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

[Magnus Danielson]

Hej Ulf,

On 2020-08-26 09:09, Ulf Kylenfall via time-nuts wrote:
> Paul,
>
> A 60 MHz xtal oscillator divided by 12 and comparedwith the incoming 5 MHz. Then there is this thing
> how to acheive a good symmetrical FM injecting
> the 137 Hz signal without affecting the loop filterparameters.
>
> Keeping the original design with modern typesof semiconductors is less work/effort, provided that the
> unit keeps working...
> The A7 original design used a non selective2nd harmonic detector. And uA 709's.That one was easy to motivate an upgrade.
>
> I am a bit lazy otherwise.

OK, If you do a PLL loop of 60 MHz oscillator, well then you can just
inject the 137 Hz into the loop just at the phase detector, assuming
it's above the PLL bandwidth. If you the PLL bandwidth is higher, then
you are on a 6 dB/Oct slope, but sure, for smaller range you can just
compensate with the drive level. This is a known technology to do
phase-modulation. You can be pretty lazy doing it after you gone through
the trouble of doing a PLL-loop.

Using a higher frequency oscillator with low noise can reduce noise
compared to the doubler/trippler chain. One could consider if not an
even higher frequency should be considered.

If one plays that game, one should also consider the strategy of
synthesizing two frequencies that then is combined at 6.8 GHz, as the
noise of the two signals adds. Synthesizing a single frequency would be
preferred, but then again, working within the limits of the current
system might just work well enough. We can improve however by leaving 60
MHz and see if we can find a more suitable higher frequency that would
work equally well, as the need for low noise at higher frequencies has
increased for similar enough purposes.

>
> As mentioned, the semiconductors that have failed
> in A3 were all dated 1968.
> At the time without knowing the exact types of the JFET'sI replaced them with 2N4416 only to discoverthat that was the type HP specified in a laterrevision of the A3.

Good to know. Thanks.

Cheers,
Magnus