[Elecraft] To PLL mod or not

[Ron D'Eau Claire]

We're taking bits and pieces out of previous messages. No wonder it's
getting confusing.

I define "side tone" as the locally-generated audio frequency that you
hear whenever you press the key to transmit CW. It is not affected by
drift of either the LO or the BFO. It is affected only by what you chose
in the St P command from the menu. 

What I call the LO is using classic superheterodyne receiver
terminology. It is the "local oscillator": the oscillator that
heterodynes the incoming signal to the i-f. In the K2 it is what is
often called the "PLL" because the K2 happens to use a phase-locked loop
to generate the LO signal. 

Finally, the BFO or "beat-frequency oscillator" is the oscillator that
heterodynes the i-f to audio. It runs close to the i-f frequency.  The
BFO frequency is what determines the audio frequency we hear in the
speaker when a CW signal is centered in the i-f bandpass. It is set up
using CAL FIL so that we hear a beat frequency equal to the sidetone
when a signal is centered in the i-f bandpass. That means that the BFO
frequency must be changed with different filter bandwidths, which is the
whole point of running CAL FIL. In SSB reception the BFO frequency
determines how the SSB signal will line up with the i-f filter passband
to produce the most pleasing audio quality. But let's stay with CW for
this discussion.

My original point was that while drift in either the LO or BFO require
retuning to put the received signal beat frequency back zero beat with
the sidetone frequency, the effects of LO and BFO drift are different
and easy to identify.  My apologies for not reposting the whole message
again with the replies. It has caused confusion. Here's what I was
saying:

IF the signal is tuned in 'properly', that is, so the CW note is zero
beat with the sidetone AND the LO drifts in frequency (which, of course
means that you must re-tune slightly to adjust for the change in pitch
and put it back zero beat with the sidetone frequency) there will be NO
change in the position of the signal in the filter bandpass. No matter
how narrow the i-f filter bandpass is set and not matter how badly the
LO might drift, when a signal is tuned in so it is zero beat with the
sidetone frequency, it will still be centered in the i-f filter
bandpass. 

IF the BFO drifts, the pitch of the received signal will change. Again,
it can be placed back to "zero beat" with the sidetone frequency by
adjusting the tuning control, but now the signal will have shifted in
relationship to the i-f filter bandpass. If the BFO drifts far enough,
the signal may move completely out of the i-f filter bandpass.

Sorry for the confusion. 

And, yes, you are absolutely right. If the BFO frequency drifts we can
adjust the receiver tuning to put the signal back in the center of the
i-f bandpass, but now we will hear a different beat frequency in the
speaker. The beat frequency will no longer be zero beat with the
sidetone frequency. But that's not an acceptable procedure because it
will move the transmitter frequency since the same oscillators are used
for both, unless we are using SPLIT mode to give us two independent
LO's: one for receive and one for transmit. That's WHY we zero beat the
incoming signal to the sidetone frequency. It puts the K2 transmitter
frequency exactly on the received frequency. So the first step in tuning
in a signal is always to zero beat the incoming signal to the sidetone
frequency. 

Ron AC7AC
K2 # 1289

-----Original Message-----
Hi,


> A drift in local oscillator frequency is the same as changing the main

> tuning dial. It won't change the relationship of the signal to the 
> filter bandpass.

Altering the main tuning, either by intent or VCO drift, will move a
signal within the passband of the filter, it has to, how could one move
on to another signal otherwise.

> Changing the BFO should be the only way to change the position of the 
> signal in the filter passband, if I understand how the thing works
that
> is...

With respect to zero beat or your desired sidetone.  VCO drift will also
alter the side tone frequency, as in normal tuning, but not the
relationship between the BFO and the filter.

We are dealing with two separate issues here, hence the need for two
cures.

1.  To remove the necessity of retuning to keep the signal within a
narrow filter. 2.  To remove the problem of BFO drift causing a change
in side tone frequency whilst keeping a signal within a narrow filter.

It has to be looked at both ways, whatever effects receive with also
effect transmit, that is the transmit signal will also drift.

This morning at switch on I tuned onto a birdie just above 28MHz (150Hz
filter), probably 7th harmonic of the KPA100 bus clock.  After 45
minutes I had to retune up 210Hz.  Interesting but useless 'cos it
introduced a third variable.

Regards,

Mike VP8NO
#1400

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