[time-nuts] Usefulness of high end counters for ADEV plots of oscillators

jpbridge at aol.com jpbridge at aol.com
Sat Jan 31 10:38:42 EST 2015

Hi Rick,

Thank you, I was intending to use an external reference and it is useful to know that the OCXO options are not worth paying a lot extra for.

Since I'm looking at refurbished or ebay instruments the presence or otherwise of the OCXO options is a matter of luck - if they are there it doesn't push up the cost as it would on a new unit, but it all depends on if the original purchaser needed one or not.

The one thing that the OCXO option seems to buy you is a narrower PLL locking range which presumably means that less noise/jitter is added to the external reference.

I am more a time acorn than a time nut at present so I'm not yet at the stage of doing serious ADEV measurements but I want to be able to assess (comparatively) different OCXOs that I have acquired and also different GPSDO approaches. Perhaps for my uses it is sufficient to have the curve from about 100 seconds on.





-----Original Message-----
From: Richard (Rick) Karlquist <richard at karlquist.com>
To: jpbridge <jpbridge at aol.com>; Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Sat, 31 Jan 2015 0:14
Subject: Re: [time-nuts] Usefulness of high end counters for ADEV plots of oscillators

First of all, the oven oscillator option of the 53230
is no where near as stable in ADEV as a 10811 for example.
The counter itself is 1 or 2 orders of magnitude better
than the built in timebase.  So don't waste your money
on the OCXO option when you, as a time nut, undoubtably
already own something like a 10811.

When I was still with Agilent, I made innumerable measurements
of ADEV with a 53230 down into the low parts in 10^11, which
was the DUT ADEV, not limited by the counter.  I vaguely
remember measuring a 10811 as a sanity check and using
the internal OCXO (not knowing any better).  After wasting
a lot of time, I eventually measured one 10811 against
another and discovered that the ADEV floor was down to
1E-12 at least, and that the internal OCXO was junk.

About the only good thing about the 53230 is that it
is a self contained box that makes ADEV measurements and
displays them in real time without requiring an external
computer with software.  In 1974, HP made a computing
counter (5360 I think) that did this.  Customers loved this
box, but the HP engineers hated the box.  Therefore, no
HP/Agilent counter ever did ADEV again, until the product
line was offshored to engineers who didn't know any better
and put the feature back in.

For serious ADEV measurements, you want two DUT's offset by
a few 100 Hz and mixed, as originated in the HP 5490 system.

Rick Karlquist N6RK

On 1/30/2015 8:38 AM, James via time-nuts wrote:
>   Though I'm new to the list I've lurked for quite a while

  and from reading various posts I am in a slightly confused

  state as to whether buying an expensive counter (eg Keysight 53230A

  or a Tek fca 3100) will be useful as a measurement tool for

  developing a GPSDO.

> Given a one shot measurement resolution of 50 psecs (on the Tek which is a 
pendulum CNT91) means that the uncertainty is around 50E-12 at 1 sec or 5 x 
10^-11 or 10^-10 in round numbers for a ADEV at 1 sec? For this noise floor to 
get well below 10^-11 (the sort of ADEV of an OCXO) requires the interval to be 
increased to nearer to 100 seconds?
> So does this mean that an expensive counter allows useful ADEV plots from 100 
seconds on but not the lower time frames? (By useful I mean able to measure down 
to and below 10^-11 not down to 10^-14!)
> The extra cost of the 53230A over the 3100 gets down to 20 psecs so possibly 
reduces the period to a bit less than 100 seconds but still above 10 seconds 
> Does paying extra for an OCXO gain significantly on this basis?
> Have I got the basic numbers right, and if one of my main aims is to have a 
good instrument for playing with GPSDO development will investing in such an 
expensive (for an individual hobbyist) instrument buy me a useful measurement 
capability or would it just be good for measuring long term frequency and say 
1pps jitter from the GPS?
> Sorry for the long post,
> James
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