[volt-nuts] External Freq Reference Mod for 3458A?

Poul-Henning Kamp phk at phk.freebsd.dk
Fri Jun 5 15:49:13 EDT 2015

In message <COL130-W405514A623341979406897DFB20 at phx.gbl>, Tom Knox writes:

>Does any of the Volt-Nuts brain trust have any experience adding
>an external Frequency Reference on the 3458A.

The only reference I have seen to this is that NIST modified a HP3458A
this way in order to be able to sample a sine-wave approximation from
a Josephson junction voltage standard after the transitions had settled.

>There is a 20MHz reference on the A/D board but externally clocking
>it has affected linearity collecting data off a 10VDC reference
>that looks a little like a hanging bridge I have seen before between
>GPS and oscillator sync. Is there another clock?  Thanks for any

You want to be 100% sure you don't inject any ground reference into
the A/D board.  Whatever you do, you *must* isolate the signal.

Agilent isolated the serial port between the A/D and the main
processor with 20cm of (plastic) optical fiber, and I think a
significant clue should be taken from that:  Optocouplers and
transformers are unlikely to be good enough.

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.

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