Isotemp OCXO44-12 as GPSDO Oscillator - Page 1
Forum section: electronics / projects

author: cncjerry
author_rank: Unknown
timestamp: November 11, 2017, 08:05:25 pm
content: I have the subject oscillator. I also have a Lucent REF0/REF1 GPSDO already but during a recent sale of TAPR GPS units, I was given one as I CNC cut the case end plates. The TAPR unit outputs a 1PPS signal. So having the Isotemp OCXO that I took out of the Tek 11301 I couldn't repair, I thought what the heck, make another GPSDO.Here's the rub, I don't see a control voltage on the Isotemp OCXO. It has an 18V input for the oven and oscillator, what's call the Logic line, ground and the 10Mhz output. The logic line I assume is for some 5v logic circuits that are inside the oven. There is also a 74LS00 nand gate chip that is probably a buffer mounted on the Tektronix board that mounts the OCXO.You tune the OCXO using a screwdriver on the side of the oven housing. I guess I can open the oven up and see if it is just a voltage divider inside and maybe the screw adjustment is really just for a 10 turn pot. If so I can just drive that with a DAC. But I opened it the other day and it didn't look like it was going to be easy as it had some foam inside that looked a little dry.Anyway, is this common on OCXOs to not have a control voltage? Anyone play with these things? Is that screw adjustment on a variable resistor or a cap?I planned to use the 1PPS from the TAPR GPS to gate an STM32F7 board that counts the 10Mhz signal and then discipline the oscillator over something like 1,000 seconds, maybe shorter initially.thanksJerry

author: texaspyro
author_rank: Super Contributor
timestamp: November 12, 2017, 01:33:16 am
content: There are lots of mechanically tuned OCXOs (pot and/or cap). And most HP equipment that uses the 10811 OCXO that has an EFC input do not use it and rely on the mechanical tuning to set the freq.If you want to build a GPSDO, get a decent OCXO. I'm rather partial to the UTC / Oscilloquartz 8663 DOCXO because it is small and has decent performance. Oh, and I bought a pile of them dirt cheap years ago.

author: cncjerry
author_rank: Unknown
timestamp: November 12, 2017, 01:55:12 am
content: It's just that I have the GPS and the OCXO so I thought I might be able to put them together.

author: texaspyro
author_rank: Super Contributor
timestamp: November 12, 2017, 02:07:11 am
content: Quote from: cncjerry on November 12, 2017, 01:55:12 amDepending upon what's in the OCXO, it might be doable. But it might also compromise the performance of the OCXO. If the adjustment is a cap, I'd forget about it. If it is a pot, it had a better chance of working. The 8663 has a VREF output pin that you can connect to the EFC pin via a pot. I stuck a lot of these the Tek DC5010 counters to replace the Isotemp OCXO's or to add an OCXO to the ones with the standard oscillator. The Isotemp OCXO tuning adjustment might just be a pot / vref... I've never opened one.

author: cncjerry
author_rank: Unknown
timestamp: November 12, 2017, 02:48:10 am
content: i tried to open it and the foam is a little brittle so I bailed. I would rather use it as-is for clocking a DDS for an SDR I am building than ruin it. I'll pull it apart again to see if it uses a cap or a Pot to set the freq. and go from there. There is a lot of code out there that I can use to make another GPSDO if I can discipline this with voltage.

author: texaspyro
author_rank: Super Contributor
timestamp: November 12, 2017, 04:01:02 am
content: Quote from: cncjerry on November 12, 2017, 02:48:10 amIf it's tuned with a cap, you could hook a screwdriver to your CNC and send gcode commands to turn the cap and discipline the oscillator that way.

author: cncjerry
author_rank: Unknown
timestamp: November 12, 2017, 08:57:16 pm
content: dont laugh, I thought of hocking a stepper to it.I tried to take it apart again last night and would have to dig all the foam out for no guarantee. I guess I am going to bail on it.

author: texaspyro
author_rank: Super Contributor
timestamp: November 12, 2017, 09:01:52 pm
content: Ok, I admit it... I did that on a piece of equipment. Adjustment was via a knob. It needed to be tweaked way too often. An AVR and a stepper turned it into a whole new game.

author: jpb
author_rank: Super Contributor
timestamp: November 13, 2017, 08:43:45 am
content: The datasheet:http://n4iqt.com/ocxo/isotemp/OCXO44-12a.pdfsays that the mechanical control is a multi-turn trimmer so to make it voltage controlled you'd need to add a variac in parallel but I don't think it would be worth it (except for the challenge). The ADEV for 1 sec is only 10E-9 from the datasheet and it would be much easier just to buy an OCXO off ebay. You can get new/old stock ones for around £25 at 10MHz eghttps://www.ebay.co.uk/itm/10MHz-OCXO-/222682451055Or if you go for funny frequencies such as 13MHz (I tend to do loony things like buy these as they are cheap) you can get one for closer to £10:https://www.ebay.co.uk/itm/OCXO-Vectron-13MHz-High-Stability-Oven-Oscillator-New-old-Stock/272806056479Of course there are a lot of second hand ones around as well though it is hard to know what condition they are in and if they are getting near end-of-life for frequency drift (i.e. whether they can be tuned to exactly 10MHz).At least your OCXO is tunable, there are quite a few around that have no tuning at all.

author: cncjerry
author_rank: Unknown
timestamp: November 14, 2017, 04:22:14 am
content: I was trying to do this with parts on-hand. I have the OCXO and the GPS. I already have a GPSDO though I am thinking of buying another Lucent RFTG-u REF0/REF1 pair now that I this on my mind.