Need ideas to derive 10 MHz standard from 26 MHz OCXO - Page 1
Forum section: electronics / projects

author: wb0gaz
author_rank: Frequent Contributor
timestamp: August 16, 2016, 12:42:13 pm
content: Given a 26 MHz OCXO module (pletronics), I'd like to derive a 10 MHz standard signal for use with a couple of test instruments that have external reference in.The catch is that I'd like to preserve whatever spectral purity is present in the 26 MHz signal (my assumption is that it's "good enough", maybe not valid but that's my going-in assumption), as the resulting 10 MHz reference would be used for signal analysis and synthesis, not just frequency counting.Approaches I've considered -1. Phase locked loop synthesizer with 2 MHz (26/13) reference - phase noise?2. Divide then multiply (26 feeding / 13 then feeding * 5 and filtered at 10 MHz) - spurs?3. Pulse-swallowing divider/prescaler (26 divided by 1.3 then divided by 2) - how to design the divide-by-1.3 stage? output jitter/spurs?4. DDS with 26 MHz reference and 10 MHz output - spurs?The motivation is this - the 26 MHz OCXO (DIP size, with frequency control input) was cheap at the time, and 10 MHz OCXO (bought new) is much more expensive. Besides, it's an interesting challenge.Any suggestions?

author: CJay
author_rank: Super Contributor
timestamp: August 16, 2016, 01:47:13 pm
content: There's an old joke which, I think, shows the whimsy and humour of the Irish.A tourist on a driving holiday in Southern Ireland becomes lost so when he spies a crusty old farmer leaning on a gate watching his cows he decides to stop and for directions to Donegal.The farmer ponders for a few minutes, strokes his chin and answers 'Well, if I wanted to get to Donegal, I wouldn't start from here.'One of the time nuts will be along in a while to explain a method which will get you to Donegal but it will likely be circuitous, have multiple potential problems, possibly involving crocodiles and volcanoes.Meanwhile, I'd have quietly hopped off and bought a 10MHz OCXO from a source that tests them and offers a guarantee.

author: bobaruni
author_rank: Regular Contributor
timestamp: August 16, 2016, 01:50:40 pm
content: That's going to be difficult, no matter what you do, you will get jitter and noise.The only idea I have is if you build a low noise multiplier tuned at the 5th harmonic (130Mhz) then divide it down by 13 (this is the first possible integer division) you will have a phase and frequency locked signal without most of the drawbacks of DDS and PLL but you will still have some impurity although dividing by 13 will alleviate some of that.

author: MosherIV
author_rank: Super Contributor
timestamp: August 16, 2016, 02:06:38 pm
content: The only thing I can come up with is to clock a STM32 microcontroller and use one of the timer peripherals.Clock the timer peripheral directly from the clock source (the default is to use the PLL higher freq internally generated clock).Use the pre-scaler to divide down.Set the timer to output on a IO pin.The problem with this is that the output is a square wave.

author: Scrts
author_rank: Frequent Contributor
timestamp: August 16, 2016, 02:24:05 pm
content: Check SiLabs production:http://www.silabs.com/products/timing/clock-generator/si53xx/pages/si53xx.aspxorhttp://www.silabs.com/products/timing/clock-generator/si533x/pages/si533x.aspx

author: Kleinstein
author_rank: Super Contributor
timestamp: August 16, 2016, 05:11:21 pm
content: For a PLL the phase noise depends on the components. With a VCXO phase noise can be rather low. Wide band it's the VCO (thus use a VCXO) and close in it's the reference phase comparator. With a low cross over the phase comparator noise can get really low.One could still use the old way of synchronizing a 10 MHz (crystal) oscillator by adding a little of the 26 MHz and than have an extra filter to remove the allready small 26 MHz residual.The µC Version is very simple, but phase noise of the chip internal PLL (VCO) is usually not that great.

author: edavid
author_rank: Super Contributor
timestamp: August 16, 2016, 05:51:13 pm
content: Quote from: Kleinstein on August 16, 2016, 05:11:21 pmBut if he has to buy a 10MHz VCXO, the project makes no senseOP, have you priced a 10MHz OCXO?http://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_sop=15&_nkw=10mhz+ocxo&rt=nc&LH_BIN=1

author: CJay
author_rank: Super Contributor
timestamp: August 16, 2016, 05:59:29 pm
content: Quote from: edavid on August 16, 2016, 05:51:13 pmHave you seen the state of those OCXOs? No way would I trust one of them to even oscillate, let alone be anywhere near reliable.While I would buy an OCXO I would pay a bit more to get one with a warranty.

author: edavid
author_rank: Super Contributor
timestamp: August 16, 2016, 06:07:57 pm
content: Quote from: CJay on August 16, 2016, 05:59:29 pmI have bought several used OCXOs from China, and so far they have all arrived in good electrical and mechanical condition. You can see from the seller feedback that others customers are happy also. As with any eBay purchase, it's a good idea to test them on arrival though.(And where could you buy an OCXO, where you would get a better warranty than eBay's Buyer Protection?)

author: CJay
author_rank: Super Contributor
timestamp: August 16, 2016, 07:09:29 pm
content: Quote from: edavid on August 16, 2016, 06:07:57 pmI've bought lots of things from China, apart from some obviously fake transistors (genuine pic, completely different when arrived) and some misdescribed RAM I've never had a problem but those OCXOs look like they've been removed from boards with a hammer and chisel.Given the way they're removed from boards (guillotined out in some cases) and handled afterwards the will have suffered high degees of mechanical shock which can severely affect their performance.I'm sure the eBay Buyer Protection works for some but I've lost outt twice now and do not trust it.

author: edavid
author_rank: Super Contributor
timestamp: August 16, 2016, 07:14:19 pm
content: Quote from: CJay on August 16, 2016, 07:09:29 pmAs I said, the OCXOs I have purchased have all been fine, with no chisel marks.Anyway, what's wrong with this one?http://www.ebay.com/itm/MORION-MV197-10Mhz-OCXO-OSCILLATOR-SQU-ARE-WAVE-SC-CUT-12V-/290755604472

author: dannyf
author_rank: Super Contributor
timestamp: August 16, 2016, 10:10:40 pm
content: If you are willing to relax on phase noise, there are a few ways to do it.With that said, it is quite easy to derive a low frequency signal, like 1pps, from pretty much any frequency source, like 26mhz.

author: alank2
author_rank: Super Contributor
timestamp: August 16, 2016, 11:17:25 pm
content: This is on eBay right now:http://www.ebay.com/itm/Isotemp-OCXO-23-11-10MHz-Sine-Wave-Oscillator-5V-/171831243622?hash=item2801f0b366:g:mtIAAOSwMmBVhFD1I have a few of the 26 MHz ocxo's you have and while it was very jittery, I once made some code for an AVR for a tiny25 - I think I got it to work but I can't recall for sure without counting up the clocks!Code:.include "tn25def.inc"out TCCR0A,0sbi DDRB,1sbi PORTB,1;haltloop:cbi PORTB,1sbi PORTB,1cbi PORTB,1sbi PORTB,1rjmp loop

author: David Hess
author_rank: Super Contributor
timestamp: August 17, 2016, 10:53:41 pm
content: Is there something special about the 26 MHz source? If so I would phase lock a 10 MHz crystal oscillator to 26 MHz / 13. Otherwise I would use an independent 10 MHz oscillator.Test instruments usually phase lock their internal oscillator to the external 10 MHz reference so the phase noise of the source is not as important as the frequency accuracy.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: August 18, 2016, 02:28:34 am
content: With exception of CJay (wow, what a waste!), I appreciate the various replies and ideas!Dave

author: CJay
author_rank: Super Contributor
timestamp: August 18, 2016, 06:53:09 am
content: Quote from: wb0gaz on August 18, 2016, 02:28:34 amMy apologies for trying to inject a little humour into an obviously humourless subject.There are ways to get to 10MHz, as others have said, you could use a dual modulus PLL to get pretty damn close, similarly you could also use a DDS clocked from it, similarly the Si535x and Si570 series chips are useful, along with numerous other 'clock generators', but they all have their own problems and hardly worth the effort if you can buy a cheap, second hand OCXO at the correct frequency which will outperform anything you can build on the bench because you run into all sorts of jitter problems which can take weeks to even identify, let alone fix.IMHO, far better solved with a cheap OCXO of the correct frequency, but the timenuts mailing list is rather scathing of the eBay sellers and having looked at the first couple of pages of results I am totally with them, hence I suggested buying fom a source that offers a guarantee and doesn't require you to ship things back to China.It'll save you weeks of unnecessary pain if all you're trying to do it run a few bench instruments.

author: dannyf
author_rank: Super Contributor
timestamp: August 18, 2016, 12:41:42 pm
content: 'once made some code for an AVR for a tiny25'Two main draw backs:1. It is a luck to hit the desired frequency.2. The loop overhead will introduce jitter.I recently looked at a MCU that has a fractional pll. I don't recall what it is now, unfortunately.

author: MosherIV
author_rank: Super Contributor
timestamp: August 18, 2016, 12:45:21 pm
content: More of a question for than a replay.Since I seem to be more orientated to digital based circuits and I cannot for the life of me figure this outGoogle did not seem to help, not directly as least.How would you do a divide by 13 circuit please?

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 18, 2016, 12:49:51 pm
content: Quote from: MosherIV on August 18, 2016, 12:45:21 pmCount 13 input pulses, toggle output, repeat.

author: rstofer
author_rank: Super Contributor
timestamp: August 18, 2016, 01:49:33 pm
content: In the world of FPGAs, it is common to have to create clocks of various frequencies. It's done all the time.Suppose you put your 26 MHz into an FPGA (slow by FPGA standards) and use one of the Digital Clock Manager features to multiply the clock up to 260 MHz and then divide it back down to 10 MHz. This is no big deal. Then all you do is bring the 10 MHz clock out of the device and you're done.Practically, it is necessary to do a bit of homework. The clock manager doesn't like low frequencies in the PLL portion of the circuit. It may be necessary to multiply by a bigger number to get the frequency high enough and then divide by a bigger number to get the result. Instead of 260 MHz, it may have to be 520 MHz or something like that.Using an FPGA will absolutely work. But it's pricey! Chapter 3 here:http://www.xilinx.com/support/documentation/user_guides/ug472_7Series_Clocking.pdfThere are clock manager chips that do the same thing. I had one on an old Spartan II FPGA board years ago. I don't seem to be able to find it via Google. Maybe it's a part of history.You don't want to divide by an odd number, the duty cycle will never be 50%. Dividing by 26 works because you first divide by 13 (which won't be symmetric) and then divide by 2 to clean it up.Just buy another OCXO! This is a problem that doesn't need solving. Unless you can find that clock manager chip...

author: edavid
author_rank: Super Contributor
timestamp: August 18, 2016, 03:08:14 pm
content: Quote from: CJay on August 18, 2016, 06:53:09 amIf you buy a new OCXO from a distributor, and it doesn't work, it will be a hassle to return it, if it's even possible, and you will have to pay return shipping.If you buy an OCXO from eBay, and it doesn't work, you just have to file a claim online, and you will get your money back painlessly, without returning it.

author: CJay
author_rank: Super Contributor
timestamp: August 18, 2016, 05:34:08 pm
content: Quote from: edavid on August 18, 2016, 03:08:14 pmYou have a vastly different experience to me. I've had two sets of dodgy parts from eBay and twice I've lost money (only around £15 or, I suppose, about $25) as well as having to return them. Maybe the rules are different in the US?

author: C
author_rank: Super Contributor
timestamp: August 18, 2016, 07:02:54 pm
content: How about feeding the 26 Mhz to a mixer.Use 16 or 36 for other input to mixer.The difference is 10 Mhz via a low pass filter.The 36 Mhz would be better choice due to low pass filter.Using a switch based mixer the 16 or 36 is digital and could be created.The question then Is a mixer output followed by low pass better then creating 10 Mhz directly?

author: grumpydoc
author_rank: Super Contributor
timestamp: August 18, 2016, 07:51:16 pm
content: Quote from: CJay on August 18, 2016, 05:34:08 pmFixed that for you

author: dannyf
author_rank: Super Contributor
timestamp: August 18, 2016, 08:46:41 pm
content: A decade counter to 13 and resets itself.Dividing by 2.6 as requested by the OP, can be a struggle.

author: rstofer
author_rank: Super Contributor
timestamp: August 18, 2016, 09:08:50 pm
content: Finding a 4 bit counter with synchronous reset is no problem. Just decode state = 12 and use it as the reset signal. Dividing by 13 is pretty easy. It's the multiplying by 10 that's the problem. Multiply by 10, divide by 13 and then divide by 2 - nice clean square wave.http://www.nxp.com/documents/data_sheet/74LVC163.pdf

author: chris_leyson
author_rank: Super Contributor
timestamp: August 18, 2016, 09:28:44 pm
content: If you want to preserve the OCXO phase noise, I wouldn't bother trying to derive 10MHz from 26MHz unless of course the phase noise is really bad and then you might have a chance. Maybe divide by 13 and multiply by 5. Theoretically the divider will reduce the phase noise but that all depends on the logic family you use, faster is better as a general rule. Then you have to consider the buffer amplifier between the OCXO and divider, how much pase noise does that introduce and the 5x frequency multiplier will increase the phase noise. You have to have really clean power supplies to avoid the AM to FM conversion in anything non linear and the best I could do with minimal component count a small AT cut crystal and a divide by 4 divider was -150dBC/rootHz at 1kHz offset. A CT cut crystal would have been better but bonkers because of the cost. What is the phase noise spec for your 26MHz OCXO ?

author: chris_leyson
author_rank: Super Contributor
timestamp: August 18, 2016, 10:07:07 pm
content: QuoteAgreed, 3x, 5x, 7x and 9x is easier, Fourier, but a theoretically perfect 5x multiplier will increase the phase noise by at least 5x. If I needed a low phase noise 10MHz source I wouldn't even consider PLLs, multipliers or mixers, I would just buy a 10MHz oscillator with the required phase noise spec or build one, a Schottky clamped Pierce is a good place to start. Why is phase noise so important anyway it's not like you've got cosited transmitters and receivers, if it's just a standard for frequency measurement then phase noise doesn't matter, it will average out.

author: katzohki
author_rank: Frequent Contributor
timestamp: August 18, 2016, 11:59:53 pm
content: If you don't mind telling us, what is the end use for this 10MHz? Perhaps that would help narrow down what you really need? My guess is radio, judging by your user name.Sorry to jump on the eBay bandwagon, but here is the results of mine in the image. Also tested with spectrum analyzer. There are a lot of decent shops (sure, avoid China if you're nervous) in the US and more recently Israel. It was an Oscilloquartz. They look worse for wear, but the OCXOs tend to hold up to some abuse pretty well from my experience.Edit:To add, many of these units have tuning inputs available and so even if it is a little off on receipt, you should be able to finely tune it in. Depending on the accuracy of your equipment, of course.

author: bingo600
author_rank: Super Contributor
timestamp: August 19, 2016, 08:01:54 am
content: I'd try a STM32 as mentioned earlier in this thread , a cheap tssop20 F0'PLL: Divide by 13 , Mul by 10 --> SysClock 20MhzSet up a timer for divide by 2 , and you got a 10Mhz square.You would get some jitter , and maybe temperature variations.But it's cheap , and maybe even fun.You'd have to adjust the 5v out of the pletronics to 3.3v before feeding it to the STM.The real way us a 10Mhz OCXO as suggested by others here./BingoPs: I have 5 of those Pletronics 26Mhz , and they have nice specs (even efc) - see J.Beales notes attached

author: dannyf
author_rank: Super Contributor
timestamp: August 19, 2016, 07:30:41 pm
content: QuoteIt was PIC32MX1xx/2xx's refernce oscillator. It allows a divider with a M/1024 fraction -> not exactly useful here but may be for other frequencies.

author: German_EE
author_rank: Super Contributor
timestamp: August 20, 2016, 07:35:43 pm
content: Feed your OXCO output into a two way power divider:Output 1 is divided by 2 to give 13 MHzOutput 2 is divided by 13 to give 2 MHz then by 2 to give 1 MHzThe 13 MHz signal is mixed with the 2 MHz signal to give 11 MHz and then passed through a 11 MHz band-pass filter. This signal is then mixed with the 1 MHz signal to give 10 MHz and passed through a second band pass filter to give the signal you want.However........................The output will be of very low quality even if you use the best filters available.A DDS is not an option since 10 MHz is very close to the 13 MHz Nyquist limit when using a 26 MHz clock

author: rstofer
author_rank: Super Contributor
timestamp: August 21, 2016, 02:06:51 am
content: ICS525R-02 but I leave the calculation to others. There used to be online calculatorshttp://www.mouser.com/ProductDetail/IDT/525R-02ILF/?qs=SmUuHNCnblrPufgFV3bBXw%3D%3DI'm not sure this will work but i remember using it on a Spartan II FPGA to derive the baud rate clock.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: August 22, 2016, 12:37:14 am
content: OK, I'm convinced that using 26 MHz OXCO (it is a part that is/was used in GSM cellular base stations which for whatever reason have 13 MHz as a core frequency) is not suitable for my application, given that I want to preserve spectral purity of the OCXO (it will be used as a non-disciplined timebase for some RF test gear). Complexity of the numerous alternatives seem inevitably likely to degrade phase noise significantly, which is not desirable for this application.I'll pursue 10 MHz OXCO alternatives.Thanks for all of the discussion!Dave

author: CJay
author_rank: Super Contributor
timestamp: August 22, 2016, 09:38:05 am
content: Quote from: German_EE on August 20, 2016, 07:35:43 pmunless of course the DDS has an internal multiplier, the AD9851 for instances has a 6x multiplier which would make the Nyquist limit 78MHz.But of course, it'd still need filtering and it'd not be anywhere near as clean as the original OCXO.

author: German_EE
author_rank: Super Contributor
timestamp: August 22, 2016, 05:37:01 pm
content: Just figured out a way to do it:1) A 10 MHz VCO is divided down to 1 Hz and used as the timebase for a frequency counter.2) The input of the counter is the 26 MHz OCXO3) If the VCO is on frequency it will count to 26,000,000 or 18CBA80 Hex, if the VCO is low in frequency the count will be higher and if the VCO is high then the count will be low.4) Feed the counter output into a CPU that has a built in DAC to do the math, feed the DAC into the VCO

author: hli
author_rank: Frequent Contributor
timestamp: August 22, 2016, 06:02:16 pm
content: TI has some nice PLL Clock Synthesizer / Multiplier / Dividers, for example theCDCE706. They run their internal PLL with a n:m divider from the input frequency, which is what you seem to need.

author: alank2
author_rank: Super Contributor
timestamp: August 23, 2016, 06:38:57 pm
content: So I received the two 10 Mhz ocxo's I ordered from eBay and they are fantastic. They were quite a bit larger than I thought they would be, I guess when they wrote "cent." on the diagram they mean center not centimeter. All measurements are in inches! Anyway, really nice part. Starts out using 275mA @ 5V and eventually drops to 140-150mA. The metal screw on it is just a cover - removing it exposes a tiny internal multiturn screw to adjust which is quite fine. I connected my Rb output to the trigger input on my scope and was able to adjust it so the wave would barely move left or right. It takes about 8 minutes for it to warm up, but once it does, it is very nice. BTW, don't bother trying to offer anything less than $15 on it. I tried $10 and was ignored for 2 days and then $12 and was ignored for 2 days even after sending a message. I don't get the idea that they are too familiar with eBay and how to use it perhaps. But...it will take an offer of $15 immediately which is half price. I ordered 3 more today so I am good to go on 10 MHz ocxo's.

author: edavid
author_rank: Super Contributor
timestamp: August 23, 2016, 07:34:35 pm
content: Quote from: alank2 on August 23, 2016, 06:38:57 pmHe is talking about these new OCXOs being sold by Isotemp Research:http://www.ebay.com/itm/Isotemp-OCXO-23-11-10MHz-Sine-Wave-Oscillator-5V-/171831243622Great deal for people who don't want to take a chance on used parts (but note that there is no warranty beyond eBay's buyer protection).Also, it doesn't have a VFC input, so you can't use it for e.g. a GPSDO.

author: alank2
author_rank: Super Contributor
timestamp: August 23, 2016, 08:10:42 pm
content: Quote from: edavid on August 23, 2016, 07:34:35 pmThis is correct, there is no steering it other than the multiturn pot.

author: CJay
author_rank: Super Contributor
timestamp: August 23, 2016, 09:28:49 pm
content: Quote from: German_EE on August 22, 2016, 05:37:01 pmSimilar concept to this one:http://g4oep.netii.net/gsppll/gpspll.html