[time-nuts] question Alan deviation measured with Timelab and counters

Bob Camp kb8tq at n1k.org
Sat Jan 17 22:25:35 EST 2015


Hi

I’m a little concerned about the measurement setup here. Based on the quoted text, there have been a few messages in the thread that have not showed up here. 

=========

The “start” input on the counter is defined below. The “stop” input is not defined. Is the counter running in time interval mode or in frequency mode? 

IF it’s in time mode - what is the stop hooked to? 

IF it’s in frequent mode - what is the gate time set to ? 

Is the “standard input” the missing front panel input or is it the external reference input on the back panel? 

=========

I’m looking at the data on the link:

http://www.ptp-images.com/affiche-directement-l-image-kccsz71c9a.html

In both cases the slope is roughly 1/tau. Both plots end up in the 3 to 4 x 10^-13  range at 100 to 300 seconds. That’s suspiciously good performance for a rubidium or a GPSDO. Which is what makes me wonder about the setup.

The Blue plot (1 pps?) ran for 18 minutes and has 1,114 points in it. The Pink plot ran for about 9 minutes and has a bit over 500 points in it. Both seem reasonable for a 1 pps to 10 MHz sort of setup. That may explain part of my confusion above. 

Again - I apologize if this all got explained in a post that went missing here.

Bob


> On Jan 17, 2015, at 7:34 PM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> 
> Bonsoir Stéphane,
> 
> (For Stéphane's unedited reply, look in the Current state thread)
> 
> On 01/17/2015 12:36 AM, Stéphane Rey wrote:
> > Hi,
> >
> > I've took the time to read carefully your long and detailed message Magnus and this was very interesting. I've learned many things that have enabled me to investigate further. Ah yes, you're right saying that the more you fall into these things, the more you discover that you have to learn. Recently I've worked a lot on PLLs and I've actually learned a lot on special care to ensure low noise.... Very interesting. By the way I'm still working on this topic to improve again the noise (currently on a 3 GHz LO)
> 
> Good that you found it useful.
> 
> > Here are some experiment results : http://www.ptp-images.com/affiche-directement-l-image-kccsz71c9a.html
> >
> > 1.       Setup #1 (blue plot)
> >
> > HP5370A
> >
> > standard input from HP GPSDO
> >
> > EXT input not connected, internal Arming 0.4s rate
> >
> > START input from 10 MHz distribution unit RacalDana 9478 Rubidium
> >
> > 2.       Setup #2 (pink plot)
> >
> > HP5370A
> >
> > standard input from HP GPSDO
> >
> > EXT input not connected, internal Arming 0.4s rate
> >
> > START input from DUT (10 MHz homemade GPSDO)
> >
> > I'm not sure this is the proper way to connect everything... but this is the setup providing the lowest ADEV... which is between 1E-10 and 1E-13. But is the truth ?
> >
> > I feel strange the two plots having the same decreasing path along a linear slope (I mean linear on the log-log plot) ... I'm not sure of what I'm measuring ? Could this be the system measurement floor ? By the way how to measure the ADEV floor of a system other than having a source greater than the measurement system ?
> 
> The slope is to be expected from the ADEV matching white PM noise, because it behaves similar enough to it on the ADEV plot.
> Please see the Allan Deviation wikipedia page, where I amongst other things added the power-law noise section with a handy table.
> https://en.wikipedia.org/wiki/Allan_variance
> 
> > What could be these oscillations on the homemade (not by me) GPSDO  ?
> 
> Could be. Do use the phase plot (press p) to see if you have visible wobblings or something.
> 
> > I've tried to downmix the DUT 10 MHz to few kHz using a SR DDS generator and a double balanced mixer from minicircuit via a low pass filter tuned at 100 kHz, but the level wasn't high enough for the counter (which I found strange as it was already nearly 200mV). I hadn't anything in hands to make a squarer quickly so I've just added a Minicircuits RF amplifier. The level was good but the ADEV has jumped to 1E-6. The signal was noisy already on the oscilloscope which I know is for sure the cause.
> 
> Did you set the original frequency to 10 MHz in TimeLab?
> As you mix-down, TimeLab needs to be told what the original frequency was, it will calculate the gain and adjust the scale accordingly.
> Check your Acquire window.
> 
> > I need to make a squarer. I was hesitating between several methods : using a CMOS gate, but this will increase the flicker noise from what I've read, using an amplifier and clamping diodes or a fast comparator which might create some noise around the trigger point... Any recommendation there ?
> 
> Check out the Wenzel two-transistor design:
> http://www.wenzel.com/documents/waveform.html
> 
> Buffer the output with an inverter or so.
> 
> > I'll try to make this squarer next week to continue my investigations
> 
> The joy of lab sessions.
> 
> Cheers,
> Magnus
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