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

Bob Camp kb8tq at n1k.org
Sat Oct 1 20:58:25 EDT 2016

```Hi

> On Oct 1, 2016, at 8:13 PM, Bob Stewart <bob at evoria.net> wrote:
>
> Hi Bob Albert,
>
> I'm having trouble following it, as well, and I started it.  So, let's back up.  Completely eliminate the use of the term "DMTD".  I didn't post that.

(see way below for why we are confused)

> I'm only interested in measuring phase with the 3456A, which is a voltmeter.
>
> So, I have two disciplined OCXOs, a DBM, an LPF, and a 3456A.  The question is this: "Can I use the 3456A to measure the phase angle (not phase noise) more precisely than the 5370A" under very specific circumstances; i.e. the frequencies are known to be very, very close between the two inputs?  One of the obvious limitations on getting a directly convertible phase output is that the mixer only allows me to directly measure 180 degrees of phase difference, and without some external referent, I won't know which 180 degree spread that is.  So, I suspect that I will need part of the answer from the 5370A (or even a 5335 or 5334).

With one DBM, probably not. With the two DBM’s you started out with below the answer might be a bit different.

If the mixer(s) are saturated, you get a pretty linear phase slope over a limited range. If they are not saturated, you get something more like a a sine wave. Either way, as you move off of zero phase, the sensitivity drops. With an RPD-1 you get about 1 volt per radian if I remember correctly (running into an open circuit). With a 100 ns / 2 pi radians sort of cycle, you have around 16 ns per volt. That all sounds pretty cool. The 5370 at 0.02 ns is only about 1000X better. The 3456 will measure millivolts.

You quickly run into the same noise issues as the limiter in the DMTD or running a counter off the mixer. That plus the TC of the mixer and the non-linearity … lots of excitement.

>  And even then there's the problem of getting the two measuring devices (the DBM and the TIC) to agree on where phase zero is.  But I think I can easily get over the phase zero problem with software, and this will need software in any case.
>
> Bob
>  -----------------------------------------------------------------
> AE6RV.com
>
> GFS GPSDO list:
> groups.yahoo.com/neo/groups/GFS-GPSDOs/info
>
>      From: Bob Albert <bob91343 at yahoo.com>
> To: Discussion of precise time and frequency measurement <time-nuts at febo.com>; Bob Stewart <bob at evoria.net>
> Sent: Saturday, October 1, 2016 6:46 PM
> Subject: Re: [time-nuts] Measuring phase with an HP 3456A?
>
> I am having trouble following this thread.  I assume we are trying to measure phase noise, but of course the result includes the noise of the local oscillator(s).  Isn't the 3456A a voltmeter?  I have one of those.  In AC mode it has a bandwidth of more than 100 kHz and measures true rms.
>
> Someone please take the time and trouble to explain what is being done here.  At this point I am imagining a mixer and a local oscillator and some unknown source.  Quadrature?  How do you accomplish that?  Doesn't that require a 90 degree phase shift?  With an analog phase shifter, or some more modern scheme?  A diagram would be helpful.  I am interested in this partly because I also want to measure phase noise (I can do it with a communications receiver or a deviation meter, both of which I have), and partly because I feel as though I am being left behind technologically speaking and want to keep up better than I have.
>
> Ever since data streams seem to take less bandwidth than that of the signals whose information they contain, I have been thoroughly confused.  My classic textbooks don't have stuff like this.  When someone talks about quadrature, it looks to me as though they are waving hands in the air, saying some incantations, and then come up with miracles.  I know it works but I surely would love to understand it.
> Bob
>
>
>    On Saturday, October 1, 2016 4:24 PM, Bob Camp <kb8tq at n1k.org> wrote:
>
>
> 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

that sure *looks* like the term 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).

that sure says two DBM’s, pretty much a DMTD

>>>  So, as I watch this, I think the obvious question: can this

“this” appears to be a DMTD.

Bob

>>> 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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