[time-nuts] Phase measurement of my GPSDO

Thu Apr 16 21:44:07 UTC 2020

An OP37 may  not function well as a diode clamped limiter since the loop gain is <5 during limiting.
A unity gain stable opamp would likely be better.

Bruce
> On 17 April 2020 at 09:26 Bob kb8tq <kb8tq at n1k.org> wrote:
> 
> 
> Hi
> 
> The OP-37 is fine as long as the gain is over 5X. Under that gain level, you 
> need to run an OP-27. The OP-07 is pretty noisy. 
> 
> Limiter circuits have been covered in a lot of detail. One quick read is at:
> 
> http://www.ko4bb.com/getsimple/index.php?id=bruces-zero-crossing-detectors <http://www.ko4bb.com/getsimple/index.php?id=bruces-zero-crossing-detectors>
> 
> That page and the others Bruce did go into a lot of detail on the how and 
> why of this kind of limiter. There are other resources out there as well. 
> 
> So yes, there are circuits that work better for this than others. 
> 
> Bob
> 
> > On Apr 16, 2020, at 3:46 PM, Tobias Pluess <tpluess at ieee.org> wrote:
> > 
> > Hey Bob
> > 
> > awesome, I actually have found some OP-07 and OP-37 in my junkbox! going to
> > build an amplifier now for my mixers. Is it a wise idea to add diodes to
> > prevent the amplifier from clipping?
> > For example I would have done it similar to this
> > 
> > https://www.allaboutcircuits.com/technical-articles/an-op-amp-limiter-how-to-limit-the-amplitude-of-amplified-signals/
> > 
> > but I don't know whether this is a timenuts-grade circuit.
> > 
> > 
> > Tobias
> > 
> > 
> > On Mon., 13 Apr. 2020, 23:53 Bob kb8tq, <kb8tq at n1k.org> wrote:
> > 
> >> Hi
> >> 
> >>> On Apr 13, 2020, at 5:06 PM, Tobias Pluess <tpluess at ieee.org> wrote:
> >>> 
> >>> Hi Bob
> >>> awesome, thanks! of course it is 1e6, not 1e7, I got a mistake :-)
> >>> 
> >>> Maybe I have some good OpAmps for this purpose in my box. I will try it!
> >> of
> >> 
> >> You need something that is quiet (like the OP-37) and has a pretty good
> >> slew
> >> rate. Past that, there are a lot of candidates. The TI OPA-228 family is
> >> one.
> >> 
> >>> course I saw that my setup was not ideal as there was a bit of noise on
> >> the
> >>> signals which I guess does lead to some jitter in the trigger circuit and
> >>> therefore decreases my measurement noise floor.
> >> 
> >> Typically a good limiter takes you from 3 or 4 digits up to 6 or 7 good
> >> digits.
> >> Net result is a measurement that’s good in the vicinity of parts in 10^-13
> >> 
> >>> 
> >>> Can you say something about how it would be done using a TIC?
> >>> I don't have two identically good counters, but the HP 5335A could be
> >> used
> >>> as TIC, couldn't it.
> >> 
> >> The standard way of doing the test is to run two counters / two TIC/s /
> >> two whatever’s.
> >> I know of no practical way to do it with a single 5335.
> >> 
> >>> 
> >>> And the offset source I used is not directly the HP 10811, but the HP
> >> 8663A
> >>> Signal generator internally uses a 10811 as reference source. But I
> >> didn't
> >>> wait for days for it to warm up properly. (Should I?)
> >> 
> >> The 8663 synthesizer adds a *lot* of crud to the 10811. Regardless of how
> >> you
> >> use the 10811, it needs to be on for a while. How long very much depends
> >> on
> >> just how long it’s been off. Best to keep it on all the time.
> >> 
> >>> 
> >>>> Fun !!!
> >>> Yea, of course! :-)
> >>> I already implemented the ADEV, MDEV and TDEV calculations in Matlab by
> >>> myself. I use TimeLab to see what numbers I should expect, and then I
> >> want
> >>> to compute it all myself in Matlab because I want to see how it actually
> >>> works. ;-)
> >> 
> >> Be careful any time you code this stuff for the first time. It’s amazingly
> >> easy
> >> ( = I’ve done it ….) to make minor errors. That’s in no way to suggest that
> >> you should not code it up yourself. I generally do it in Excel or in C.
> >> 
> >> Bob
> >> 
> >>> 
> >>> 
> >>> Best
> >>> Tobias
> >>> 
> >>> 
> >>> 
> >>> On Mon, Apr 13, 2020 at 10:50 PM Bob kb8tq <kb8tq at n1k.org> wrote:
> >>> 
> >>>> Hi
> >>>> 
> >>>> Ok, first the math:
> >>>> 
> >>>> If your offset oscillator is 10 Hz high at 10 MHz, you have a:
> >>>> 
> >>>> 10,000,000 / 10 = 1,000,000 : 1 multiplier in front of the DMTD
> >>>> 
> >>>> You get to add a 6 to what Time Lab shows you.
> >>>> 
> >>>> If you are getting an ADEV at 1 second of 1x10^-4 then that multiplier
> >>>> gets you to 1x10^-10
> >>>> 
> >>>> So, what’s going on?
> >>>> 
> >>>> You can’t feed the mixer outputs straight into a counter. The counter
> >>>> front
> >>>> end does not handle LF audio sine waves very well. You need to do an
> >>>> op-amp based limiter. A pair of OP-37’s in each leg ( or something
> >>>> similar)
> >>>> should do the trick.
> >>>> 
> >>>> Second, the offset source needs to be pretty good. A 10811 tuned high
> >> with
> >>>> both the mechanical trim and the EFC is a pretty good choice to start
> >> out.
> >>>> 
> >>>> If you only have one counter, simply ignore the second channel. You are
> >> now
> >>>> running a single mixer. It still works as a comparison between the
> >> offset
> >>>> oscillator
> >>>> and your DUT.
> >>>> 
> >>>> If you want to do it properly as a DMTD, then you set up two counters.
> >> One
> >>>> to measure mixer A and the other to measure mixer B.  Set them both up
> >> to
> >>>> measure frequency. Time tag the data files so you know which reading
> >>>> matches up with which.
> >>>> 
> >>>> Fun !!!
> >>>> 
> >>>> Bob
> >>>> 
> >>>>> On Apr 13, 2020, at 3:18 PM, Tobias Pluess <tpluess at ieee.org> wrote:
> >>>>> 
> >>>>> Hi again Bob
> >>>>> 
> >>>>> I tried to do some measurements with a DMTD!
> >>>>> In my junk box I found a little PCB from earlier experiments on that
> >>>> topic,
> >>>>> with a power splitter and two SRA-3H mixers, it was even already wired
> >>>> for
> >>>>> the DMTD configuration. So I gave it a try!
> >>>>> As "transfer oscillator" I used my HP 8663A signal generator, and set
> >> it
> >>>>> high in frequency by 10 Hz. To the two mixers, I connected the two
> >> 10MHz
> >>>>> signals and at the mixer outputs, I put a little lowpass filter with
> >>>> 100Hz
> >>>>> corner frequency.
> >>>>> The output signals from the two SRA-3 mixers are almost 0.5Vpp, so I
> >>>> tried
> >>>>> to feed them directly into the HP 5335A TIC and used the TI mode to
> >>>> measure
> >>>>> the delay between the two signals.
> >>>>> This gives 10 readings/sec, which I try to process with TimeLab.
> >>>>> It does give some interesting graphs, but I don't know yet how to
> >>>> correctly
> >>>>> set up TimeLab for this kind of measurement. I.e. now, I get an ADEV in
> >>>> the
> >>>>> order of 1e-4 (at tau=1sec) to 1e-5 (at tau=500sec). So does that mean
> >> I
> >>>>> simply need to multiply this with 1e-7 to get the *real* ADEV at 10MHz?
> >>>>> this would mean that my real ADEV is in the range of 1e-11 to 1e-12,
> >>>> which
> >>>>> is indeed my target value, BUT I expect that things are not that
> >> simple.
> >>>>> (i.e. what if I didn't set the transfer oscillator high by +10Hz but
> >> only
> >>>>> by 9.9Hz for example).
> >>>>> Can you give some hints on that?
> >>>>> Of course I also did the noise floor test (i.e. I fed the 10MHz signal
> >>>> into
> >>>>> a power splitter and connected the two outputs to my DMTD with two
> >>>>> different lenghts of cables. This gave results starting at 1e-4 going
> >>>> down
> >>>>> to 1e-7, maybe it would have gone even lower but I measured only for a
> >>>>> couple of minutes.)
> >>>>> 
> >>>>> Can you give some hints on that?
> >>>>> 
> >>>>> Best
> >>>>> Tobias
> >>>>> HB9FSX
> >>>>> 
> >>>>> 
> >>>>> 
> >>>>> 
> >>>>> On Fri, Apr 3, 2020 at 1:45 PM Bob kb8tq <kb8tq at n1k.org> wrote:
> >>>>>> 
> >>>>>>> Hi
> >>>>>>> 
> >>>>>>> The quick way to do this is with a single mixer. Take something like
> >> an
> >>>>>>> old
> >>>>>>> 10811 and use the coarse tune to set it high in frequency by 5 to 10
> >>>> Hz.
> >>>>>>> 
> >>>>>>> Then feed it into an RPD-1 mixer and pull out the 5 to 10 Hz audio
> >>>> tone.
> >>>>>>> That tone is the *difference* between the 10811 and your device under
> >>>>>>> test.
> >>>>>>> If the DUT moves 1 Hz, the audio tone changes by 1 Hz.
> >>>>>>> 
> >>>>>>> If you measured the 10 MHz on the DUT, that 1 Hz would be a very
> >> small
> >>>>>>> shift
> >>>>>>> ( 0.1 ppm ). At 10 Hz it’s a 10% change. You have “amplified” the
> >>>> change
> >>>>>>> in frequency by the ratio of 10 MHz to 10 Hz ( so a million X
> >> increase
> >>>> ).
> >>>>>>> 
> >>>>>>> *IF* you could tack that on to the ADEV plot of your 5335 ( no, it’s
> >>>> not
> >>>>>>> that
> >>>>>>> simple) your 7x10^-10 at 1 second would become more 7x10^-16 at 1
> >>>>>>> second.
> >>>>>>> 
> >>>>>>> The reason its not quite that simple is that the input circuit on the
> >>>>>>> counter
> >>>>>>> really does not handle a 10 Hz audio tone as well as it handles a 10
> >>>> MHz
> >>>>>>> RF signal. Instead of getting 9 digits a second, you probably will
> >> get
> >>>>>>> three
> >>>>>>> *good* digits a second and another 6 digits of noise.
> >>>>>>> 
> >>>>>>> The good news is that an op amp used as a preamp ( to get you up to
> >>>> maybe
> >>>>>>> 32 V p-p rather than a volt or so) and another op amp or three as
> >>>>>>> limiters will
> >>>>>>> get you up around 6 or 7 good digits. Toss in a cap or two as a high
> >>>> pass
> >>>>>>> and low pass filter ( DC offsets can be a problem ….) and you have a
> >>>>>>> working
> >>>>>>> device that gets into the parts in 10^-13 with your 5335.
> >>>>>>> 
> >>>>>>> It all can be done with point to point wiring. No need for a PCB
> >>>> layout.
> >>>>>>> Be
> >>>>>>> careful that the +/- 18V supplies to the op amp *both* go on and off
> >> at
> >>>>>>> the
> >>>>>>> same time ….
> >>>>>>> 
> >>>>>>> Bob
> >>>>>>> 
> >>>>>> 
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