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| Injection locking the 10Mhz OCXO to external reference (upgrading a FY6600) |
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| Johnny B Good:
--- Quote from: Labrat101 on May 27, 2020, 08:00:45 pm ---Yup. I replaced that resistor yes i know it was a 33ohm . I just used a 1% 33ohm 1/8w cut one wire to about 1/4inch bent it at right angles and soldered it onto the chip leg and the the remains of the track going to the were the old xtal was . and left the resistor vertical. going to my tcxo . temp job .. it works fine ^-^ .. Also your board layout is a bit different to my one nothing major . Did you noticed those empty pins on the cyclone they are all clock inputs to the internal PLL . I was reading though some of the manual for this chip .. Then I suddenly realized its a Clone on your photo it shows the same number as far as i can make out . There should be 4 banks of PLL with xtal switching incorporated I was hoping to see if I could use one of the other inputs. the input that is used now is clk 1 n0 the next one up is switchable with a 180 phase shift . I am not sure you can see it on your scope . in glitch mode I can see the 4ns jitter is Negative so we need a positive to cancel . In the documents it says you can have 2 xtal inputs and the internal DAC will flick between the inputs . they are mark n & p This is a little out of my field of knowledge . I might try it .. What can Happen .. Rub lamp .. Get 3 wishes ... Poof ... 1. Magic Smoke ... 2. It will work .. 3. Does nothing. .. --- End quote --- I'd been a little concerned that you might have misidentified that resistor. I was just a little bit puzzled that it had been placed so far from the XO's output pin when I looked at those photos since they're often used as a 'stopper resistor' to prevent unwanted feedback or instability issues (I found I needed to connect to those 3N502s's output pins with a 100 to 330 ohm resistor on my solderless breadboard layouts to stop them oscillating at hundreds of MHz). That Cyclone FPGA IC is a closed book as far as I'm concerned (I'd had a look at its data sheet and my eyes glazed over (MEGO) after just a few dozen pages of a hundred or more page document). That's way too much complexity packed into just one LSI chip for me to get my head around these days (or even back in the good old days of Z80 cpus and their support chips when I still had sufficient motivation to study their inner workings). Kit like that is normally programmed/configured using software tools (I don't have) to do all the donkeywork. I doubt even those who are programming them (FeelElec for one) fully understand their inner workings so I see little point in trying to nibble away at the edges of this technology at an amateur level whilst there are plenty of other aspects of the FY6600 that I do understand and can do something about. --- Quote from: Labrat101 on May 27, 2020, 08:00:45 pm ---... My power rails are all showing no noise even on the scope lowest settings. I have Tantrum caps on the 78xxs --- End quote --- Well, decoupling caps around these Old Skool voltage regulator ICs are an essential requirement (tants or ceramics) and, considering the heat penalty you're paying for the privilege of low noise, I should bloody well hope you're not seeing any noise on their outputs. ;) --- Quote from: Labrat101 on May 27, 2020, 08:00:45 pm --- ... Your SMPS looks so clean after a year or so .. My one looked more crispy . Morning toast.. --- End quote --- I'm surprised you think so after all the experimental mods I've subjected it to over its first 12 months of my stewardship. That last and final (I'm almost sure it's the final mod ::)) with the single turn buck winding to effectively reduce the 5v secondary by one turn to efficiently boost the 12v rails without cooking the logic circuitry and the three LDOs close to the XO chip location was done some six months ago. I don't think there's much more I can do now to improve it. Any further improvements I'll be attempting will be along the lines of a replacement PSU. --- Quote from: Labrat101 on May 27, 2020, 08:00:45 pm ---... Nice photos . I am glade you liked my transformer recommendation ,I thought that might put a smile on your face .. I was born in Kingston Apon Thames. there are a lot of good transformer manufacturers around in the UK I would go for a toroidal . The R core that come from Japan are good . the one you are looking at is from china there is a big difference Q factor. and it was 18 quid .. buy local. --- End quote --- After reading your comments on R-Core and toroidal type transformers, I did an Ebay search to check pricing and availability of toroidal transformers. I found a UK supplier selling various 30VA for just £18.00 and free postage but, as with all the other toroidal transformers I'd checked out, none of them with more than two secondary windings on offer. Since I need three separate secondaries so I can use a positive buck regulator in reverse for the -13v rail and keep the 5v buck converter's ground rail separate from the analogue supplies' (the +/-13v and the OCXO's +12v) ground return, it seems I'm stuck with an R-Core transformer solution since these are commonly available with up to 4 separate secondary windings. However, I'm only weighing up my transformer options right now since I've yet to test with battery power to verify whether there's any point in replacing the original smpsu board with a quieter psu. BTW, I owe you an apology over the estimated psu requirements. It turns out your estimate of 250mA on the +/-12v rails wasn't quite as far out as I'd miscalculated (by overlooking the fact that the 20V pk-pk is the open circuit voltage with a peak amplitude of 10 volts in each direction which halves when driving a 50 ohm load :-[ :palm:). This is a peak current of 100mA per channel for 50% of the time with most alternating waveforms on each rail with a further 25mA of drive from the preceding opamps, plus 17mA vampire current from each THS3491 and, at a guess, another 8mA vampire loading for each of the two driving opamps and a further 25mA allowance for other odds and ends powered from the 12v rails, a grand total of 325mA or so peak on one or the other rail when using both channels to generate a DC voltage of 5 into 50 ohm dummy loads at the same polarity, leaving the other rail ticking over with just 75mA or so of vampire load. With a +/-13v supply, this represents a total worst case power demand between the two rails of 5.2W whether concentrated onto just one or the other rail or shared equally between them. I have since edited my previous post to correct my miscalculations. :-[ --- Quote from: Labrat101 on May 27, 2020, 08:00:45 pm ---... I am getting 4x 3n502 @ $5 each I have notice that the price vary from country to country only the price symbol changes. and not getting clones . which are hard to spot by eye . ... The OCXO i must admit I don't remember ever using one before . always used the regular xtals & tcxo. --- End quote --- That $5 price is less than the £5 or so I'd paid for each of a pack of three last year. That trick of just swapping the $ sign for the £ sign was typical of the Tandy Stores (aka Radioshack) pricing policy back in the day before their over-priced operation went deservedly bust some thirty years ago here in the UK. I still have my Zaks "Programming the Z80" book with its $9.99 price sticker that I'd been charged £9.99 for by Tandy as a bitter reminder of that sharp practice. >:( Until I tried the game of "Chasing Will of the Wisp" trying to match the FY6600's frequency against regular XOs both before and after upgrading to a 50MHz 0.1ppm TCXO module, I hadn't really appreciated the true worth of an OCXO for its supreme stability. The TCXO upgrade had made this game a little easier to win and the OCXO a little more easier again when the only player in the game remained the rather unstable XOs I'd been playing with (a bunch of metal canned DIP XOs). Testing against another of my stock of 10MHz OCXOs to determine the true cause of the jitter between my GPSDO and the FY6600 was a whole new ball game where my aim went from trying to stop the drift from degenerating into a jumbled mess on the DSO's screen to trying to keep it within one cycle of drift for more than ten or fifteen minutes at a time in order to examine the jitter with a 1ns per div timebase setting. The main issue with TCXOs, apart from the three orders of magnitude difference in stability, is that of the 'retrace' effect upon the temperature compensation circuit's ability to hold it on frequency. If there is a large temperature excursion from low to high and back to low, the TC circuit mitigates the detuning effect of the change in temperature but not perfectly and, what's worse, when it returns to its original temperature the crystal doesn't retrace this change exactly leaving it slightly off the frequency it had started from with the tuning compensation that the TC circuit had been using at the time which it would have restored since it can only respond to the temperature changes. A TCXO is an improvement over the ordinary XO but it's a very much imperfect solution. The OCXO gets round this retrace issue by not allowing the crystal to suffer any changes of temperature once it has warmed up and reached a stable temperature somewhere around the 65 to 85 deg C mark. I'm not sure just how tight a temperature control is applied, possibly as tight as +/-0.1 deg C or, since a C cut crystal has an inflection in its tempco curve at around the 65 to 85 degree mark, a slacker +/-0'5 degree may suffice at this temperature curve inflection point. Once an OCXO has had a few days to settle out any retrace effect from the last time it had been allowed to reach room temperature,what largely remains is ageing drift which, in a GPSDO or a Rubidium standard, is disciplined out or else manually re-calibrated against a standard reference on a routine basis. Since the crystal is isolated from the slightest of changes in temperature once fully warmed up, it never has to suffer retrace except for those occasions when it is powered down and back up again. That's why it's considered bad practice to subject them to regular power cycling events since it prevents them from attaining the supreme stability and accuracy they'd otherwise be able to achieve. Of course, even abused in this way, they're still an order or two of magnitude better than the humble TCXO in stability and accuracy which might be all that is called for. --- Quote from: Labrat101 on May 27, 2020, 08:00:45 pm ---... With regards to the Buck DC DC . I have the one which is CC CV . so it can have the amps set . works really well and also I measured the noise on the HP dv/dt @ 20ns got 10mv small peeks . at that level it could even be my plug connection to scope. on the Digital its a flat line . I use one of these in my bench power supply as well 5A which also has a 12A dummy load my last project. It mark as HW-083 5A CC CV DC DC converter Ebay. they are really good. and the Amp trip is fast. ... You ask about the fan its a 6v running from the 5v line . I have no power noise. I used this as I opened the side dummy vents so I get Laminar air flow across the whole broad surface. Nice @ Cool. --- End quote --- That sounds like a reference to those buck converter kits for building a DIY bench supply using a suitable fixed voltage source (battery or 24 or 36 volt 5 or 10 amp smpsu) which include a display screen and voltage and current control knobs. I came across a bunch of them with various voltage and current capabilities on the Banggood website which looked a rather appealing way of building a modest DIY variable voltage bench supply, particularly if you happened to already possess a suitable fixed voltage smpsu and enclosure to build everything into. They were quite cheap for the 5A 15 volt units but, by the time you were looking to build a 32V 10A bench supply, the price wasn't far short of one of the Longwei 32v 10A ready made bench supplies before you'd even included the cost of a 36v 10A smpsu to power it from. I landed up buying a Longwei KW-K3010D bench supply with 4 digit voltage and current displays for just under 40 quid. Having seen a few youtube review videos, I knew I'd have to open it up to check (and remedy) the push-pull pair of HT switching transistors leaning towards each other and threatening to short out the hot ends of the switching transformer and go bang in a big way just as some other Banggood customer had described his letting out its magic smoke just an hour or so after he'd started using it (a review I didn't come across until a few months after I'd purchased mine). I'd taken the precaution of researching the product so as to be forewarned of such all too common examples of careless Chinese construction of their smpsu powered kit so had neatly neutered this particular time bomb before it had any chance to cause me any grief. Whilst I had the thing apart, I also disconnected the 120/240 volt selector switch wiring which links two points in the rectifier/smoothing circuit which converts it from a fullwave bridge with the two 200v rated smoothing caps in series into a fullwave voltage doubler for use with 120v mains since I had no immediate plans to move lock stock and barrel to a country where the domestic electricity supply is based on a 120v standard. Kit fitted with such mains voltage selector switches destined never to leave UK shores are simply a disaster waiting to happen. You might think the little smpsu board in the FeelTech/FeelElec function generators are low quality but compared to most Chinese smpsu product it's a model example of safety and (IME at least) reliability. Also, worthy of note, the design has inherent parity in the plus and minus 12 volt rail amperage ratings, perfectly matching a design requirement for powering opamps from bi-polar supply rails. You try and find any dual 12 or 15 volt rail smpsus that have such a feature (and in the unlikely event that you do succeed in this "Holy Grail" quest, please let me know). Every single one I've ever seen being offered on Ebay and Amazon and elsewhere have, without exception claimed a higher current rating for their positive 12 or 15 volt rail compared to that of the negative rail, typically in the ratio of 2 to 1 and 3 to 1 and often with the proviso that the positive rail have a minimum loading before the negative rail starts producing any voltage that comes within the claimed half a volt lower than the positive rail. That 6 volt fan mustn't be a whole lot quieter than normal with such a tiny reduction in voltage. I don't for one moment doubt it keeps everything nice and cool. ;) The little 40mm square fan I'm using is a 12 volt axial fan which I'm running very quietly off the 5 volt rail. Cutting a 38mm diameter hole in the base to accommodate it means I can use all of the existing vent slots as exhaust vents (including the rear panel vent slots covered with clear plastic ducting to force the air flow from the fan to go the scenic route before it eventually exhausts out of those rear panel (and side) slots. There's no nonsense with thermostatic fan speed control since even at reduced speed it upgrades the original, almost non existent, airflow to an actual flow of cooling air between the outside and the inside of the case, vastly reducing the otherwise excessive interior temperature it had suffered through reliance on only the barest hint of thermosyphonic effect which was completely nullified when propped up on its bail stand. --- Quote from: Labrat101 on May 27, 2020, 08:00:45 pm ---... I think that I have wasted my time .. but I have a lot. .. If I had got the Siglent i would have got bored .. But save money in the long run :popcorn: :-DD.. .. UPdate Johnny look at the Siglent SDG2000x series they show the jitter @ 1.000001mhz 1 clock and also there noise reduction is worse than mine after that small mod I did 10pf . I have no top or bottom noise on the square wave it was caused by the relay contacts and the op amp . and up to 8.4ns .. on others. so maybe we should patent some of our ideas .. :popcorn: ;D :-+ RNS --- End quote --- I downloaded the datasheets for both the 1000 and 2000 series some time ago and, for the money, I wasn't particularly impressed. The reason why I chose to spend my money on the FY6600-60M was because the 5 to 7 fold price hike for a product that was in some respects inferior to the Feeltech had seemed rather too much for what Siglent were offering, especially as the 40MHz SDG1000 was only a fiver cheaper than their SDS1202X-E which had 5 times the bandwidth, rather more controls, and a much higher resolution display panel. I just couldn't see why generating a signal could be so much more expensive than examining it (especially in view of FeelTech's achievement). Maybe someone will explain it to me one of these days. :popcorn: Since this is a hobby activity for the pleasure of learning more about electronics and modifying a cheap bit of kit into something with better functionality than the bean counters had allowed along with other little side projects, rather than an attempt to meet a commercial deadline. That FY6600 has been a much wiser investment than an over-priced ready to go function generator from Siglent. Their SDS1202X-E DSO, otoh, has been an excellent and surprisingly cheap investment by comparison. JBG |
| Labrat101:
Hi Johnny . A question was your OCXO a sine or a square wave outPut ? Because my ocxo is a square wave can I feed it directly into the 3n502 or will I have to do what you did run it thought the 74hc14 . I don't have a 74hc14 but I have its equivalent cd40106 just the voltage the voltage range differs . :-+ I am going to make your circuit any way as it looks good . and not to complicated . Was that the final drawing you up loaded ? you used 3x 150 ohm loading on your output to the 3n502 PLL to give the Y connection 50 ohm :-+ the only difference is that my OCXO is 20Mhz so I will set the PLL to 2.5x instead of 5 . every thing else should be good to go .. I have some 10MHz xtals so that part is the same . the isolation cap that's a ceramic 1uf correct as the drawing was a bit smudgy in places and 27pf .. not sure I have that odd value . That choke you used 4.6uH I can only find 4.7uH .. It will have to do I can't find a listing for 4.6uH or you made it?? But I am read to go for the build .. Still waiting for my PLL chip will be another week so that will give me time to make Magic smoke. Thanks RNS |
| Johnny B Good:
--- Quote from: Labrat101 on May 29, 2020, 05:41:39 pm ---Hi Johnny . A question was your OCXO a sine or a square wave outPut ? --- End quote --- The very first of these CQE OCXOs (which I'd blagged from a stall holder displaying this 'find' amongst a collection of 'tat' for just £4 at the Blackpool radioham mobile rally just over a year ago) was a 13MHz unit with a square wave output. It wasn't one of the 10MHz units I'd hoped to see on sale at this event but, in spite of this (and out of some frustration), I felt it would be worth taking a punt on it at the 4 quid price I'd managed to negotiate. Heck! At least it was an OCXO, even if was the 'wrong frequency' since, even if I couldn't use it to generate a 10MHz signal locked to a GPS, I could at least gain some useful insights. It turned out I wasn't at all wrong and even managed to figure out a way to use it as a precision ovenised 10MHz source to start playing with experimental GPSDO test setups. The other seven 10MHz CQE OCXOs (definitely 12v versions - I still have doubts over the voltage requirements of that first "5V" 13MHz example) are all sine wave output and this is what I'm using in the FY6600-60M. It's rather handy that these are of the sine wave type since I suspect it may have been more of a struggle to get my injection locking module to lock a square wave type via its output pin. There are two types of square wave output OCXOs, 'clipped sine wave' and the full blown square wave attained with a separate internal 'squaring' gate drive' IC to give the most perfect of square wave output possible with 50.00% duty cycle as opposed to the former, typically specced for a 45 to 55% duty cycle square wave. Both types, incidentally use dc coupled outputs, allowing them to be directly connected to a logic gate input without any need for biassing the logic gate input to the mid point and adding a DC blocking cap (often not required with sine wave oscillators which typically include such a dc blocking cap anyway - a difference that had left me scratching my head for while when I first started using the 10MHz sine output OCXOs in my GPSDO test builds on solderless breadboards). The 'clipped sine wave' type will probably be more amenable to injection locking via their output pins than the fully buffered type which you seem to have. I would suggest you run some experiments (carefully! Use a coupling cap, 100nF should do the trick, to prevent any 5/3.3 volt accidents) before committing to anything based on my design. --- Quote from: Labrat101 on May 29, 2020, 05:41:39 pm ---Because my ocxo is a square wave can I feed it directly into the 3n502 or will I have to do what you did run it thought the 74hc14 . I don't have a 74hc14 but I have its equivalent cd40106 just the voltage the voltage range differs . :-+ I am going to make your circuit any way as it looks good . and not to complicated . Was that the final drawing you up loaded ? --- End quote --- Since both parts are 3.3v, you can directly connect the OCXO to the 3N502's clk input since, afaicr, it's powered off the same 3.3v supply as the original XO IC. The drawing is pretty much the same. All I did was add a note pointing to the 27pF cap to remind me to replace it with an actual trimmer cap (33 or 45pF) so I can trim the optimum value in circuit rather than select on test outside of the module itself. It's obviously not trimmed exactly to suit the actual circuit conditions compared to my select on test setup. --- Quote from: Labrat101 on May 29, 2020, 05:41:39 pm ---you used 3x 150 ohm loading on your output to the 3n502 PLL to give the Y connection 50 ohm :-+ the only difference is that my OCXO is 20Mhz so I will set the PLL to 2.5x instead of 5 . every thing else should be good to go .. I have some 10MHz xtals so that part is the same . the isolation cap that's a ceramic 1uf correct as the drawing was a bit smudgy in places and 27pf .. not sure I have that odd value . --- End quote --- Using the 3N502 which is taking the place of the original 50MHz XO ic on the main board to multiply your 20MHz OCXO by 2 1/2 to recreate the original 50MHz clock signal to the Cyclone is perfectly fine but since it's now a 20MHz OCXO that needs to be influenced with an injection locking signal, you'll have to double up your external 10MHz reference to match the OCXO's frequency. That's the main departure between mine and your version of this injection locking module before we start considering whether it's actually possible to inject the locking frequency (20MHz in your case) via the output pin route as I did with mine. The internal buffer/squaring amp inside your 20MHz OCXOs may offer too much protection against such (normally undesired) external influences for this to work effectively, or even at all. You might have better luck injecting via the Vref pin, the EFC pin or even via the Vcc pin. In view of the filtering on these pins, you'll probably have to use a low impedance drive via a stepdown transformer which will not only reduce the impedance but also provide dc isolation from potentially damaging voltages. Given enough drive current into whatever filtering circuit is inside the OCXO on these lines, you may be able to brute force sufficient levels of EMC inside the can, bypassing any such filtering and thereby achieve your goal of influencing the actual oscillator itself. The isolation cap to the right of that 27pF cap (a 'preferred value', btw) is actually a 1nF cap. The component itself looks like a green bodied 1K resistor (1000pF), It's not overly critical and I suspect a 10nF might be a better choice (it's just that I had plenty of these 1nF caps to hand ;) ) --- Quote from: Labrat101 on May 29, 2020, 05:41:39 pm ---That choke you used 4.6uH I can only find 4.7uH .. It will have to do I can't find a listing for 4.6uH or you made it?? But I am read to go for the build .. Still waiting for my PLL chip will be another week so that will give me time to make Magic smoke. Thanks RNS --- End quote --- A 4.7uH inductor is probably close enough. I didn't have any 4.7uH inductors so took an 8.2uH or some such inductor and removed enough turns to drop its value to what the filter calculator had actually called for using my trusty Banggood LC tester (what a Ghodsend that has been! :) ). If you have an LC tester and a small collection of 4.7uH inductors, you will very likely find a 4.6uH in amongst them (the tolerance on such inductors is very slack indeed as I discovered when I started testing a bunch of them that had been lurking in my now rather neglected basement radioshack / workshop for well over thirty years. Of course, this won't be relevant at an injection locking frequency of 20MHz :( Even if you were using a 10MHz sine wave output OCXO, you could probably, in view of the 10MHz series resonant Xtal's presence, do away with that inductor and the 2nd 100pF capacitor. The 150 ohm with 100pF RC LPF filter should still be kept to protect the Xtal from being hammered with odd harmonics which could excite a strong third overtone (with associated spurii) response from a Xtal operating in its series resonant mode. I reckon you've got quite bit of testing to do with those 20MHz OCXOs of yours before you can figure out the best way (if, indeed there is an actual way) to inject your reference frequency into any of these OCXOs. I've never tried testing this with my one and only 13MHz square wave OCXO so I don't really know just how difficult (or easy) a task this will be. I wish you all the best in this endeavour. JBG |
| Labrat101:
Hi JBG I build it as per diagram without the last stage ie the transistor. This is what I got without any alterations .. I have not got the PLL yet .. What do you think is it correct ?? The cd40106 was on the 3.3v for test But I think it should go to the 5v . its will operate 0.9 - 15v as per spec sheet . The time between the Pip & the valley is 4ns exact . It Had only been running about 20 mins so it ocxo was still stabilizing a bit There is copper stripping underneath for ground plain . there are 2 more pics but I got a MAX error :palm: i found this but it does not tally with your calculation maybe I'm missing something . (yup a few of my screws ) >:D http://www.calculatoredge.com/electronics/bw%20pi%20low%20pass.htm What calculator did you use ?? |
| Johnny B Good:
It's difficult to see exactly what you've built onto that 0.1 inch matrix board, so I can only assume that you've assembled a cutdown version using a dog slow cmos version of a 74HC14 to do some initial testing whilst awaiting that pack of 3N502 ICs you've had on order from your supplier. The 20MHz pulses coming out of the CD40106 you're using look like just what you'd expect to see out of Old Skool cmos crap substitutes for TTL logic chips. You should be using more modern cmos versions such as the 74HC or 74HCT range (or even the original 74 series if needs must - example: the 74HC193s just couldn't cut the mustard with a 26MHz clock signal in my GPSDO and I was forced to use "The Real Thing", aka an actual 74193, for this task). It's well worth downloading and studying the datasheets on any ICs, especially old cmos ICs before soldering any into your test boards (in particular, look at any timing specs - hint: scan down the extreme right hand column of each table to hunt down units mentioning ns or MHz ;) ). I had enough concerns over propagation and transition delay timings at just 10MHz (the 26MHz clock example was just an inescapable consequence of dividing a 13MHz OCXO' square wave output by 1.3 requiring just one "Special IC" in the circuit). You're dealing with a clock speed of 20MHz which makes such dynamic performance of the logic gates used literally twice as important. When I was looking for datasheets for a CD40106 IC, all I could find were datasheets for the much slower buffered output versions (B suffixed) quoting typical and max delays at a Vcc of 5v of 140 and 280ns for the TI parts (Intersil only quoted the max values, not bothering to tease its customers with vague promises of a faster typical performance). It looks like there may never have been an unbuffered version and I can't locate where I've hidden my CMOS Logic data book to check on this. :( TBH, I can see you struggling to persuade these 20MHz OCXOs to respond to the injection locking method. That's not to say you won't find a way since injection locking by accident is one of the designer's worst nightmare scenarios, requiring careful consideration to avoid this, including the use of extremely well buffered oscillator outputs to minimise this risk. However, you might find it easier to use a PLL to feed a trimming voltage to the EFC pin normally controlled only with a well padded out multi-turn trimpot that's normally used to facilitate manual calibration of test equipment blessed with an OCXO to provide it with an internal highly accurate and stable free running clock oscillator reference. In this case, you'll need to effectively disconnect the PLL output's disciplining control voltage feed from the EFC circuit in the absence of a valid external reference signal. This assumes that you don't want to just give in and go for the "Simple Change-over switch and never mind the glitch!" technique. ;) I may have needless concerns over this glitchy way of simply using a changeover switch to go between the internal and an external reference but, if nothing else, this injection locking module does obviate the need for such a switch, automating as it does, the whole change-over process to a simple matter of merely connecting and disconnecting the external reference to seamlessly effect the change-over. There is a pushbutton switch in my circuit but this is only to effect a change-over between terminated and unterminated mode which would normally be a once in a blue moon operation rather than a regular chore to switch between the external and internal reference clock sources. That PLL suggestion does have the merit of allowing you to phase lock an "Injection Locking Immune" OCXO to an external reference, retaining all the benefits of injection locking (glitchless automatic change-over to and from an external reference) so may be a more rewarding avenue for you to follow with these 20MHz OCXOs. Anyway, have a think about this before you commit to what might prove in the end to be an utter folly. By all means experiment with the injection locking idea but be prepared to check out other options. JBG |
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