[time-nuts] HP 1938 revisited (Richard (Rick) Karlquist)

[wje]

Must be coupling in via the EFC, then. The phase shift does correlate 
precisely with the 1Hz LED. The EFC is just connected to the center tap 
of a 1meg pot, which is connected to +VREF and EFC shield.
But, this is just for testing so I could trim the 1938 to my reference 
to check drift, of which there isn't a whole lot!

For a real circuit, I'll have to be a bit more careful, now that I know 
this isn't an internal loop artifact. The EFC sensitivity is higher than 
my trusty 105B's.
>
> The AFC loop (as opposed to EFC) is a purely analog loop in which there
> is no mechanism for a 1 Hz oscillation.
>
> Now if you are talking about the EFC, that is another story.  Depending
> on how you are driving the EFC, you can pickup noise from any number
> of sources.
>
> I vaguely remember there was an LED that flashed at 1 Hz if everything
> was working OK.  You might see if that is the source of what you are
> seeing.
>
>
> Rick Karlquist N6RK
> Designer of E1938A
>