# [time-nuts] Measuring phase with an HP 3456A?

Bob Camp kb8tq at n1k.org
Sat Oct 1 16:14:15 EDT 2016

```Hi

What is the beat note coming out of the DMTD?

Put another way:

DMTD involves three oscillators. Two are on roughly the same frequency and the third is
offset from the other two. The difference frequency is typically something like 10 Hz.
It does not *have* to be 10 Hz, but that is one way to do it.

So, moving on using 10 Hz (which may be wrong):

If you are at (say) 10 Hz, you get a 1x10^6 “error multiplication” on the output. One cycle
at 10 MHz gives you one cycle at 10 Hz. The one cycle is 10% of 10 MHz, it’s 0.1 ppm
of 10 MHz. You get a 10 degree phase change at 10 Hz for each 10 degree phase change
at 10 MHz.

The 10 Hz offset limits your phase noise process. The upper (or lower) sideband wraps around
at 10 Hz and then starts dumping back into the other sideband’s data. You also need to have a
signal processing chain that will tolerate the carrier being “in band”. Between the two … not
such a great way to do it.

It’s *much* easier to simply hook up a single mixer (half of what you have already) and look at
the two sources in quadrature. Then the sidebands line up. The carrier is gone. The dynamic
range can be *much* less.

Bob

> On Oct 1, 2016, at 2:32 PM, Bob Stewart <bob at evoria.net> wrote:
>
> I've been spending a small amount of my time looking into making a sort of hybrid DMTD with a pair of DBMs up front feeding the stereo input to a sound card.  So, I got the 100KHz LPF back from Oshpark and hooked it up to my scope for verification - an obvious step.  Then I hooked it up to my 3456A just for grins.  (The two DBM inputs are 10MHz outputs from two different GPSDOs).  So, as I watch this, I think the obvious question: can this measure phase angle better than the 5370A?  I guess I need to send it through a full 100ns of phase change to get a calibration value.  So, who's been down this road and what did you discover?
> Bob
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```