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

Bob Camp kb8tq at n1k.org
Sat Oct 1 19:24:32 EDT 2016

```HI

DMTD = Dual Mixer Time Difference

Single Mixer = what is commonly used for most things.

If you have a single mixer setup, just put the two inputs in quadrature, attach to a sound card and you have all you need for phase noise.

Bob

> On Oct 1, 2016, at 4:30 PM, Bob Stewart <bob at evoria.net> wrote:
>
> Hi Bob,
> I don't have a DMTD breadboarded up for testing.  This was just a test of the new LPF using only a single Mini Circuits ZLW-1H DBM, and things kind of progressed from looking at the output of the LPF on the scope to "I wonder what I would see on the 3456A?" sort of thing.  I'm running a holdover/recovery test on the code and hardware changes to get a reliable 1PPS from my GPSDO, so there is some very slow movement over the range of 0 to 100ns.
>
> Bob
>
>      From: Bob Camp <kb8tq at n1k.org>
> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com>
> Sent: Saturday, October 1, 2016 3:14 PM
> Subject: Re: [time-nuts] Measuring phase with an HP 3456A?
>
> 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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>
>
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