[time-nuts] Re: DMTD Questions

[Alex Denner]

Hi Chris,

Wow, that is interesting---we did similar measurements.  I have
included my version of the same measurement.  I just did the
measurement yesterday and I was still thinking about the result as the
additive phase noise is less than I would have guessed it would be.
What do you think, is one of our versions approximately correct?  My
additive plot is somewhat similar to the plot that starts lower in
your graphs (the one you identified as the 8663a.)  First, as is
typical with phase noise plots, one has to use a little bit of poetic
license to compare them.  Also, I am using a different reference which
obviously makes a difference.  (I can try doing the measurement with a
10811A if it helps you.)  My reference used is an Endrun ultra-stable
LPN oscillator disciplined by GPS.  I've also included a phase noise
plot of the reference itself. Interestingly, it is pretty much on
spec. (The Allan deviation is excellent but it doesn't usually quite
get to the specs listed.)

As you know, the design of a DDS is such that it cannot output most
frequencies EXACTLY...ie, because of the divisions and multiplications
it does it can't get exactly to any given frequency.  So, when using
it to adjust between frequencies that are very close together, the
traditional DDS design may not perform very well--frequency resolution
is basically =(clock speed)/(phase accumulator size).  As you probably
also know, the Trueform in the 33522B has a modified design which I
have not ever seen fully described but as I understand it, Trueform
uses something like a 64 bit phase accumulator with an interpolation
scheme that allows the waveform generator to vary to more closely
match the desired frequency.  Perhaps you or someone on this list can
explain in detail how this works....I don't understand it.

Another question: is it better to use a 33522B or 33622a for 10MHz
offset purposes?  The latter has a higher clock speed (1G vs 250 M)?
and better jitter specs (1ps) but have been told that it might have
less precise frequency resolution at 10MHz (meaning the residue after
the least significant digit of microhertz)..  Does anyone know the
details about these generators?

Of course, measuring the effects of tiny offsets are hard to assess
and measure bc of the long time scales needed to resolve say 10
microhertz.

I have been trying out the Keysight 5047A phase noise analyzer....in
my opinion it is a great instrument that is much more flexible than it
seems.  It can do a lot of different types of measurements.  The noise
floor isn't as good as some of the more expensive phase noise
analyzers but it is quite flexible and has a number of functions that
aren't advertised.

With all that in mind, I am trying to see if there are phase noise
differences from small changes in frequency right now.  I am also
testing out a bunch of other instruments including the SRS CG792 clock
generator which uses a different frequency generation technique which
is described clearly in its manual.  I find it to be very good in
early testing.  (Note as a clock generator it outputs a square wave.)
There are other approaches such as the SRS SG384 which uses RAFS to
set frequency exactly to anything in its range within exactly 1
microhertz. I don't have a Spectradynamics EOG (frequency resolution
of 10^-20 over a small range) but would like to test one. ;-)

Alex




On Sat, Jul 11, 2026 at 10:33 AM Chris Hastreiter <chastreiter at gmail.com> wrote:
>
> Alex,
>
> The "additive" measurement method you're describing sounds exactly like what I used to measure the residual noise of my synthesizers.  In the case of my 8663A I used its internal reference but measured the phase noise of its output referenced to the "ref out" connector on the back.  In the case of my 33250A I used an external 10811A as the reference.  Here are my results, it looks like the 33522B is on a similar order of magnitude. (the 8663A is the lower noise trace on my plot)
>
> I've also included the absolute noise measurement plots for comparison.
>
> Chris
>
> On Thu, Jul 9, 2026 at 4:50 PM Alex Denner <alex.denner at gmail.com> wrote:
>>
>> By coincidence, I happen to be looking at one small part of your
>> project.  I was curious about using a DDS to do frequency offsetting
>> for another purpose.  In these graphs, I used the Keysight 5047A (the
>> AnaPico PN analyzer) cross-correlation phase noise analyzer.  I looked
>> at the phase noise of a free running Keysight 33522B vs the same unit
>> with an external reference.  Below are the two phase noise curves from
>> 0.1 Hz to 5 MHz (10 MHz carrier).  One can see the free running 33522B
>> adds a decent amount of jitter if using it for a synthesized offset
>> generator.  I was estimating jitter by the difference in the two
>> integrals. (I think the graphs have 100 correlations with some spur
>> removal and averaging, I could try to find the details if anyone
>> cares.).
>>
>> However, I thought of an interesting experiment: I could run an
>> "additive" phase noise plot of the 33522B where one channel is the
>> external reference and the second channel is the 33522B's (locked to
>> external reference) output where the output is set to be in quadrature
>> to the external reference entering the 5047A.  It is not really an
>> additive phase noise measurement in the classic sense but it may
>> measure the total phase noise contribution of the DDS synthesis chain.
>>
>> As you know, the unreferenced HP 8663a with the OCXO is quite a bit
>> better---probably some combination of the OCXO, better synthesis
>> chain, and better power supplies, etc.  I will also include a phase
>> noise plot of my 8663a from 0.1 Hz to 5 Mhz at a 10 MHz carrier.
>>
>> Alex
>>
>>
>> On Thu, Jul 9, 2026 at 4:42 AM Chris Hastreiter via time-nuts
>> <time-nuts at lists.febo.com> wrote:
>> >
>> > Time-nuts,
>> >
>> > I have been working on some modifications to Corby’s DMTD design [1] in an
>> > effort to further improve its noise floor and had a couple questions for
>> > the group.  The performance I’ve been able to achieve so far is shown in
>> > the below plot with the blue trace being the noise of the ProTIC counter
>> > itself, the pink trace being the combined noise of the counter and my zero
>> > crossing detector (both channels driven by a 10Hz sine wave), and the
>> > remaining traces the full system noise floors with 4 different offset
>> > oscillator options.  All noise floor measurements were taken with a carrier
>> > frequency of 10MHz and an offset of 10Hz.  I believe that much of the noise
>> > floor degradation at long tau is due to thermal effects in the first stage
>> > amplifier of the ZCD as I haven’t yet gotten the system installed into an
>> > enclosure and I can worsen the issue by thermally disturbing the LT1028 of
>> > the first stage.
>> >
>> > [image: image.png]
>> >
>> > My first question is in relation to mixer noise which is a noise source
>> > that I have not yet considered.  I am using the same SBL-1 mixer as Corby’s
>> > original design but [2] implies that some mixers exhibit lower flicker
>> > phase modulation than others and I haven’t seen any literature or
>> > specifications that point to any devices other than the 10514A in that
>> > paper.  They also discuss adding capacitive loading to the mixer to further
>> > reduce noise referencing another paper that I have yet to be able to find.
>> > Is there any expectation that changing to a mixer different than the SBL-1
>> > will improve my results and should I consider capacitive loading on the
>> > mixer?  If so, how would I determine an appropriate amount of capacitive
>> > loading?  I currently have it loaded with a bridged-T filter holding 50
>> > Ohms of impedance across the frequency spectrum.
>> >
>> >
>> >
>> > My second question is in regard to the common oscillator noise
>> > contribution, specifically for the use case of a synthesized offset
>> > oscillator.  The below plot shows the results I’ve gotten so far with some
>> > sources that I already have, and as can be seen the system noise floor
>> > degrades much faster with the synthesized offset than with the 10811A.
>> > However, it would be advantageous for me to be able to use a synthesized
>> > offset for tests where the DUTs differ slightly in frequency and the system
>> > will phase wrap.  The reason for this is that when using a free-running
>> > oscillator the phase wraps are very difficult to remove automatically
>> > because the divided phase wrap will differ from the expected with any
>> > frequency error in the offset oscillator.  Do you have any suggestions for
>> > synthesizers or other offset techniques that would help with this?  Using a
>> > 10811A and a slow PLL as a cleanup loop on the synthesizer does appear to
>> > work well in the below plot, but looking at the first plot in this email at
>> > least in my initial implementation there seems to be some low frequency
>> > oscillation in the PLL impacting the noise floor.  I’ve also included a
>> > plot showing measurements I’ve taken of the residual noise for the two
>> > synthesizers used as well as a comparison of two of the 10811A oscillators
>> > that I’m using.
>> >
>> > [image: image.png]
>> >
>> > [image: image.png]
>> >
>> > [1] https://www.eevblog.com/forum/metrology/dmtd-board/
>> >
>> > [2] https://ieeexplore.ieee.org/document/5075023
>> >
>> >
>> >
>> > Thanks,
>> >
>> >
>> >
>> > Chris
>> > _______________________________________________
>> > time-nuts mailing list -- time-nuts at lists.febo.com
>> > To unsubscribe send an email to time-nuts-leave at lists.febo.com
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