Integrating GPSDO references inside counter? - Page 1
Forum section: electronics / general

author: pizzigri
author_rank: Regular Contributor
timestamp: August 09, 2021, 12:10:06 pm
content: Hi everyone,I was shopping for a good OCXO reference for a HP 53131A. While I settled for a clone of the original Option 010/012 built by RoadRunner, I found on the bay the following buy it now: "Agilent/HP 53131A/53132A/53181 GPS Disciplined High Stability Time Base"https://www.ebay.com/itm/373569791970I am intrigued, because it is indeed a nice idea, and it does provide for an active antenna (not clear if it is included or not). I could not figure out how the FW of the counter interacts with it, how is calibration carried out or if it is simply considered as an external reference by the meter (the seller did not reply to my questions), although it connects as a "normal" Opt 010 with the flat cable.What do you think?I am including a few pics from the original post, note the status LEDs on the 53131A window.ETA: I have no affiliation or interest connected to this item or the seller!!

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 09, 2021, 06:07:15 pm
content: That does look neat. I would of hoped for a set of flat zeros using GPSDO. As you say it doesn't tell you much about adjustment.

author: TheSteve
author_rank: Unknown
timestamp: August 09, 2021, 06:27:29 pm
content: Perhaps the cal wouldn't matter at all for 10 MHz - the GPSDO module likely ignores any correction voltage the cal applies and just spits out a GPS synced 10 MHz at all times.

author: Cerebus
author_rank: Super Contributor
timestamp: August 09, 2021, 07:28:12 pm
content: In the 53131A/53132A the official HP option board references all override the crappy "no-option" internal reference. When the option board is used as the reference clock it is calibrated by supplying a 10MHz input on Channel 1 from a frequency standard and running the calibration procedure from the menus. This procedure adjusts the option board's VCOCXO by setting a DAC that controls the EFC voltage input to the VCOCXO. One it hits agreement with the input standard it stores the DAC value in non-volatile memory and you're done.From this point onward the option board VCOCXO just free runs and supplies the timebase for the rest of the counter.The way these GPSDO add-ons have to work is by replicating the 'supplying a clock' bit of the option board. They don't need calibration because it's the job of a GPSDO to constantly recalibrate itself using GPS. Running the counter's calibration, even if the third party option card replicated or simulated the DAC from the option card, would achieve nothing.However, you might need to initially fool the 53131A/53132A into running the calibration just so that it thinks it's calibrated. If you insert one of the timebase option cards in a 53131A/53132A that's never had a timebase option card it'll say "UNCALIBRATED" on the front and do nothing, so a one-time fake calibration may be necessary.Quote from: mendip_discovery on August 09, 2021, 06:07:15 pmThe specifications section of the 53131A/53132A manuals have a fearsomely detailed section that allows you to work out the likely error limits that you can expect them to measure to. It's too complex for me to want to get into a worked example here. But, to give you a flavour of the errors other than pure timebase accuracy that mean that you will only see a 'perfect' result by chance: The spec for the 53131A has a best resolution of 650ps. An error of +650ps on a 10MHz clock would come out as 10,000,000.01 Hz. Add a systematic uncertainty specified as 350ps best case, 1.25ns worst case, due to trigger jitter and other factors, and it's clear that you're not always going to get 10,000,000.00 Hz as the answer.I've got a 53132A with the super-duper option 012 "ultra-stability" option board in it. The OCXO on the 012 board is good enough, according to the specification, that it doesn't drift more than 3 parts per billion in a month, and I've tested mine against a GPSDO and it exceeds specs. If the 53132A is fed from its own reference out, it wanders one or two counts in the last digit between measurements. Sometimes it's all 9s, sometimes 10,000,000.000 0, sometimes 10,000,000.000 1. That last digit represents 10 parts per trillion - which is about 1/1,000,000th of half a bee's dick.The OCXOs used in those GPSDO references probably have 0.05 ppb 1 second stability if they've one of the decent stratum 3E OCXOs that are kicking about in fair quantities from the Chinese used ex-telecom market.

author: pizzigri
author_rank: Regular Contributor
timestamp: August 09, 2021, 09:53:19 pm
content: This is extremely interesting....I guess that your description is quite realistic, a GPSDO dosent need any calibration (that's the whole point of getting one in the first place) so it just needs to fake a cal routine in the 53131A...the idea has real merit IMHO, it would avoid very costly double ovens that also draw a lot of current (although I don't know if a GPSDO wouldn't be just as "greedy").I mean, a highly regarded GPSDO reference here in the forum is Leo Bodnar's and it doesnt even use an OCXO, but a good TXCO as far as I know... so very affordable-portability is out even with a 010/012 opt board, it takes hours before it stabilizes, so it makes senseI was intrigued because i was toying with the idea of getting a second BG7TBL small "cigarette pack" GPSDO and hacking it to place it inside the counter - and then, I found this!I already spent the money on a OCXO for my counter, but maybe if some forum member gets one, make a report here?

author: ZGoode
author_rank: Frequent Contributor
timestamp: August 14, 2024, 11:36:35 pm
content: Anyone ever end up trying one of those internal GPSDO units for the 53131A family? I have a 53131A that doesn't have any options, so it isn't the most accurate counter. I'm wondering if I should consider the GPSDO upgrade instead of trying to source one of the option 010 or 012 boards

author: ivo
author_rank: Regular Contributor
timestamp: August 17, 2024, 05:45:56 am
content: Quote from: ZGoode on August 14, 2024, 11:36:35 pmAlso consider anexternalGPSDO module that you feed in via REF IN, if things tend to sit on a bench 24/7. This could be a more flexible setup. There's numerous DIY and ready-to-buy GPSDO projects around to construct. AFAIK if you're on a budget you can just chuck a standard 2nd hand eBay 10MHz OCXO of whichever quality you like inside it to upgrade over a TCXO, and fit a pot / reference you twiddle yourself to 10Mhz (provided you already have an accurate source), should you want it to simply be a more accurate self-contained box.

author: ZGoode
author_rank: Frequent Contributor
timestamp: August 18, 2024, 01:43:14 am
content: Yeah, thats kinda what I'm leaning more towards after looking into it now. I've started my own GPSDO design based on the LARS GPSDO for a eurorack. I ended up buying a board from Gaurav Singh over at CircuitValley.com and a MV89A from Ebay. I'll just have to wait until I have a working GPSDO to calibrate my 53131A

author: eplpwr
author_rank: Regular Contributor
timestamp: August 19, 2024, 06:04:53 am
content: Quote from: ZGoode on August 18, 2024, 01:43:14 amI have the same setup for my 53132A and it is working flawlessly. I guess I got a good (used) MV89A from eBay, and after a retrace for about a week, it has shown almost no drift (10% of spec). Every 3 months I do a recalibration against GPSDO but I could get away with doing it yearly.