Hi If your only instrument is a counter.
— and — You never measure past 1x10^-10 with that counter — and — Measurements that bounce around with a standard deviation of the difference between readings of 1x10^-10 are ok. — then — No, you don’t need anything better than a 1x10^-10 ADEV. Most people would be bothered by a counter that has an typical jump of 1x10^-10 between every reading, so most would want a standard that’s a bit better than that. In addition, if you want to guarantee accuracy of a reading, you probably want something that’s 5X to 10X better than the level that stops the reading jitter. Simply put - ADEV is not standard deviation of frequency. Your frequency counter measures frequency. Going from one to the other means you want to have better ADEV than you might think. Bob > On Jan 9, 2015, at 10:42 AM, steph.rey <steph....@wanadoo.fr> 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 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 >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
