Does the physical orientation of an OCXO affect stability and performance? - Page 1
Forum section: electronics / projects

author: enut11
author_rank: Super Contributor
timestamp: January 17, 2024, 04:24:43 am
content: I am revisiting an old 10MHz project with a view to improving its performance.I have an Oscilloquartz OCXO model 8663-XS mounted in an Extron video distribution amplifier box.My question relates to the orientation of the Oscilloquartz module. Currently it sits on its side against a vertical panel of the box.So, does this affect the oscillator performance or would it be better on the bottom of the box with the label facing upwards?enut11

author: Andy Chee
author_rank: Super Contributor
timestamp: January 17, 2024, 06:56:15 am
content: One particular orientation doesn't necessarily perform better than the other. But because orientation does affect frequency, one should calibrate and trim the OCXO in its intended operating position.

author: pqass
author_rank: Super Contributor
timestamp: January 17, 2024, 07:12:50 am
content: According to Dave (below), changing the orientation by 90° results in a 4 mHz change @10MHz (0.4ppb).But if you don't change the orientation after calibration, then altitude (gravity) for a horizontal vs. vertically-oriented xtal and barometric effects are orders of magnitude less than temperature effects.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 17, 2024, 11:15:00 am
content: you could say how the temp inside the ocxo is dissipated or how it climb thru the parts in one direction vs the otherbut we talk about some big can as senn in some video and pictures, i doubt it would be that much in smaller casingsfor sure you need a warm up too lolli would need to try that in some huge racal dana cans inside an hz counter like the 1991 1992 lollbut i would think gravity, air humidity, physical factors could intervene ? since a crystal is fundamentally a vibrating device

author: rhb
author_rank: Super Contributor
timestamp: January 17, 2024, 01:34:38 pm
content: Orientation matters because of air convection. The pair of ebay CTI OXCOs I boxed up for portable 1 ppb references drifted very noticeably when placed so the OXCO was vertical until I put a small bit of polyester batting in the cases. After that I could detect no change watching the OXCO and GPSDO trigger signal in any orientation.It's fair near impossible to tune the ebay units dead on, but once they are close you can measure frequency error very accurately by timing how long it takes for a 180 phase shift.Have Fun!Reg

author: bostonman
author_rank: Super Contributor
timestamp: January 17, 2024, 04:05:19 pm
content: Orientation would affect the g forces on the crystal because the force would push on the crystal differently. Turning while it's powered would certainly affect stability, but, if it's calibrated in one orientation, then turning will cause it to change.

author: rhb
author_rank: Super Contributor
timestamp: January 17, 2024, 04:48:18 pm
content: I've done the experiment multiple times. Have you done it once? Alternatively, can you show the mathematics to quantify the effect? Gravity is perhaps a theoretical influence, but short of a NIST or NPL level experimental demonstration, I find the assertion that the effect is significant relative to the myriad of other factors dubious.Alfred Loomis used quartz oscillators to demonstrate the effect of the moon on pendulum clocks back in the '30s. The British clockmaker Loomis visited in London made the state of the art clocks of the day, dual, temperature compensated, pendulums in a vacuum. Loomis was a wealthy former Wall St. financier. The clockmaker was on the verge of throwing Loomis out of his shop when Loomis pulled out his checkbook and said, "I'd like to buy two." The clockmaker was floored as they were made to order at 5000 pounds a piece back when a pound was many multiples of a dollar. If we use the $1 USD = 1 oz of silver basis of American coinage of the day those were$60,000 USDclocks.To put this in a modern context, a cesium clock should show the effect of the moon onanyquartz oscillator. The gravity vectors are additive.Have Fun!Reg

author: bostonman
author_rank: Super Contributor
timestamp: January 17, 2024, 04:54:01 pm
content: QuoteNo, I haven't done it (just) once, actually many years working with atomic clocks.Maybe orientating the oscillator differently won't show on typical measurement equipment, but, if we connect it to a counter referenced to a Cesium atomic clock, then the affects would be obvious.If the crystal is orientated in the X-axis direction, and a measurement is taken, then changing the orientation so it's now being affect in the Y-axis will certainly cause it to resonate slightly different.

author: fourfathom
author_rank: Super Contributor
timestamp: January 17, 2024, 06:13:55 pm
content: Do be aware that regardless of the orientation, every time you apply power to an OCXO (or any crystal oscillator) it will settle out to a different frequency, even when there is no change in temperature. This is called "retrace", and also shows up as the crystal temperature changes -- the frequency trajectory will be different for the heating vs the cooling cycles. Even in an OCXO there are temperature variations beyond the obvious start-up change.And this is also the case for the silicon MEMS oscillators (SiTime, etc.)So if you are evaluating the effects of orientation, be aware of the retrace effect as you power-cycle the thing. This is one reason I like GPSDOs -- I get to use someone else's cesium standard.FYI, here's a plot of a TCXO frequency vs temperature, where I cycle the temperature through large and small changes. You can plainly see the effects of retrace. The frequency isn't merely a function of temperature, but also of history. And no, the frequency does not eventually revert to the midpoint. The timescale on this chart is day:hour:minute:second, so these are slow cycles. I had initially suspected that there was some thermal transient issue (the xtal temperature change lagging the measured chamber temperature), but the retrace offset persisted even if I held the chamber temperature constant for many hours.

author: bostonman
author_rank: Super Contributor
timestamp: January 17, 2024, 06:20:12 pm
content: QuoteYes, you are correct.

author: enut11
author_rank: Super Contributor
timestamp: January 18, 2024, 12:21:17 am
content: Quote from: rhb on January 17, 2024, 01:34:38 pmHi@rhb. What equipment is required to measure a 180deg phase shift?

author: rhb
author_rank: Super Contributor
timestamp: January 18, 2024, 02:04:53 am
content: QuoteA 2 channel oscilloscope. You time how long it takes for the rising edge to travel from the rising edge to the falling edge of the reference signal. Half a 10 MHz square wave is 50 ns. So if it takes 100 seconds for the leading edge of one trace to reach the falling edge of the other, the periods differ by 500 ps. Very easy to do if you overlay the 2 traces.My ebay CTI OXCOs will hit 1 ppb about 10 minutes after power on. Many others have reported the same. They are better than the OXCOs in my HPAK gear. My 5386A is under power continuously. If I hook one up to the 5386A, after 10 minutes it will read the same frequency as the last time to +/- the last digit. The OXCO from China is closer to 10 MHz than the one in the 5386A because the adjustment is finer resolution. The 5386A OXCO takes much (2-5x) longer to stabilize.I'd like a citation showing the OXCO orientation variation due to gravity. Seems to me that ballistic missiles would have timing issues when launched as that's many times the acceleration of gravity. Do they put cesium clocks in ICBMs? After all, cost is not an issue.FWIW I was not power cycling the OXCO. I just set the enclosure on different faces and timed the shift. As I was pushing the GPSDO accuracy limit I did not pursue it further. The little bit of batting reduced the frequency difference to the point I didn't see any point to pursuing the matter further. I'm much more concerned about crystal aging which CTI states as 0.5 ppb after 10 years of operation and supply and control voltage noise levels.When I crunched through the CTI data sheet, the stated 0.05 ppb/s was close to the variation that the noise from an LM 399 produces. To quote a map I don't recall, "There be dragons in this region."The OXCO would need to be in a hard vacuum so that there was no possibility of air convection during a comparison of orientation vs frequency. Though there would still be the possibility of convection inside theOXCO unless it was also in a vacuum.I have bought 20x of the CTI OXCOs with the intent of seeing if 4 of them can be used to achieve 0.1 ppb/yr or better. Achieving ~ 1-2 uVrms noise levels is seriously non-trivial.I like papers with lots of math. This topic clearly requires it.Have Fun!Reg

author: bostonman
author_rank: Super Contributor
timestamp: January 18, 2024, 04:36:25 am
content: I don't have a phase comparator box or a chart recorder, else I'd try graphing the phase over time.A graph was posted above. May I ask what was used to graph the frequency?

author: fourfathom
author_rank: Super Contributor
timestamp: January 18, 2024, 05:57:24 am
content: Quote from: bostonman on January 18, 2024, 04:36:25 amThe graphs I posted? Those came from a C# program I wrote that works with a Time Interval Counter of my own design. Here's some info about that counter (and how I use it to measure very small frequency deviations quickly and accurately):http://wb6cxc.com/?p=275The C# program also interfaces to a temperature controller I made for the "Reptile Incubator" (an inexpensive TEC heating/cooling chamber) . Dave has a video about using this incubator. I ripped out the original controller and installed my own design giving me USB/serial-port control and monitoring of the chamber. It can't do temperature extremes, and the temperature ramp is quite slow, but it is still very useful.If I were smarter I would write something more generic in Python to do the graphing and control, but I already know C# and can't be bothered to get good at Python.

author: forrestc
author_rank: Unknown
timestamp: January 18, 2024, 07:08:46 am
content: Quote from: rhb on January 17, 2024, 04:48:18 pmVarious publications (including slides from a NIST presentation I attended a few years ago) indicate the changes related to rotating a crystal oscillator in a 1g field to be on the order of a couple of ppb. I'm guessing this varies depending on the specifics of a given crystal, and no, I don't have the math, although the most cited paper on this seems to be "R. L. Filler, "The Acceleration Sensitivity of Quartz Crystal Oscillators: A Review,"IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 35, No. 3, pp. 297-305, May 1988". The charts in this paper show a +4-0ppb in one axis, a +-2ppb in a second, and a +3-1ppb in a third. And lots of math regarding the effects of acceleration on crystal oscillators.But you're 100% correct; this effect is relatively minor when compared to all of the other sources of "misbehavior." Retrace (i.e., a power cycle), in particular, often has a bigger effect than turning a crystal on its side. So, if you're concerned about the accuracy of turning an oscillator on its side, you probably shouldn't be power cycling it, subjecting it to shock or vibration or any of a dozen other causes of frequency change (including letting it age).On the other hand, if you have a precisely tuned OCXO on your desk that is stable enough that a ppb of error matters, you might not want to flip it over. One example of this might be a high-quality GPSDO where the disciplining algorithm has done a good job of trimming out all the causes of aging. Flipping it over during operation will likely throw the clock out of whack until the disciplining algorithm can bring it back into tune.Now I think about it, it might be a fun experiment to tune two of my rubidium oscillators such that they are well within a ppb of each other and then flip one over. I know the physics package should eventually bring the frequency back into range, but seeing how quickly it does this would be interesting. (For those that don't know, a rubidium oscillator is often a rubidium-disciplined-OCXO - or at least the ones in my lab are designed that way).

author: ch_scr
author_rank: Frequent Contributor
timestamp: January 18, 2024, 07:33:30 am
content: Quote from: bostonman on January 18, 2024, 04:36:25 amTimelabis a free software that can do that as well, and it can interface to almost any counter in "talk only mode". Reasonably priced counters can't do time interval or phase directly for a 10MHz signal though, only for signals of lower frequency. I've tried to implement aDMTD, which mixes down the phase difference between a Ref and Dut from 10MHz to 10Hz (with the help of an 9,99999MHz "offset Ref"). The time interval / phase can than be measured precisely even with a standard 35Y old universal counter. The resolution is alotbetter than watching edges on a scope, one can seemuchslower drift, directly. Trying to "null" the drift between Ref and Dut, even a "range reduced" (flanked by 2 fixed resistors) 10-turn pot isn't enough - tilting one OCXO slightly offers amuchfiner adjustment.It's not all roses though, noise from the offset Ref get's into the measurement (so a "good" ocxo is needed there as well) and implementing the thing properly isn't exactly easy as well, I'm on Rev4, after 3 years, giving this a prod now and then, and Imightbe slowly getting there. The one Corby showedherelooksdeceptivelysimple.

author: forrestc
author_rank: Unknown
timestamp: January 18, 2024, 08:04:18 am
content: Quote from: ch_scr on January 18, 2024, 07:33:30 amOne option is also a TAPR TICC in combination with a TAPR TADD-2 (or two).The TICC is a 60ps resolution timestamping counter. It has two inputs and it works really well to compare the phase of two 1PPS-style signals. Basically it outputs a timestamp every time it sees a transition on one of it's inputs. With the TICC it gets fairly easy to do phase drift type comparisons (if the timestamps have a consistent delta, then they're not drifting in phase with each other). The output can be fed into timelab. The TICC is my preferred weapon of choice when I'm looking at the relative phase of 1PPS signals from GPS-derived time sources. And I have a bit of addition when it comes to accumulating timestamping counters.The TADD-2 is a 10mhz to 1Hz divider with around 1ps jitter. It's useful to produce a 1HZ signal which can be used with the TICC or other timestamping counters (53131A 53132A, etc). It isn't all that useful if you need absolute phase difference measurement, but if you're looking at phase drift it's definitely useful.One thing I haven't tried which seems to be somewhat the rage nowadays is that there is now firmware for the nanoVNA-H4 hardware which can apparently do phase measurements down to the sub-ns (ps?) level. Since I haven't played with it I can't vouch for how well it works... Seehttps://www.tinydevices.org/wiki/pmwiki.php?n=TinyPFA.Homepage. I need to figure out if my NanoVNA is the H4 iteration, and if not get me one to play with.

author: ch_scr
author_rank: Frequent Contributor
timestamp: January 18, 2024, 09:04:05 am
content: Oh yeah, TinyPFA is definitely the cheapest way to "tip a toe" into this. Also easier to get going, no need to set up PC software first.

author: rhb
author_rank: Super Contributor
timestamp: January 18, 2024, 03:25:01 pm
content: @forrestcMany thanks for the citation! I'm rather surprised at the size of the shifts as I was making ppb observations with my DSO and the bit of batting eliminated a very repeatable shift of several ppb when I changed the orientation.I clearly need to drop an OXCO a few times. Seriously, I have a 12 bit Owon scope with enough memory that I should be able to observe the gravity effect as a step change in frequency if I drop it a few feet onto a pillow or with some other means of gently stopping it such as a string over a pulley attached to a spring. The latter would be more repeatable. Use an electromagnet to release the OXCO just after triggering the scope with the delay equal to the fall time. I can set up a 14' drop in my shop using the pulley arrangement. That should give about 1/2 second of free fall before the deceleration spring took effect.Have Fun!Reg

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: January 18, 2024, 03:30:36 pm
content: I have both a NanoVNA H4, and a TinyPFA. They both work the same (same hardware, just load the appropriate firmware to swap functions).Be careful to only buy a NanoVNA or PFA from an authorised supplier, there are Chinese fakes around that don’t work properly… they are sold at a similar price to genuine ones!Used by itself, the display of the TinyPFA is very “busy” and hard to view. However, the free TimeLab software works fine with a TinyPFA.You need a very stable reference for one input of the TinyPFA as it produces a comparison between the inputs. I have a rubidium for reference purposes.With regards to the orientation of ovened oscillators, I used to be involved as a field systems engineer in radio systems and used both frequency counters and signal generators with ovened oscillators. It made no difference whether the instruments were horizontal or vertical, and there was no reference to orientation in their manuals.Old ovened oscillators might have loose insulation, some have the electronics packed in foam which can deteriorate over time. I have an old Racal 1998 counter that seems to occasionally jump in frequency and could do with the oven opening to investigate.The attached image shows two GPSDO 10 MHz outputs on a combined plot against a rubidium oscillator using a TinyPFA. One is a home made Lars Walenius type (red trace) and a commercial Leo Bodnar one (green trace).SJ

author: bostonman
author_rank: Super Contributor
timestamp: January 18, 2024, 03:52:47 pm
content: Everything affects a crystal and oscillator. Certainly it's less obvious in systems not requiring precise accuracy, but, when measuring PPB when referenced to a Cesium atomic clock, you begin seeing all the jumps.

author: bostonman
author_rank: Super Contributor
timestamp: January 18, 2024, 06:42:38 pm
content: When I worked in the atomic clock field, one way we measured offset: we had four channel phase boxes referenced to the house standard. They would accept anywhere from one to four units (fifth input was the reference) and would automatically switch between channels (providing it was set to auto).The output was a DC voltage that went into a strip chart recorder. We had other ways too, but this was one.