GPSDO with Rb and XO

[tkamiya]

Ed,

Thank you very much for your detailed analysis.  You are right - I am having trouble interpreting the graph.  I am pretty much a newbie-time-semi-nuts.  Your discussion of the graph really helps.  I will take time to read section-by-section tomorrow as it is getting late here in Eastern US.  (Florida)

JUST few hours ago, I finally managed to set up GPIB with TimeLab to talk to HP52132A, so I don't have to rely on serial.  I don't yet know if that will mean anything in terms of measurement.  I does allow me to use other equipment with TimeLab though. 

I am thinking, use of GPSDO as clock was a bad idea.  As they are completely different GPSDO, I have two oscillators (standard and DUT) that's yanked all over the place.  Maybe that's the missing piece in this puzzle.  I am thinking maybe I'll just use double oven Xtal.  Initial idea was, this combination of GPS and Rb will give me the stability and accuracy that I wanted.

I have a FTS4040A Cesium.  Should I use THAT instead??  By the way, the signal being compared is a sine 10MHz, and the trigger is AC coupled.  Actually, I didn't even think about that, so I will review it in depth.

Thanks again,
Taka

ps.  In a perfect world, everyone has a Hydrogen Maser or two.  Hello, tvb...

[edpalmer42]

Unfortunately, I kind of messed up my analysis.  I always use Time Interval instead of frequency for my measurements and I got mixed up because you're using frequency.     I've edited my message to correct the mixup.

Ed

[edpalmer42]

I wouldn't recommend the 4040A as your reference for a couple of reasons.  First, if you look at the graph I just posted for my 4065A, it's okay, but not stellar when it comes to ADEV - no Cesium is.  They, like GPS are frequency references.  Second, there's still the problem of using up your Cesium.  Once it's gone, you've got a really effective boat anchor!  Using it as a reference means that it will be running a lot!

Searching for a good reference is a never-ending quest.  Almost every time-nut moves from reference to reference as they find a better one.  It's like mining for diamonds - digging through lots of ore, looking for a gem or two.  They also use different references for different measurements.  OCXO or DOCXO for short time intervals.  Maybe Rb for middle intervals.  Maybe GPS or Cesium for long term measurements - i.e. make a measurement every few days to check for drift, not ADEV.  Yes, there are a few crazy people that have their own hydrogen masers, but let's remember ...."everything in moderation".... (says the guy with a Cesium reference )   

I don't know what you've got for OCXOs or DOCXOs.  The best item you've mentioned is the PRS10.  It's got decent low-Tau stability and good aging and noise.  If you look here:  http://www.ke5fx.com/rb.htm you'll see a good comparison between some of the more popular Rb standards.  The PRS10 looks quite good.  I'd just run it without trying to discipline it to GPS.  My current reference is an HP 5065A.  I can't tell you how good it really is, but it's good enough for me.  I have a few OCXOs and DOCXOs that might beat it at low Tau values, but I can't measure that low.   

Ed

[thinkfat]

Using a GPSDO as a reference for these kinds of measurements is really going to "color" your data a lot. While experimenting with my own GPSDO platform I was using one of these UCCM GPSDO modules as the 10MHz input and 1PPS from a different GPS timing receiver. You could really see the GPSDO at work, large cycles of 4500s long up- and down movement as it steered its internal OCXO around.  A bit of context can be found here:

https://www.eevblog.com/forum/projects/diy-gpsdo-project-w-stm32-tdc7200/msg2858734/#msg2858734

For this type of measurements you definitely need a more stable reference.

[tkamiya]

I have an LPRO-101 as well.  Also half a dozen of HP10811-60111, a dozen or so of oscilloquartz 8663-01.

Moderation....  got it!

[tkamiya]

..... moderation .....

I just found some forgotten Efratom FRS stuff....  (um....)

Here's what I am doing now.  Since we kind of think GPS or GPS related portion is at fault, I've decided to use something else to steer PRS10.  I have some genuine Trimble Tbolt, so I took one, used its 10MHz out to feed the counter sync port, and took its (Tbolt) PPS to feed the PPS IN of PRS10.  They are now synced up and I'm seeing an incredible result.  (kind of expected)  The PRS10 is still warming up, so I'll have to let it run for some time before I can say anything meaningful.

After this test proves GPS is at fault, I'll use an independent GPS to feed PPS IN and retest. 

[tkamiya]

I am finding out, there is a major problem with my measuring setup.  Corby is helping me with it right now (different thread).  Until this is resolved, the project itself is on hold. 

[tkamiya]

Thanks to many folks on this forum, my measuring system problem has been resolved.  It was the parameter I gave to the TimeLab software.  It was MY falut, not the equipment or software.  It was reading incoming data as Hz when it should be MHz.  Huge scaling issue.  I'm currently doing a verification run on HP53132A noise floor.  I'll have to do the same to my standard.  Sadly, that also means every measurement I took in the past contain the same error.  Luckily, data from this point on will be correct.

[jpb]

Thanks to many folks on this forum, my measuring system problem has been resolved.  It was the parameter I gave to the TimeLab software.  It was MY falut, not the equipment or software.  It was reading incoming data as Hz when it should be MHz.  Huge scaling issue.  I'm currently doing a verification run on HP53132A noise floor.  I'll have to do the same to my standard.  Sadly, that also means every measurement I took in the past contain the same error.  Luckily, data from this point on will be correct.

Easily done. I made a similar mistake when first using the Stable32 software - I entered files in Hz when that software expects frequencies to be an error ratio (f-f0)/f0 - this also made a huge difference!

[tkamiya]

My day job is a software developer / programmer at one of the major software company.  I also spent some time as a support rep.  I've learned always approach cases with major doubt in users' report, and never trust what's being said as fact.

In THIS case, the user was ME.  Sure enough, I was the cause of problem.  Doesn't surprise me, really.  But I've been using the same setup for some time.  It took me 6 month to realize my own mistake when corner case alerted me of possible issues.

It has been a good learning experience.

By the way, this project started first, because I wanted to do it.  The major motivation was, a lot of people said I cannot do it, and SRS (who made PRS10) said the feature was really meant for those who make GPDSO and Rb for living.  Well....  they are currently winning but the never counted those who are majorly obsessive!  Let's see what happens in next 12 month.  This is going to be a long project.