Trimble OCXO 'extreme' teardown - Page 1
Forum section: electronics / general

author: ve7xen
author_rank: Super Contributor
timestamp: April 27, 2013, 11:57:00 pm
content: I managed to kill this somehow. The oscillator would still run but the output buffer got pooched. Too lazy to fix it, I thought I'd rip it apart and see how it was built, for SCIENCE. This is a fairly good unit used in cell base station applications, I believe.First, the unit in question. The foam stuff is presumably to help thermally insulate from the environment:I had already cut the can open when I first broke it, so I can't show the pristine condition. The circuit board was soldered to the pass-through pins visible on the cut-off bottom of the can. Date codes are mid 1992. Here's what's inside:Bottom:Top:Inner oven can gently removed with a screwdriver and heat gun...it was definitely not hermetic, only soldered on the narrow edges, I almost thought I could pry it open.Some side shots of the construction. The two heater devices are clearly visible on either side under the can. I guess this is a single-oven device:Crystal module disassembled. Interesting ceramic hybrid boards with epoxied dies. I'm quite curious why the traces are blue and green. There aren't any traces on the bottom side, just the heat transfer pads which feel very much like paper but must be some kind of sil-pad material.I tried to cut open the crystal can and noticed it seems to be plated copper. Not sure if that's typical of 'normal' oscillators but it was interesting so I took a photo. Unfortunately I dremmeled right into the quartz crystal itself and it shattered so not much to look at inside.There are a few more photos and slightly higher quality in my Google album:https://plus.google.com/photos/100139855762220392646/albums/5871680270442769857?authkey=CM6MsI3eh6jOZA

author: JackOfVA
author_rank: Unknown
timestamp: April 28, 2013, 02:04:29 am
content: I believe the crystal is cold welded at the base and the pins exit via hermetic glass/metal seals. This is essential to prevent contaminants from entering the crystal and landing on the quartz element, thereby adding mass and causing it to drop in frequency.

author: codeboy2k
author_rank: Super Contributor
timestamp: April 28, 2013, 05:36:55 am
content: I've seen lots of these old crystals in big metal cans like this; often times the ones for SMD use are much much smaller...So are all crystals in cans hermetically sealed? Or are some crystals not, and are thus cheaper to make (and sell) ??Also, I know you can get really tiny crystals for smd, and you can get massive crystals, like this one (ok, not really massive, but by size alone, it's about 1000 times bigger than a small smd crystal.How does the crystal's size play into it's frequency and or temperature stability? I guess more thermal mass means more stable, so larger crystals are likely more temperature stable than a small smd crystal. These ovens often use large crystals, but lately I've been seeing OCXOs in smaller DIP cans like the original 4-pin DIP can oscillators.

author: JackOfVA
author_rank: Unknown
timestamp: April 28, 2013, 01:03:38 pm
content: Quote from: codeboy2k on April 28, 2013, 05:36:55 amInexpensive crystals in a metal can are usually soldered - the base and can are separate pieces and after the crystal blank is mounted, the can is soft soldered to the base. It's pretty obvious how the base is attached to the can if you look at it as there will be a solder filet around the base-can joint. If the crystal is filled with inert gas, there will usually be a small solder blob on the can to seal the hole through which the inert gas was applied.The second thing to look at is the base - the base on an inexpensive crystal will be a piece of fiberglass or phenolic impregnated paper with a metal rim. The pins go through the insulating section of the base.A high quality crystal will usually have the can and base pieces cold welded together. The base will be largely metal with glass seals where the pins go through the base.Some ultra high precision crystals use a quartz enclosure - I've only seen a couple of these as they are rare and very expensive.

author: ddavidebor
author_rank: Super Contributor
timestamp: April 28, 2013, 01:25:10 pm
content: this thing have a lot of circuitry inside!

author: ve7xen
author_rank: Super Contributor
timestamp: April 29, 2013, 05:38:42 pm
content: Quote from: ddavidebor on April 28, 2013, 01:25:10 pmYeah, I was surprised too. I wasn't expecting so much inside the inner can. I guess the ceramic hybrids are probably the oscillator itself. The rest must be just signal conditioning and the heater control, but they sure used a lot of circuitry to do it!QuoteI think you're right. The glass seals are clearly visible from inside the crystal can. It's also got that weird orange goop sealing the bottom, some kind of epoxy maybe?

author: SeanB
author_rank: Super Contributor
timestamp: April 29, 2013, 06:21:51 pm
content: The orange is a vibration dampener, it keeps the crystal leads from vibration and adding spurious noise to the output signal.

author: codeboy2k
author_rank: Super Contributor
timestamp: April 29, 2013, 08:29:06 pm
content: Quote from: SeanB on April 29, 2013, 06:21:51 pmThe physics and science of a crystal is more voodoo than an 8-1/2 digit meter

author: SeanB
author_rank: Super Contributor
timestamp: April 30, 2013, 04:25:08 pm
content: Yes, you get different results depending on drive, temp, pressure, fill gas, how it was cut, where it was mined, where in the seam it came from, how it was transported, how it was aged and how it was mounted. Then you can start to measure it.Then you get into the synthetic grown quartzes.........After all that they can be pretty good.

author: olepr01@gmail.com
author_rank: Contributor
timestamp: May 05, 2013, 07:00:24 am
content: Excellent! I have one of these, and cant really make heads or tails of it.. I was considering getting another on to tear down, just to figure out how it works, not I dont have to!Does anyone have a datasheet for this? Or any info at all? Phase-noise? What is the tuning-range (in volts and hz)?Theres also an unlabeled pin with ~6v on it next to the VFC; it seems stable, so I am assuming it is a reference voltage, for the top of the tuning range? Anyone know?Thanks!

author: gxti
author_rank: Frequent Contributor
timestamp: May 05, 2013, 06:08:52 pm
content: Yes, you can connect a potentiometer between the regulated output, VFC, and ground to get stable frequency adjust.

author: ddavidebor
author_rank: Super Contributor
timestamp: May 05, 2013, 07:16:04 pm
content: Well, you need a potentiometer with a exellent temoerature deviation.