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

Jim Lux jimlux at earthlink.net
Fri Jan 9 10:59:58 EST 2015

On 1/9/15 7:42 AM, steph.rey wrote:
> Hi Bob,
> Many thanks for your prompt and detailled answer.
> My question on applications wasn't on good ADEV where I perfetcly
> understand the need, but actually what could be the applications of
> measuring BAD ADEV (>10e-7). That was my point asking what king of
> application can we cover by measuring such high ADEV when you have
> counters with resolution not greater than 0.01Hz
> However you bring to me part of the answer when you talk about the
> reference and the way to get something cheap and better than 10e-12. I
> will investigate on DMTD. However, even if you have a beautiful Maser
> source, will you improve anything above the resolution of your counter.
> In other words, with my 0.01Hz counter, will I improve my measurement if
> I replace my GPSDO source with something much better ? I feel the
> resolution of the counter will anyway limit the ADEV floor, right ? If
> the last digit of the counter do not move how could we measure something
> smaller ?

The various heterodyne techniques (DMTD is but one) let you use your 
counter for many more digits than it has. Essentially what you do is 
beat your unknown against the standard, and count the difference 
frequency.  What you really do is put an offset in one (say 100Hz on a 
10 MHz standard) so you're accurately measuring a 100 Hz instead of a 10 
MHz signal. Your counter then gets down into the microHz.

The other approach is to use one standard to drive the ADC clock to 
sample the unknown, and then post process in software.  Once you've got 
a series of numbers, you can get infinite precision in software. 
there's a variety of schemes here too.

> The counters I'm using are not running on their own reference (OCXO or
> TCXO) but with the HP58503b which is a GPS disciplined OCXO but with
> stability in the range of 10e-11 or 10e-12 at best.
> I'm working for a big lab where possibly I could have nice piece of
> equipment but this is always easier to find alternatives solutions at
> lower price. On the application I'm working on we're looking for phase
> stability in the range of fs at several GHz. One of the project I'm
> working will use a femtosecond laser modulated at 88 Mhz that some
> people want to use as RF reference for the 3 GHz source. I'm pretty sure
> this can't achieve the phase stability requirement and I'm trying to
> illustrate this.
> However even for my ham activites where I'm trying to design low noise
> LOs, I'd like to have a tool able to measure goog frequency and phase
> stability...
> Stephane
> On Fri, 9 Jan 2015 07:48:42 -0500, Bob Camp <kb8tq at n1k.org> wrote:
>> Hi
>> Welcome to the world of trying to measure this stuff …
>>> On Jan 9, 2015, at 6:53 AM, steph.rey <steph.rey at wanadoo.fr> wrote:
>>> Dear all,
>>> I'm trying to measure Alan Deviations using Timelab and some
>>> frequency counters.
>>> The device under test is a GPSDO using a TCXO as référence
>>> I've an HP58503B GPSDO which feeds my counters. I've tried an
>>> HP5342A, 0-18 GHz, 1 Hz resolution and a Philipps PM6654C, 0.01Hz
>>> resolution.
>>> In Timelab, the plot with the HP5342A is around 10e-7 which
>>> correspond to 1Hz and with the PM6654C, the plot is around 10e-10.
>>> I would suspect that this is still the counter which limits the
>>> actual response of my device under test.
>> Yes, the counters and TCXO are limiting your measurements.
>>> My question are :
>>> - how to measure Alan Deviations with levels below 10e-12/10e-13 ?
>> How much money do you have to spend? ( There are expensive commercial
>> ways to do this).
>> No matter what, you will need a “better than” reference. That’s not
>> going to be cheap. Most of us simply get a second GPSDO and compare
>> them. The assumption is that they both are the same and you can
>> allocate the error equally between them. With three you can more
>> accurately allocate the error.
>> A DMTD is the “cheap” way to get the actual measurement done.
>>> What can be the application of measurement Alan deviation > 10e-10 ?
>>> I guess most of the low frequency
>> There are a number of systems applications that very much need good
>> ADEV. Getting into why this or that nav or com system needs it would
>> take a bit of time.
>>> - The HP53503 GPS is given to be 10e-11 / 10e-12. I guess this will
>>> limit anyway the measurement floor. I've a Rb source, but it's
>>> stability is within the same range. What kind of reference would be
>>> more suitable for such measurements ?
>> If you want to do it directly, a hydrogen maser is a good way to go.
>> That’s silly expensive. Just compare GPSDO’s, that’s a lot cheaper.
>>> - With the PM6654C on 15h measurement, I can see some frequency jumps
>>> of 800 Hz which are not relevants with the GPSDO undertest. I suspect
>>> error in data transmission. This makes the overall measurement
>>> totally wrong (10e-5). The counter is in talk only mode. I'd like to
>>> get rid of these points maybe 40-50 points out of 10000. Is there a
>>> way to do that from Timelab or the only option is to export the file
>>> and process manually the data ?
>> You can expand the data and zap the offending segments. It’s done on
>> the phase plot.
>> Have Fun.
>> Bob
>>> Thanks & cheers
>>> Stephane
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