[time-nuts] General questions about making measurements with time interval counter.
kb8tq at n1k.org
Thu Jan 12 12:04:04 EST 2017
There is a big difference between RMS and single shot. Single shot, the 5370 is a very different beast.
That’s not a big deal when you have a few thousand readings and it all averages down. Unfortunately
we all love to do runs with a very small number of points and then draw conclusions from them. As the
sample size goes down, you no longer have a 2 to 4 x 10^-11 beast, it’s more like 5X that.
> On Jan 12, 2017, at 11:31 AM, Bob Stewart <bob at evoria.net> wrote:
> Hi Bob,
> Normally I see somewhere between 2E-11 and 4E-11 at 1S tau on my 5370A, as in the blue trace on the attached plot. Am I misunderstanding your meaning? Granted, I am clocking the 5370A with a GPSDO, but I believe I see about the same thing with the HP10811. This test was 1PPS vs 1PPS on two different units.
> The plot also has a test run by Tom, in orange, using his H Maser and a Timepod to show how poor the 5370 is compared to the Timepod below about 60S tau. These are essentially apples vs apples tests.
> From: Bob Camp <kb8tq at n1k.org>
> To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
> Sent: Thursday, January 12, 2017 8:27 AM
> Subject: Re: [time-nuts] General questions about making measurements with time interval counter.
> There are a number of ways to improve the resolution (and accuracy) of your data without spending
> big piles of cash. They have been discussed here on the list many times over the last few years.
> What I’m suggesting is that you dig into that ahead of taking data. You will dive into it eventually as you
> look more and more at devices that are locked to some sort of stable reference internally.
> Ideally you would like a device with a floor 5X to 10X better than what you are measuring. For ADEV style
> data, the 5370 is a 1x10^-10 sort of device single shot (so 1x10^-9 is the limit at 10:1). With a lot of averaging
> (which is not something you do with ADEV) you can get about 5X better than that as a floor. In either case, it is getting in the way of any
> readings that are much below 1x10^-9 at one second. A low cost XO can hit that level of performance.
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