TCXOs versus OCXOs - Page 1
Forum section: electronics / beginners

author: wobbly
author_rank: Regular Contributor
timestamp: October 13, 2024, 12:20:35 pm
content: I'm contemplating building a simple OCXO that I can tinker with and try to optimise for best performance - purely as a learning exercise.My thought was to use a microcontroller running a PID controller to keep a power resistor at a fixed temperature (probably 50 °C) and thermally couple a crystal oscillator to it along with a suitable thermistor to provide feedback to the PID.I was wondering what sort of XO I should select for this project. I'm not aiming for metrology grade or anything, just something I can compare to a GPSDO using a frequency counter and attempt to get the frequencies as close as possible.Is there any benefit in using a TCXO inside the "oven"? I suspect not, because if my PID controller is any good the temperature will be damn near constant anyway, thus the TCXO isn't actually having to compensate for anything.But then again, a "normal" XO - eg, the ubiquitous metal can sort aren't designed to be "ovenised" either.I suppose a TCXOmightgive a slight advantage in the first few minutes after a cold-start - but I'm not too bothered about that. I don't mind if it takes an hour to stabilise.What would be a sensible choice here?

author: tggzzz
author_rank: Super Contributor
timestamp: October 13, 2024, 12:48:45 pm
content: If you use an XO, you will be able to see the imperfections of your OC more easily. Whether that is an advantage is your decision.I presume you have other oscillators (e.g. inside a frequency counter) that are sufficiently stable to be able to see changes in your oscillator.If you are planning on using GPS as a reference frequency, make sure the phase noise is not going to be an issue.

author: wobbly
author_rank: Regular Contributor
timestamp: October 13, 2024, 01:29:42 pm
content: Quote from: tggzzz on October 13, 2024, 12:48:45 pmGood point. Yes I think that would be useful while I'm tweaking my PID values.Quote from: tggzzz on October 13, 2024, 12:48:45 pmI have a recently restored 1984 vintage Racal-Dana 9906 with a modern TCXO as its reference. Very solid once it's up to temperature. I'll use the GPSDO to calibrate the Racal and then use the Racal as the standard for testing the OCXO I'm going to build.I've been looking at some XOs that have voltage adjustment as well as I think it would be handy to be able to carefully tune the oscillator's adjustment pin using a 10-turn pot or two (coarse & fine adjustments).

author: CaptDon
author_rank: Super Contributor
timestamp: October 13, 2024, 02:56:38 pm
content: I have a very good quality TCXO and fortunately it is trimmable with an internal 2-10pf trimmer. The unit is designed for 5VDC. I found the frequency can be pulled roughly +/- .25Hz by adjusting the VCC between 4.75 and 5.25VDC which is how I maintain compensation externally. I put the voltage regulator and some power resistors used as an ovenizing circuit as well as the thermal control circuits onto the metal body of the TCXO. It is housed in a well insulated small stainless steel box, insulated both internally and externally. I have the temperature set at 110 degrees F.It works extremely well and provides my lab reference frequency. I haven't seen the 1MHz output drift by more than one cycle of 1MHZ in a 24 hour period. I did it as a fun project and am well pleased with the results. It took several adjustments and some anticipation of how far to overshoot or undershoot the adjustments to finally get a drift factor that exceeded my expectations. The compensation adjustment is a 25 turn front panel trimmer which adjusts the VCC. The biggest drift factor of the whole unit seems to be caused by the relative temperature of the front panel trimpot.

author: edavid
author_rank: Super Contributor
timestamp: October 13, 2024, 03:05:33 pm
content: You should probably use a bare crystal rather than any kind of packaged oscillator, for more effective temperature control where it counts.Note that oven crystals are cut so the turnover point is at the oven temperature instead of room temperature.So if you really want to replicate an OCXO control algorithm, you would want to special order an oven crystal.

author: tggzzz
author_rank: Super Contributor
timestamp: October 13, 2024, 03:19:57 pm
content: Quote from: wobbly on October 13, 2024, 01:29:42 pmThe modern (CTI) devices hacksawed out of mobile basestations can be very good; there is another thread on here. For ultimate performance you need to have a low-noise voltage source driving the control voltage.

author: Phil1977
author_rank: Frequent Contributor
timestamp: October 13, 2024, 03:24:31 pm
content: Inside a well controlled oven environment the compensation circuit of the TCXO is just an error factor - it doesn't contribute to accuracy but may contribute e.g. to phase noise. There are even digital TCXOs that are of a "hit and miss"-style, meaning they just omit a few pulses per second to compensate for the temperature drift. This is okay for e.g. a clock that only shows seconds but it is more or less the worst case if you want a clean reference signal.One big DIY problem is that the scavenged OCXOs that are available from aliexpress and ebay are too cheap and too good. If I would want to get more precise then I´d put such an oscillator into an even better controlled environment. You could double-ovenize it to further reduce temperature gradient changer, you can shield it from changing mechanical stress and humidity or even air pressure changes. But probably from there on it really gets difficult to see you success - the GPDSO can be a reference for comparison only on a very slow scale.

author: wobbly
author_rank: Regular Contributor
timestamp: October 13, 2024, 03:24:38 pm
content: @CaptDon,Thank you for the insights. What kind of insulation material would you recommend?I'm thinking of using something like this for the TCXO:https://www.mouser.co.uk/ProductDetail/Fox-Abracon/FOX801BE-10.0MHZ?qs=7MCwSqnnz3KSqbSQU0NGHw%3D%3DI know that one's obsolete now but that's just an example.

author: jwet
author_rank: Frequent Contributor
timestamp: October 13, 2024, 03:34:01 pm
content: Good (only one I've seen) book on these topics is by Frerking from about 30+ yrs back. Great learning exercise. You can make something extremely stable (low phase noise) etc but without a reference, you can't make something extremely precise long term (spot on frequency). This is a big topic and there are a bunch of guys usually called "time nuts" that play with these circuits. Google "allen variance" or crystal aging and it will lead you to them.Maxim (my old employer) makes a clever TCXO and clock called the DS3231 which you might find interesting. It measures temp vs. an oven to keep power low. It has a state machine, temp measurement and look up tables internally that pushes the oscillator with capacitor arrays (digital VXO like). You end up with something like 2 ppm max over 0 to 40 C. Most people just want an accurate RTC but you can look at internal temps, raw frequency out, correction registers, cap array values, etc and even push the oscillator around yourself. BTW, the tempco of crystals is usually the shape of an inverted parabola called a "bow"- this can make driving temp as the independent variable challenging for a control system.Have Fun.

author: David Hess
author_rank: Super Contributor
timestamp: October 13, 2024, 05:44:44 pm
content: Quote from: edavid on October 13, 2024, 03:05:33 pm^ This ^OCXOs use a crystal cut (SC?) where the turnover point, that is the zero temperature coefficient point, is at the intended oven temperature. So you will never get the same performance by ovenizing a common crystal or TCXO because they have their turnover point at about 25C.What could be done however is to use a standard 25C crystal or TCXO with a small fixed heater and a thermal control loop connected to a Peltier cooler. This way the crystal or TXCO temperature can be fixed near 25C.

author: edavid
author_rank: Super Contributor
timestamp: October 13, 2024, 06:01:13 pm
content: Quote from: David Hess on October 13, 2024, 05:44:44 pmLow end OCXOs use AT cut, since it's cheaper:https://blog.bliley.com/anatomy-of-an-ocxo-oven-controlled-crystal-oscillator

author: Phil1977
author_rank: Frequent Contributor
timestamp: October 13, 2024, 06:14:29 pm
content: I think most of the integrated OCXOs (like the ubiquitous CTI parts) are AT - because it is cheaper and for many applications like communication base stations it´s good enough. A AT-OCXO is still around one order of magnitude more stable than the best TCXOs.Here is a nice comparison table about different oscillator options:https://www.meinbergglobal.com/english/specs/gpsopt.htm

author: fourfathom
author_rank: Super Contributor
timestamp: October 13, 2024, 07:53:50 pm
content: Quote from: Phil1977 on October 13, 2024, 03:24:31 pmYes! When I was designing my own frequency counter this is one thing that had me scratching my head, thinking that my digital circuitry was glitching. I used to design TCXO circuits for transmitters and receivers and these used old-fashioned analog compensation, with capacitors having various temperature coefficients. But some of the cheap TCXOs now use a digital compensation technique as Phil1977 mentions.Another issue is "retrace" which is kind of a temperature / frequency hysteresis. XO, TCXO, OCXO, silicon MEMS oscillators -- they all show this. There is also a related effectively random frequency shift that occurs when power-cycling, even at a fixed temperature.I discuss some of these issues in a presentation I gave to my local ham club (pdf:http://wb6cxc.com/wp-content/uploads/2023/03/Freq-Stability.pdf) the XO / TCXO stuff starts on page 22.

author: coppice
author_rank: Super Contributor
timestamp: October 13, 2024, 09:38:20 pm
content: Quote from: wobbly on October 13, 2024, 12:20:35 pm50C is an unusually low temperature to choose. It really restricts where the oscillator can be placed. Most commercial ovens run a good deal hotter.

author: VK4AMG
author_rank: Newbie
timestamp: October 13, 2024, 10:55:30 pm
content: Folks may find my Gippsland Microwave Conference paper of 2019https://vk4amg.blog/gpsdo-traps/ and available on YouTube(youtube.com), interesting.

author: TimFox
author_rank: Super Contributor
timestamp: October 13, 2024, 10:58:26 pm
content: Quote from: coppice on October 13, 2024, 09:38:20 pmIf I remember correctly, a typical oven temperature in olden times was 85oC.

author: fourfathom
author_rank: Super Contributor
timestamp: October 14, 2024, 04:24:18 am
content: If you wanted to build an OCXO using a TCXO running at 20 deg C nominal, instead of a heater use a TEC element. Heat or cool, as appropriate for the ambient temperature. There are better ways to make an OCXO, but this would be an interesting project.

author: BrianHG
author_rank: Super Contributor
timestamp: October 14, 2024, 05:02:15 am
content: Quote from: fourfathom on October 14, 2024, 04:24:18 amWouldn't that be running the specific TCXO's crystal optimized for low drift at a higher temperature in a cooler point where the crystal has a much larger temp/hz ppm drift?I guess if you cut open the TCXO and replace the crystal inside for a normal AT cut crystal optimized for minimum drift at room temperature to make your 20 degree design most stable.

author: jonpaul
author_rank: Super Contributor
timestamp: October 14, 2024, 05:32:28 am
content: crystal cut determines turnover temp and need for oven or not,see AT, SC, etc.https://blog.bliley.com/crystal-cut-types-at-vs-scBest ref is a GPS DSO, we use Leo Bodnar.https://leobodnar.com/shop/index.php?main_page=product_info&products_id=234No need for any xtals....j

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: October 14, 2024, 06:53:40 am
content: Quote from: wobbly on October 13, 2024, 12:20:35 pmThere's no need for a uC. That involves firmware development and gets the project too messy with adjusting the three coefficients as well as starting and debugging the code. All it needs is a heater and thermistor in the oven and an op-amp outside of it with a pot to control the temp. No matter how you do it, the original problem of determining the best operating temp remains. I built a synthesized signal source with home-built OCXO that ran at 20 MHz. That was then div by 2 to get a square 10 MHz that I could use for other things in my little shop.I determined the best temp by putting the oscillator into a counter (referenced to a fairly stable 2ndary source) and recording the frequency out of the oven as I increased the temp control by small amounts each time. I let it settle for 15 min each step. I ran the temp upwards and downwards this way and saw that the frequency tracked the curve coming down as well as going up.It was not important just WHAT frequency was being generated but how it changed as the temp was ramped up or down. The curve I got was part of the classic T-F dependancy curve with two turnover frequencies and an inflection point between them. You can see that dependancy here:https://www.jauch.com/blog/en/ask-the-engineer-how-temperature-sensitive-are-quartz-crystals/By adjusting the temp control to the most horizontal point you get on your measurements then you have the optimum temp and you are done.My schematic for the oscillator is below.

author: fourfathom
author_rank: Super Contributor
timestamp: October 14, 2024, 06:55:19 am
content: Quote from: BrianHG on October 14, 2024, 05:02:15 amI was thinking of the typical TCXO, which (unlike an OCXO) probably uses an AT cut crystal and the temperature compensation components are designed to operate around room temperature. That's why I suggested the TEC heater/cooler -- to hold the TCXO at "room temperature".This is not going to be a great OXCO, but the OP did say:QuoteAnd my suggestion was with that in mind. Yes, for better performance you can get a surplus OCXO, or a GPSDO, or a Rubidium clock (I've several of each of these). I have a friend with Cesium references, but I'm notthatmuch of a "time nut".

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: October 14, 2024, 09:56:24 am
content: An alternative method of improving the compensation of a TCXO is to measure the temperature close to it and apply a correction. This idea is used as a means of reducing the drift on an Elecraft KX3 amateur radio transceiver. The KX3 has a basic 1ppm TCXO fitted.Within the KX3 is a temperature sensor which can be read by the internal processor. You calibrate the system by placing the radio in a fridge, then bring it up to room temperature and finally heat it with a hair dryer, all the while the processor measures the frequency of an external stable signal and stores the offset needed for every degree C of temperature. The routine is covered in this document:https://ftp.elecraft.com/KX3/Mod%20Notes%20Alerts/KX3%20Custom%20VFO%20TC%20rev%20A9.pdfThe end result is a noticeable improvement in stability, which I particularly noticed when I used one extensively in Spain during their hot summers.SJ

author: CaptDon
author_rank: Super Contributor
timestamp: October 14, 2024, 01:39:01 pm
content: Here is some additional data for my homebrew ovenized TCXO. Original device,Microsonics Weymouth, MassModel K1085A-1T1D1.000 MHzSet at 25C 0.0HzSer# 074I bought two of these many years ago. The one that I ovenized is running at 110F and is wrapped in about 2" thick fiberglass attic insulation, then is inside a stainless steel container which is also wrapped in about 1" thick insulation. I think my heater resistors which are thermal epoxied onto the TCXO create 3 watts of heat peak and are controlled by a TMP01 I.C. I show an average 'On' time of 2 seconds out of every 15 seconds for about .38 watts of average heater power. The TCXO itself is marked 27ma. @ 5vdc. You can observe slight phase shift during heater cycling but over a 24 hour period the total gain or loss is about 1 cycle. It did take several small adjustments over a couple of days to achieve final stability.

author: fourfathom
author_rank: Super Contributor
timestamp: October 14, 2024, 04:03:00 pm
content: Quote from: Solder_Junkie on October 14, 2024, 09:56:24 amThis can indeed improve the frequency stability, but I was trying this and that's when I ran into the "retrace" phenomenon. This looks like a frequency hysteresis as the temperature changes. At first thought I was seeing differences in heating and cooling time delays between the oscillator and the temperature sensor, but no matter how closely I coupled them, or slowed the temperature change rate, the effect remained. All crystal (and silicon MEMS) oscillators exhibit this.Here are some measurements I made when trying to stabilize the TCXO on a little transceiver (I ended up designing a GPSDO-driven clock multiplier to replace the TCXO):link:http://wb6cxc.com/wp-content/uploads/2023/01/TCXO-QDX-no-case-idle-3.pngThe upper chart shows freq vs temperature. The oscillator is 25 MHz, but my homebrew counter includes a digital mixer that here is effectively shifting the signal under test down to 100 Hz, so add 24,999,900 Hz to the displayed value.The middle chart shows frequency vs time.The lower chart shows oven temperature (deg C) vs time (day:hour:minute:second). The chamber is one of the TEC "Reptile Incubators" that Dave mentioned on one of his videos). I ripped out the controller guts and put in my own so I could automate my measurements.

author: jwet
author_rank: Frequent Contributor
timestamp: October 14, 2024, 04:57:16 pm
content: From what I've read, hysteresis is what limits how good you can get with all these techniques. Few things have < 1 in 10^7 kind of "non" hysteresis.The reference I gave above bears this out. It looks like the author did a lot of consulting for Army ETDL in Ft. Monmouth, NJ. Pretty unique content, covers a lot ground including uP based techniques (using a toaster size computer with an 8080 in with assembly code listings!). Its available as a free PDF online. Here's a hopefully old but working link to the PDF. You can buy it on Amazon and elsewhere pretty cheap.www.introni.it/pdf/Frerking%20-%20Crystal%20oscillator%20Design%20and%20Temperature%20Compensation%201978.pdfCheersJohn

author: wobbly
author_rank: Regular Contributor
timestamp: October 14, 2024, 05:01:26 pm
content: Thank you to all who have kindly proffered information and suggestions, I appreciate your time.I think it is apparent that I have massively underestimated the difficulty of making an OCXO. I need to do a lot of research on this.

author: fourfathom
author_rank: Super Contributor
timestamp: October 14, 2024, 05:14:11 pm
content: Quote from: wobbly on October 14, 2024, 05:01:26 pmIt's easy to make an OCXO. The hard part is figuring out your actual requirements and meeting those. For many applications, factors like retrace are insignificant. For others, we start looking for other sources (rubidium, cesium, GPS, and the emerging (and amazing) technology being developed at the NIST.Wander and jitter / phase noise are other factors to consider. Building your own GPSDO is also instructive, and you get to look at the effects of update rate in phase and frequency feedback loops as well as oscillator stability and noise. But if you just want to have a fairly inexpensive and pretty damn good clock source, just get a Bodnar. It's not perfect, but nothing is so I always remember my design motto: "good enough is perfect".

author: wobbly
author_rank: Regular Contributor
timestamp: October 14, 2024, 06:38:22 pm
content: I do have a Bodnar in factThe small "Mini" unit. This evening I've been using it to dial-in my Racal-Dana 9906's home-made 5 MHz internal reference.My 9906 is quite extensively modified*but I've managed somehow to get it to measure the Bodnar's 10 MHz output (Channel A, set to "HF" mode with a gate of 100 seconds). Here's the data output from the Racal's serial output...Code:FreqGate: 100 secOVERFLOW000.00000FreqGate: 100 secOVERFLOW000.00000FreqGate: 100 secOVERFLOW000.00000... where 000.00000 actually means 10,000,000.00000 Hz because the Racal only has 8 physical digits on it's display (hence "OVERFLOW").And as I type this, it's just rolled down to (99999)999.99999 Hz due to slight variation in room temp.I'm very pleased as I've never had it this close to perfect before.Perhaps I don't need an OCXO after all!---*modifications to the Racal-Dana 9906:1. Tear-down and scrubbed clean of cigarette smoke from filthy previous owner!2. Installed silent fan to provide internal air circulation (the display drive circuitry was running hot).3. Replaced the thermally unstable original 5 MHz internal reference with a modern TCXO with filtering and signal shaping to mimic original.4. (My favourite) - serial data output using the built-in rear proprietary data port. Built from scratch from the datasheet. I have not seen anyone on the internet with another of these.

author: fourfathom
author_rank: Super Contributor
timestamp: October 14, 2024, 07:11:42 pm
content: My freq counter is/was a 1970's vintage HP5326A, with nixie tubes and a good OCXO (as well as an external reference input). It served me well, and then a couple of the nixie displays went bad. Since I wanted more capabilities in my counter, rather than trying to repair the HP I designed and built my own counter, using an FPGA as the guts, as well as a SAMD uController for the period to frequency calculations, as well as the FPGA and user interface management. It's a four-channel timestamper with 10ns resolution, so I can do all types of multichannel measurements and comparisons. As I mentioned, I also included (in the FPGA) a digital local oscillator and mixer / filter, for down conversion of the inputs, giving me fast measurements at good resolution. I describe the thing here:http://wb6cxc.com/?p=275My counter has an internal TCXO for casual use, but for more precision I connect an external OCXO or GPSDO.

author: wobbly
author_rank: Regular Contributor
timestamp: October 16, 2024, 03:42:13 pm
content: I think I might be getting thetime-nutaffliction.About 2 years ago I built a very primitive 10 MHz reference from parts I found for cheap on the interwebs. A cheap sealed-can OCXO (definitely a part taken from a recycled product). I added an ATTiny85 to measure the temperature of the metal can and tell me when it's temperature is stable (light turns on when the ATTiny measures the exact same temperature for 60 seconds in a row).I have spent the last 3 days running my Leo Bodnar GPSDO into my Racal Dana 9906 UCT to calibrate the Racal to 10 MHz. I am happy to trust the Bodnar to be a perfect 10 MHz source after a few hours of uptime. The results are fantastic - The Racal is as solid as a rock.And then I swapped from the Bodnar to my home-brewed 10 MHz gizmo and I was astonished to find that (after several more hours of settling and warm-up time) the frequency was onlyabout 30 micro-Hertz off from the Bodnar!.This will impress nobody I'm sure, but as far as my feeble electronics career goes, this is almost as happy as I've ever been(Shout-out to the Racal-Dana 9906 by the way. What a unit!).Isn't technology AMAZING?!

author: tggzzz
author_rank: Super Contributor
timestamp: October 16, 2024, 04:16:50 pm
content: Quote from: wobbly on October 16, 2024, 03:42:13 pmGood for youYes indeed tech is amazing - and that we can afford it without a bank loan!Scopes: power-vs-time. Spectrum analysers: power-vs-frequency. But what's the instrument for frequency-vs-time? A Modulation Domain Analyser or Time Interval Analyser, "of course".My HP/Agilent 53310A is no longer made, and has two very annoying features: while a decimal pointisdisplayed in frequencies, a comma separator isn't. With most instruments that doesn't really matter, but when the displayed frequency is 10.0000000020MHz, it can be difficult to work out the significance of the "2"Have a look at some of the screenshots inhttps://www.eevblog.com/forum/testgear/hpagilent-53310a-modulation-domain-analyser-frequency-time-interval-analyser/msg3194292/#msg3194292The other irritation is that is doesn't display Allen variationContinue to have fun; many people here will gently and insidiously encourage you towards ever more ridiculously extreme goalsStart with the TEA thread.