"Percy Picacity" wrote in message
...
In article ,
"gareth" wrote:
"Percy Picacity" wrote in message
...
Once you have tuned the radio (VFO) to get the wanted signal at the
centre of the crystal passband,
There is no passband, it is a single sharp peak.
Sorry that is a passband, unless it is an ideal infinitely narrow
filter, in which case you would not be able to hear the morse characters!
you can set the BFO to taste and
altering the phasing of the crystal will not alter the IF frequency of
the wanted signal so won't alter the beat note.
If the phasing shifts
the crystal pass band significantly (which it probably won't) you might
need to retune the radio (VFO) slightly and then the beat note would
alter so you might have to adjust the BFO to taste again. But the BFO
won't alter where the signal is in the passband, and the crystal
phasing
won't alter the beat note when it is adjusted. Only changing the VFO
could do that.

Sorry OM, but you're way off topic.
A single-Xtal series resonant XTal has a sharp peak, and then a deep null
at
the parallel resonant frequency, and the phasing control adjusts the
position of the null.
My assumption, which is where I came in, is that the BFO would be centred
between
the two frequencies so that an interfering signal at the audio image
frequency would
be nullified.
The crystal nulls the signal that *leads to* the audio image, not the
audio image itself. The only effect of putting the BFO half way between
the wanted and unwanted signal is to give them the same beat note and
therefore make them harder to distinguish. If the BFO is elsewhere they
will have different pitches. But the position of the BFO frequency has
no effect on the the crystal nulling the unwanted signal. If you
actually wanted to null the audio image (or any other audio frequency)
you would need to use DSP.

You're still missing the point that in addition to the peak response, there
is
also a deep null.