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 <kb...@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....@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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