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

Magnus Danielson magnus at rubidium.dyndns.org
Sat Jan 17 19:34:34 EST 2015

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 : 
 > 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.

 > 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:

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.


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