[time-nuts] Dead 5065A - Some comments - Some questions... (III)

[Bob kb8tq]

Hi

> On Aug 27, 2020, at 10:35 AM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> 
> --------
> Bob kb8tq writes:
> 
>> Common wisdom is that there is a filter in the cavity match that
>> is specific to 60 MHz.  Putting in 120 MHz apparently does not work
>> well. I haven’t tried it so I’m just passing on what I’ve heard.
> 
> Driving step-diodes is a bit of an art, and the relationship between
> driving frequency, AC power and DC bias is very far from trivial,
> as the "real" parameter, if I have understood it right, is the
> slew-rate through the "magic" voltage.
> 
>> The “other” signal (normally 5.3x MHz) is mixed with the multiplied 60 MHz.
> 
> As far as I have been able to make out, both the 137Hz and 5.3x MHz
> are "mixed" through their amplitude modulation of the DC-bias.

At least in the 5065, the modulation signal feeds into the (A3) multiplier board and
hits a phase modulator running at 5 MHz. It’s that phase modulator that generates
the PM on the output. 

In my manual the stage is labeled “10 MHz Multiplier and Phase Mod”. The dirty work 
is done by CR3,4,5, and 6. They are driven by the “mod level” pot. 

Since the modulator is early in the string of stages. I doubt that a lot of AM is *supposed*
to get through to the output. 

Bob

> 
> My Personal DDS plan is to simply drive the step diode at 59.953... MHz
> +/- the modulation pattern, and forget entirely about the 5.3x MHz and
> 137 Hz.
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by incompetence.