the IF is what keeps you from hearing more than 
one harmonic of the calibrator. If the calibrator has a harmonic every 
megahertz the RF selectivity will usually be plenty to insure only one 
harmonic gets through.  Also, the BFO almost certainly has a range of 
only a few kilohertz.
     As far as knowing where the BFO is for calibrating, you must 
calibrate the BFO to make sure its at the nominal center frequency of 
the IF. The handbook will have a procedure for this. Some receivers have 
fixed, crystal controlled BFOs for SSB. If you calibrate the dial with 
one the indicated carrier frequency will be the effective carrier of the 
signal rather than the center of the passband. Often this is desirable 
but you should know what you are getting.
On 7/7/2016 11:45 AM, Robert  Eleazer wrote:
> I have an embarrassingly simple question.  I understand how with a BFO
> in operation you can sometimes tune off to one side of an AM plus
> carrier signal and hear it without the whistle.  I recall listening with
> my Kenwood TS-820S once and hearing a guy running what I think was a
> T-195.  He even said that many people did not know he was transmitting
> straight AM rather than SSB.
>
> But if you have a bunch of harmonics coming from a calibrator, say in 1
> MHZ steps, how do you know you are on frequency, given that the whistle
> is 1 KHZ or so off?  And if you zero beat the signal and adjust the
> tuning pointer, how do you know if you are above or below the signal?
>
> Thanks,
>
> Wayne
> WB5WSV
>
>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL