[time-nuts] 10MHz Rubidium reference source for frequency counter

[Karen Tadevosyan]

Charles, John and Bob, 
Many thanks for your advices.

1) Charles Steinmetz wrote: "I would suggest trying a few of the oscillators again (no need to try them all), using the Isotemp OCXO as the time base for the Pendulum by connecting it to the Pendulum's REF input and selecting External Reference from the front panel menus.  This will tell you if there is a problem with the internal time base in the counter."

Other external reference signal (from R&S FSQ) for Pendulum CNT-91 tested. 
ADEV result is the same.  If so CNT-91's internal OCXO is good enough.

2) Charles Steinmetz wrote: "Are the input signals triggering the counter very stably near the middle of their peak-to-peak voltages?"

Checked that the input signals triggering the counter very stably near the middle of their peak-to-peak voltages.
CNT-91 has an automatic trigger level set up and an indication of triggering.

3) Charles Steinmetz wrote: "Does your sample interval give the counter time to process each new sample?"  

As I saw TimeLab and CNT-91 set up a sample interval automatically when I press a "Monitor" button in TimeLab.
I got a sample interval 0.04sec.

4)  Charles Steinmetz wrote: "Are you sure your TimeLab setup is correct?"
 
Not sure 100%. 
May be John can check - https://dl.dropboxusercontent.com/u/21338179/hamradio/OCXO_Adev/ocxo%20adev%20ti%20mode%20config%2020141102.jpg

5) John Miles wrote: "Also consider taking TI readings rather than frequency readings."
 
Mode has been switch from Frequency to TI - result is much better now. It was the main improvement in my measurement. Thanks a lot!
Picture of ADEV OCXO result - https://dl.dropboxusercontent.com/u/21338179/hamradio/OCXO_Adev/OCXO%20ADEV%20TI%20mode%2020141102.jpg

6) Bob Camp wrote: "One of the things that will drive them a bit nuts are sub-harmonics. If your OCXO’s are 5 MHz doubled to 10 MHz, they will have sub-harmonics (5 MHz, 15 MHz, 25 MHz etc). These “extra” signals are not what the counter is expecting to see on a 10 MHz signal."
   
Harmonic level from OCXO checked - maximum harmonic levels (MV89A) are below -41 dBс

7) John Miles wrote: "I have no experience with the Pendulum counters but I wouldn't expect them to do much better than 1E-10 at t=1s regardless of how you set up the measurement, just based on their single-shot resolution spec in the 50-100 ps region."

I got this result 1E-10 at tau=1s with my tested OCXOs and my measure equipment.
I am not sure that I need better measure stability for my current applications (WSPR, VHF/UHF synthesizers and LO).      

Thanks a lot for your advices.
Karen, ra3apw


> Date: Fri, 31 Oct 2014 19:39:35 -0400
> From: Charles Steinmetz <csteinmetz at yandex.com>
> 
> Karen wrote:
> 
> >All OCXOs have been tested in free-running mode after 40-50 min of
> >switch-on/warming.
> >
> >All OCXOs have been installed in isolated close box to exclude external
> >airflow.
> >
> >Cases of all OCXOs had not any contact with external metallic parts of the
> >box.
> 
> Then it appears that something is wrong with your method of measuring
> the ADEV.  I would suggest trying a few of the oscillators again (no
> need to try them all), using the Isotemp OCXO as the time base for
> the Pendulum by connecting it to the Pendulum's REF input and
> selecting External Reference from the front panel menus.  This will
> tell you if there is a problem with the internal time base in the
> counter.  [NOTE that the REF input needs a sinewave between 0.1 Vrms
> and 5 Vrms.  If the Isotemp has a square-wave output, choose another
> OCXO that fulfills the Pendulum specification.]
> 
> If those tests produce ADEV measurements of 1e-11 or better at 1
> second, the internal time base is too noisy for what you are trying
> to do.  I expect that the internal time base will prove to be OK, but
> you need to verify that before proceeding.
> 
> However, I note that in the manual, the internal time base of the OPT
> 19/90 counter is rated for a Root Allan Variance of 1e-10 at tau = 1
> second and 10 seconds (see p. 8-15 of the 2014 User's Manual,
> Rev.18).  OPT 30/90 is rated at 1e-11, and OPT 40/90 is rated at
> 5e-12.  So, you may be getting about all of the performance the OPT
> 19/90 can deliver, if its OCXO just meets specification.  (HP5370Bs
> are also specified at 1e-10, but they often deliver ADEV = 1e-11 or
> 1e-12 at 1 second.  I do not have enough experience with Pendulum
> counters to know what kind of stability is typical.)
> 
> Assuming that the test above (with the external REF) gives similar
> results to what you posted today, there is something wrong with your
> measurement setup.  Are the input signals triggering the counter very
> stably near the middle of their peak-to-peak voltages?  Is the
> counter adjusted to give maximum resolution for the sampling you are
> doing?  Does your sample interval give the counter time to process
> each new sample?  Have you read Chapters 3, 4, and 5 of the User's
> Manual carefully, and set up the counter properly for your
> measurements?  Are you sure your TimeLab setup is correct?  (I'll
> leave it to John to comment on that.)
> 
> Best regards,
> 
> Charles