Hi Everyone,

Some recent tests I have made on my HP 3586B Selective Level Meter have
confirmed that the detected audio drifts a lot as a function of ambient
temperature. A day/night change of 1.5 degree Celcius was clearly visible on
Spectrum Lab samples. I could actually find out how many times and when the
central home furnace had cycled in-out overnight. It gave a neat plot which I
post here: https://www.onlinefilefolder.com/3sFcUl8Z8i8zy7

So I have been looking at modifying my HP 3586B SLM so that it
becomes entirely
synchronized off the external 10MHz reference. As you probably know,
all stages
exept the final one, the SSB LO (BFO),  are derived from the 10MHz reference.
The SSB LO chain is made of two free-running crystals used for LSB and USB
detection. In my unit (3586B, option 003), the detection LO frequencies are
13775 Hz and 17475 Hz. These two frequencies are not directly math-related to
any other internal reference (at least I could not personally find any). BTW,
the IF LO rate is 15625 Hz.

I have substituted a function generator instead of the internal
oscillators just
to see how much the off-centering would affect the audio quality,
and found out
that this works well as long as it is not too far off the designed
LO rates. The
closest 10MHz-derived integer rate I could find that will work for audio tone
measurement is divide-by-726 (13774.104... Hz) in LSB. The closest USB rate
would be divide-by-572 (17482.5... Hz), much farther than the LSB one. So the
LSB rate difference of around 0.9 Hz can be neglected when keying in the
frequency and listening to regular radio signals. When making absolute audio
measurements, subtracting that delta is easy to achieve in post processing.
Besides, the frequency and amplitude measurement capabilities of the
instrument
are not altered by this mod. To me this would seem like an
acceptable compromise
that is simple to implement and would add long-term stability. My
intent is to
have a single chip (in this case, an 8-pin PIC divider) do the
trick, as we are
dealing with tTL level signals here.

But can I do better?
How much more complex?
Can I avoid PLLs?
All mixer stages become synchronized to a single source. Is this an issue for
reliable audio detection?
Am I missing something here?

As always, I truly enjoy your feedback and am quite convinced I will learn
something new once more.

Thanks,

Bert, VE2ZAZ

Hi Everyone,

Some recent tests I have made on my HP 3586B Selective Level Meter have
confirmed that the detected audio drifts a lot as a function of ambient
temperature. A day/night change of 1.5 degree Celcius was clearly visible on
Spectrum Lab samples. I could actually find out how many times and when the
central home furnace had cycled in-out overnight. It gave a neat plot which I
post here: https://www.onlinefilefolder.com/3sFcUl8Z8i8zy7

So I have been looking at modifying my HP 3586B SLM so that it
becomes entirely
synchronized off the external 10MHz reference. As you probably know,
all stages
exept the final one, the SSB LO (BFO),  are derived from the 10MHz reference.
The SSB LO chain is made of two free-running crystals used for LSB and USB
detection. In my unit (3586B, option 003), the detection LO frequencies are
13775 Hz and 17475 Hz. These two frequencies are not directly math-related to
any other internal reference (at least I could not personally find any). BTW,
the IF LO rate is 15625 Hz.

I have substituted a function generator instead of the internal
oscillators just
to see how much the off-centering would affect the audio quality,
and found out
that this works well as long as it is not too far off the designed
LO rates. The
closest 10MHz-derived integer rate I could find that will work for audio tone
measurement is divide-by-726 (13774.104... Hz) in LSB. The closest USB rate
would be divide-by-572 (17482.5... Hz), much farther than the LSB one. So the
LSB rate difference of around 0.9 Hz can be neglected when keying in the
frequency and listening to regular radio signals. When making absolute audio
measurements, subtracting that delta is easy to achieve in post processing.
Besides, the frequency and amplitude measurement capabilities of the
instrument
are not altered by this mod. To me this would seem like an
acceptable compromise
that is simple to implement and would add long-term stability. My
intent is to
have a single chip (in this case, an 8-pin PIC divider) do the
trick, as we are
dealing with tTL level signals here.

But can I do better?
How much more complex?
Can I avoid PLLs?
All mixer stages become synchronized to a single source. Is this an issue for
reliable audio detection?
Am I missing something here?

As always, I truly enjoy your feedback and am quite convinced I will learn
something new once more.

Thanks,

Bert, VE2ZAZ