MV89A OCXO Ref pin  - Page 1
Forum section: electronics / projects

author: BiOzZ
author_rank: Regular Contributor
timestamp: August 29, 2013, 07:09:08 am
content: I bought on ebay an MV89A oven controlled crystal and i wired it up simply (3300uf caps on 12v input and 1000uf on the 5V because of issues with my powersupply that this seemed to fix ... also a simple short probe loop on the output for now) but its time for me to take it into a calibration lab and have it tuned ... i have no idea what to do with the REF pin and i dont want to screw it up and blow the feeim just guessing you take a 10k 20 turn pot or something and wipe 0-5v from the vin or use a good voltage reference or somthing but i need some conformation and the datasheet was fairly useless on this and i cant find really any docs on this topicanyone know? anything else i should know like if i have to filter or AC couple the output?thanks!

author: ve7xen
author_rank: Super Contributor
timestamp: August 29, 2013, 07:37:57 am
content: Where are you feeding +5V? These are powered from +12V only.The simplest Vtune circuit is a 10-turn pot between Vref and GND with the wiper to Vtune, though you can do better. It's all down to how much time and parts you want to throw at it.I wouldn't bother with measuring it super precisely until you know how you're going to use it. Orientation and ambient temperature in the enclosure will both have effects on frequency.

author: BiOzZ
author_rank: Regular Contributor
timestamp: August 29, 2013, 07:48:53 am
content: Quote from: ve7xen on August 29, 2013, 07:37:57 amim feeding it in to Uin as shown herehttp://www.morion.com.ru/catalog_pdf/MV89-OCXO.pdf... "with external control voltage range (Uin)" is not exactly descriptive is this not necessary?thanks for the cal info!ambient temperature effects an OCXO? ... but i want to have this tuned as it will function as my current 10mhz ref and it looks like its a bit banged up and removed from old equipment so i want to take care of issues that might be from that and i already have cal lab time to calibrate my voltage and current refs and they told me i could do this with no extra fee

author: BiOzZ
author_rank: Regular Contributor
timestamp: August 29, 2013, 07:57:44 am
content: i ran it without the 5V line and it seams to work fine ... i feel stupid but what's Uin for?

author: ve7xen
author_rank: Super Contributor
timestamp: August 29, 2013, 08:19:05 am
content: Uin is the control voltage. You feed it a voltage between GND and Uref (e.g. with a pot between them) and it will pull the frequency slightly. With 5V on it, it should've been approximately at the maximum frequency.If you have anything reasonably trustworthy to measure it with, I'd check that it seems possible to get it in spec with a pot there. I got one of these same OCXOs on eBay that was not tunable to 10MHz.

author: BiOzZ
author_rank: Regular Contributor
timestamp: August 29, 2013, 08:27:00 am
content: Quote from: ve7xen on August 29, 2013, 08:19:05 amso im confused do i put the pot to adjust it between ground and vref or between Uin and Vref or what if i want to tune the output?if i dont use anything should i just leave those 2 pins floating?

author: ve7xen
author_rank: Super Contributor
timestamp: August 29, 2013, 08:52:37 am
content: Maybe it will clarify things for you if I mention that the Uref is an output voltage (~5V). It's supposed to be used as the reference voltage to tune the oscillator at Uin.You'd put a pot between Uref and GND with the wiper attached to Uin, then tuning the pot will apply a variable voltage between Uref and GND to the Uin pin.You should be okay floating it, but it may drift more. If you don't want to vary the frequency I would divide Uref with a pair of resistors to generate a Uin voltage of ~ 1/2 Uref and tune it around the middle of the range.

author: BiOzZ
author_rank: Regular Contributor
timestamp: August 29, 2013, 09:11:45 am
content: Quote from: ve7xen on August 29, 2013, 08:52:37 amOHHH ok .. confusing that they call it Uin ... but thanks allot for clearing that up and this is very helpful!

author: mamalala
author_rank: Unknown
timestamp: August 29, 2013, 07:59:28 pm
content: Quote from: BiOzZ on August 29, 2013, 07:48:53 amThe issue here is not so much with the OCXO then, but the tuning pot instead. If you connect an external tuning pot, it will be subjected to temperature changes, and thuns change it's value ever so slightly.The pdf you linked to says very little, but some OCXO have an adjustment pot built in, that you can reach by removing a screw. Such OCXO can then be tuned to the proper frequency while not using the external tuning voltage (for example, the Piezo 2940210 has such an "internal" tuning pot).Greetings,Chris

author: calmtron
author_rank: Regular Contributor
timestamp: August 29, 2013, 10:12:47 pm
content: Check the manufacturing date of your MV89 if possible. They have a very small frequency pull-in-range compared to the aging rate, so it might be hard to tune it to 10.000000 (and so on) MHz if that's the goal.I'm currently playing around with one trying to make a basic GPSDO. Going to feed Uin with a DAC referenced to Uref, I guess that's the standard method. My MV89 is close to impossible to tune to 10.0 MHz though :/