Bob et Stéphane,

The "Collins hard limiter" is a fancy squarer circuit.

In short, the Collins hard limiter is what I hinted about in my earlier message. Rather than hitting the hard limiter of the comparator directly, with the slew-rate issue I gave.

Collins observed that if you provide a linear amplifier (with limiting), you can increase the slew-rate of the signal. At the same time you will add noise from the amplifier. Now, to limit the noise going forward, you bandwidth limit the amplifier, which is fine as long as it supports the outgoing slew-rate of that amp. This helps you a bit and many times a single linear stage suffice to make the trigger noise sufficiently low, but then you can cascade these, and for many research DMTD setups has some hand-hacked variant. Collins then made a systematic approach to optimize the bandwidth and gain for a number of stages. Collins did however not think about amplifiers with different noise-contributions, but Bruce Griffiths have then generalized it further. This makes sense, since for higher slew-rates, you need faster amps, but their noise-contribution will be fairly low considering the high slew-rate coming into it.

The original Collins article is not freely available. Therefore I recommend you to dip your nose into Bruce pages:

http://www.ko4bb.com/~bruce/

Zero Crossing Detectors and Collins:
http://www.ko4bb.com/~bruce/ZeroCrossingDetectors.html

paper:
http://www.ko4bb.com/~bruce/GeneralisedCollinsHardLimiterPaperV3B.pdf

So, "squaring up" with 74AC04 or comparator and similar stuff might be an interesting exercise, but you end up doing the same exercise as going straight into the counters comparator. Try to amplify yourself out of the situation, try different gain settings. Make the habit of measuring the slew-rate (at the trigger point).

It's interesting to note that not many counter support the feature of measuring slew-rate directly, the closest one usually get is to get the rise-time, but it needs scaling with the difference of the start and stop trigger-points, and you want them not at 10%-90% but closer.

Cheers,
Magnus

On 01/24/2015 11:36 PM, Bob Camp wrote:
Hi

If you go back in the archives and look for the discussions on “Collins hard 
limiter” they will lead you to some other areas to consider when trying to 
square low frequency sine wave signals. The quick summary is that you need a 
series of bandwidth limited limiter stages ahead of what ever you use as a 
squaring circuit. There are a number of ways to make these stages, each with 
their benefits and drawbacks. The limiting process has a much larger impact on 
the results than the choice of squaring circuit.

Bob

On Jan 24, 2015, at 3:07 PM, Stéphane Rey <steph....@wanadoo.fr> wrote:

Hi guys.

After several experiments I could discover that the "bad" ADEV from the two 
GPSDO DUT are due to GPS lock losses. This is probably because the antenna is outside the 
windows but half the sky is hidden. We can see the on the frequency plot the sharp change 
of 0.5Hz and the locking. Good point.

I'm now trying to evaluate various architectures of 2-channels squarers and a 
DMDT. For that I'm designing a PCB with 4 squarers : simple 74ac04 gate biased 
at VCC/2, a LT1016 comparator, the transistor based differential amplifier from 
Winzel and the one from Charles. I will add two balanced mixers (minicircuits), 
IF filters and amplifiers.
Does anyone has an idea of what I could add for this evaluation ?

Cheers
Stephane


-----Message d'origine-----
De : time-nuts [mailto:time-nuts-boun...@febo.com] De la part de Stéphane Rey
Envoyé : mardi 20 janvier 2015 23:15
À : 'Discussion of precise time and frequency measurement'
Objet : Re: [time-nuts] question Alan deviation measured with Timelab and 
counters

Hi,

Following the tests results in the previous email, today I've performed 
additional measurements showing that the repeatability of the GPSDO DUT is not 
great but is coming from the design. I've tested several over sources and 
repeatability is correct.

I can already make some measurement. Good !

Now I'd like to improve. First I'm going to implement a squarer and then I will 
work on the DMTD... I'm thinking to make a setup on the table, and possibly 
make a small PCB then.

Any comment for the tests results of yesterday here under ?

Cheers
Stephane


-----Message d'origine-----
De : time-nuts [mailto:time-nuts-boun...@febo.com] De la part de Stéphane Rey 
Envoyé : lundi 19 janvier 2015 22:32 À : 'Discussion of precise time and 
frequency measurement'
Objet : Re: [time-nuts] question Alan deviation measured with Timelab and 
counters

Hi

Here are the results of today's experiments. plots and TIM files attached to 
this email.


Setup #1 : dark blue
I've done again the floor measurement with same conditions : HP58503 for 10 MHz 
Standard, 1PPS for the EXT gating and the Rb on channel A. Same result 
(hopefully)

Setup #2 : Pink
Then I've made what Magnus has suggested, i.e. using the 1 PPS on Channel A, 
the Rb on channel B and internal gating.... The ADEV has increased by more than 
1 order of magnitude. I guess this confirms the 1PPS stability is lower than 
the 10 MHz

Setup #3-6 : Dark Green, Red, Light blue and Dark yellow.
I've measured several times the GPSDO DUT with SEParate inputs. 1PPS on EXT, Rb 
on channel A and DUT on channel B. This gives 4 different plots... When 
starting the measurement the plots starts directly at different values... Mmmm 
very strange. Is it coming for the setup of the GPSDO ? To be investigated 
further with other sources. This is the plan for tomorrow. However the overal 
shape of the plot sounds relevant to me.

Setup #7-8 + #9 not showed here
I've tested the suggested splitted same signal on both inputs with 1m coax for 
channel B. I've discovered that when swaping the GPSDO on the standard input 
and the Rb on the channel A I have a slight difference. In order to confirm 
I've made two time each measurement and this confirms that having the Rb on 
channel A and GPSDO on the standard input gives the lowest ADEV. The setup #9 
which is the same than the light green gives the superimposed plot on that 
one... So what does it mean ? One of the two sources is better than the other, 
but which one ?


Some other comments :
- Swaping signals between channel A and B gives the same ADEV (setup #4 and 5, 
light blue and red)
- On some measurement on the GPSDO DUT, (not displayed here), I could see 
during the measurement suddenly an increase of one order of magnitude. The 
HP5370A do not show any difference (the time interval value continues to move 
with a beat but visually impossible to quantify if the value between two values 
has increased. No explanation for that. I'll redo the test with some other 
sources to check if it comes for the measurement system or the GPSDO DUT

In conclusion,
1. swaping the Rb and HP58503 doesn't give the same result. The GPSDO has 
standard seems the best (or the Rb measured) 2. the measurement on the GPSDO 
DUT gives different results with nearly one order of magnitude difference but 
shape is still the same.
3. the 1PPS must be connected on the EXT gating input

What do you think ?

Cheers
Stephane


-----Message d'origine-----
De : time-nuts [mailto:time-nuts-boun...@febo.com] De la part de steph.rey 
Envoyé : lundi 19 janvier 2015 16:44 À : Discussion of precise time and 
frequency measurement Objet : Re: [time-nuts] question Alan deviation measured 
with Timelab and counters

Actually I'm working in the RF department of a big lab, designing RF  
electronics mainly in microwaves range. I'm luckilly having some tools  around 
to play with and a lot of components like  
mixers/amplifiers/couplers/splitters/attenuators, ... almost whatever  the 
frequency is up to several tens of GHz.
At home since the last 20 years I could as well get nice instruments.
The next two measuring tools really missing and for which I'm limited  are the 
phase noise and stability measurement and possibly a good  standard. My 
Effratom FRK Rb is old and probably not the best from a  phase noise and 
stability point of view but until now has never been  characterized. Otherwise 
I've almost everything I need up to 40 GHz I  guess.

I'm doing further measurement right now which sounds much much more  
consistent. I will share tonight.
Cheers
Stephane



On Mon, 19 Jan 2015 08:59:58 -0500, Bob Camp <kb...@n1k.org> wrote:
Hi

On Jan 18, 2015, at 5:12 PM, Stéphane Rey <steph....@wanadoo.fr>
wrote:

Bonsoir Magnus (Are you in Sweeden ?)

Being able to measure high stability and low phase noise is
definitely a need for me as I'm trying to design low noise
synthesizers and I'm already reaching the limits of my current tools
for phase noise and I can't afford an E5052 for my own. At work I've
one but I will probably not stay after august. And anyway I need such
tools in my lab at home…

If you have tools at work, the best possible thing to do is to get
some oscillators / standards characterized. If you *know* what this or
that oscillator is doing in terms of ADEV or phase noise at this Tau
or frequency offset, it’s much easier to figure a lot of this out.

The most basic way to do phase noise in the basement is with a single
mixer setup running into some sort of audio FFT device. A sound card
can be used or an audio spectrum analyzer. Parts are < $100 to get one
setup once you can do the audio measurements.

For ADEV, a DMTD or it’s cousin, the single mixer is the easy way to
go. The single mixer does not get a lot of discussion these days. It
is much easier to set up than a DMTD. It does require an offset
oscillator. Once you have a single mixer phase noise setup, you are
about half way to a single mixer ADEV setup. Cost for one is < $100 in
parts. You already have a counter to collect the data out of it.

In both cases you are running a comparison device. Having a
characterized OCXO to compare to is a really nice thing.

Bob



As low-noise and stable synthetizers depends on the standard used, I
need as well to measure them as well...

Let's start with this simple experiments and once I will understand
the ins and outs I will try to improve. I know techniques of
cross-correlations and you've already talked about DMTD that for sure
I will have to come to...

Good night
Stephane

-----Message d'origine-----
De : Magnus Danielson [mailto:mag...@rubidium.se] Envoyé : dimanche
18 janvier 2015 22:46 À : Stéphane Rey; 'Discussion of precise time
and frequency measurement'
Cc : mag...@rubidium.se
Objet : Re: [time-nuts] question Alan deviation measured with Timelab
and counters

Bonsoir Stéphane,

On 01/18/2015 10:34 PM, Stéphane Rey wrote:
Thanks a lot Bob and Magnus for your very helpful comments.

The HP5370a was indeed in TI mode. By the way what is the difference
with +/-TI, the button just aside...

But I guess I understand where I've missed something : I've tried to
put the Rb on channel A and the DUT on channel B but result was
always the same but I do understand now that there is indeed a
switch to change from COMmon to SEParate and it was always on COM
meaning I believe that channel B wasn't used. This explains a lot of
things I did not understand. I'm sorry for these so basic issues
that might have been solved if I had read carefully the HP5370a manual first.

Good. This confirmation makes sense to be and Bob, now we can relax
as the mystery is solved.

So possible conclusions until now are that I have actually measured
the ADEV floor of the system rather than my DUT... which is already
nice. The second conclusion from these oscillations seen with the
GPSDO under test is that there is very likely in this GPSDO design a
systemic noise added to the 10 MHz output (power supply, PCB
coupling, ... I'll make further investigations on it later on).

It's a great opportunity to learn the tools, and once you have the
tools, you can see if you can't improve things.

I will experiment all the suggestions you made and will come back.
For information the 1PPS from the HP58503b has a positive pulse
width that is only few us length.

This only makes it hard to view on a scope, but long enough to
reliably trigger your counter and scope.

Now, when considering that the method is to compare the DUT to an
other source, I assume then that the other source shall be at least
1 order of magnitude better than the DUT. Otherwise this will be
impossible to distinguish who is the instability contributor between
the source and DUT, right ?

For a simple setup, yes. But then we are the time-nuts, we have ways
of handling these things. :) Let's get you started with the basic
measurement, it will be a good start.

Then the second question is what kind of very stable source can be
used to measure DUT which could be Rb or GPSDO which are already in
the range of 10E-10 to 10E-12 < 100s ?

Time-nuts tend to spend their time and money getting even more stable
clocks and tools. If you have the right tool, you can measure near
and
*under* the noise-level of your reference, but not without running
into issues. One such trick is called cross-correlation, while
another is to use three-corner hat techniques.

Cheers,
Magnus



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