General > General Technical Chat

Injection locking the 10Mhz OCXO to external reference (upgrading a FY6600)

<< < (3/6) > >>

Johnny B Good:
 Arthur Dent explains and posted a couple of pictures very early on in the FY6600 topic thread:

https://www.eevblog.com/forum/testgear/feeltech-fy6600-60mhz-2-ch-vco-function-arbitrary-waveform-signal-generator/msg1341056/#msg1341056

 You need to read through the first 15 pages at least of this thread if you want to follow what this injection locking project has been all about (I thought you'd already read all 88 pages of it  >:D).

 I've attached an earlier version of the reverse engineered circuit schematic of the main board of the FY6600 from Der Kammi's github pages (the later version didn't show the 50MHz XO chip (QG1) in that layout). You'll find it in the lower LHS of the schematic.

 When I first upgraded the ten a penny smd XO chip to a 50MHz TCXO, I removed the original chip from the board and connected an outboard 50MHz TCXO clock module to the vacated ground and output pads. Originally, I'd intended to cannibalise the clock module for its TCXO and plant it directly in place of the original smd part using wire stilts. That idea died very soon after I took some IR temperature readings around the XO location, just 10mm away from a trio of very warm LDOs (70 deg C!!!) - the XO was showing 60 deg C. No wonder you only had to blow gently onto it to send the frequency drifting off a few tens of parts per million (in this game, 1 part per million is nothing to be proud of, indeed imho, it's more something to be ashamed of).

 What Arthur Dent had been forced to do in order to use a standard 10MHz OCXO, was to replace that crappy XO chip with a NB3N502 clock multiplier chip configured to multiply the 10MHz fed into its CLK input pin fivefold to recreate the original XO's 50MHz clock on its output pin, now soldered to the pad used by the XO chip's output pin. The same requirement also applies to the FY6800 (but not the FY6900 which already uses a 10MHz XO clock chip).

 The chip layouts don't match so the usual way of dropping the NB3N502 in as a replacement for the XO is to fit it slightly cock eyed for best grounding with a minimum of additional wire links. With care and a craft knife, it's even possible to avoid additional wire links altogether if you get the angle of misalignment just right  :) .

 It may look a little 'ugly' but we're not Feeltech pretending everything was just perfect to start with by trying to disguise the 'cover up' - the presence of a hulking great OCXO rather defeats that idea of 'tidiness'. Indeed, I rather bask in the pragmatic bodging of this modification as an FU statement to Feeltech's bean counters.

 JBG

Labrat101:
Hi I must admit I had not read Arthur Dent,s blog .
I thought you were joking when I saw the Name ..  :phew: .

  I read them now thanks ..
I also did some research on the cyclone chip apparently it has built in PLL pins and a whole long thing
from Intel as to use these . These Pins located on the left & Right side of the chip . DAC sides.
and a describe how to deal with Jitter. To great length.
 Just blew my Brain to much info.
      Way over my Head . and out of my field .  (this might be a good place for injection . It might involve software as well . Not Sure .. I am not into Software unless its a Blond  >:D )
 You may Well understand it .  Its in the cyclone iv documents. Many Pages ..
 It Needs some one who knows How.
This is properly why Arthur Dent came up with a simpler work around solution. 3n502 PPL chip. OK  :-+

UPDATE:
In your schematic you are using 5Mhz oc where dent use a 10Mhz .so if you use 5mhz and then
inject 10mhz into 3n502 pll chip where is the 5Mhz going to ?? because its a multiplier .
I am getting confused . His method seem logical . unless you mixed them with the 5Mhz going
direct to the output  of the PLL and the injection to pin  of the 3n502 .
Can you please include pictures of your connections . I have got lost . on your injection .
I am sure it works . and you are correct but I can't connect it to gather in my mind.
and I have read your work over several times .
So some nice photos Please .
...
another Update.
According to the Manufactures recommendations of the 3N502. Use a 20MHZ x 2.5 =50Mhz
will reduce jitter due to the 48mhz internal clock of the chip.
Or as ArtherDent had also figured out. or use 2x  3N502  .  4Mhz X 2.5 =10mhz fed into the other  PLL to give the 10mhz x 2.5 =50mhz . Which runs the PLL at its optimum .
..  So I don't Get Why you have all those really good OCXO and then use a regular 10mhz Xtal
as per your circuit diagram . Sorry . If you already had the PLL installed .
 Two of those really nice ocxo with another PLL and a Buffer chip would have sufficed . (200PPb)
 This equipment will not last 1.8million years. Maybe 15,000hrs as the life time of the smd caps.

....

  I now really starting to think this project has Gone past the specs of the Big boys equipment.
  If feelelec or feeltech was to but all this info and produce a product of $150 we would have a
 Roles Royce engine in a cheesiness case. and a bigger screen  :-+

Johnny B Good:
 Sorry for the delay in getting back to you. I've been embroiled with a rather weird "Theremin" issue with my GPSDO these past three days which has been driving me slowly round the twist. >:(

 When I first started this GPSDO project, I was making do with 3 1/2 digit DMMs to check voltages, including, once I got the whole thing soldered down onto veroboard and nicely boxed up, the EFC voltage which had started off around the 3.28 volt mark (as much resolution as a 3 1/2 digit DMM allows at this voltage). Reading to the nearest ten millivolts just wasn't good enough so I bought myself a 10000 counts DMM earlier this year which would at least resolve the EFC voltage in 1 millivolt increments.

 Just recently, I hit upon the idea of measuring the difference between the EFC voltage, now hovering around the 3.315 volt mark, and a fresh CR2032 cell (3.334v), allowing me to monitor voltage variations in tenths of a millivolt. The lithium coin cell is a remarkably stable voltage reference all things considered, showing less than a millivolt variation day to day over the 3 or 4 deg C room temperature variations and certainly stable enough to show the minute to minute and hour to hour tuning corrections being applied to the OCXO. However, flushed with my success at wringing another digit's worth of resolution out of my cheap chinese 10000 counts DMM, I thought I'd experiment with a high precision voltage reference, aka the humble TL431 typically to be found in almost  every cheap smpsu based wallwart that's been manufactured by the billion over the past two decades.

 I managed to find one in a jar of salvaged wallwart parts. A quick breadboard test confirmed it was still working and, as the data sheets implied, with a very small tempco over the 25 to 50 deg C range. The initial accuracy wasn't an issue, I was going to include a trimpot in the feedback network to raise its natural 2.495v to exactly 3.000 volts anyway. The real concern had been over how stable it would be over the temperature range inside my GPSDO which runs some 10 or 11 degrees C above room temperature (somewhere in the region of 30 to 40 degrees C, excluding heatwave conditions (coincidentally, the flattest part of the tempco curve given in all the datasheets - it's as if they'd set it this way by design).

 Right now, the tempco is the least of my problems since I seem to have stumbled upon yet a third "Theremin Fault". Just in case you don't know what a "Theremin Fault" is, it is the situation where you can effect an undesired change in an electrical gadget just by waving your hand near to said gadget or by touching it or by connecting a cable  to it even when it's just a connection to the ground terminal of the socket in question or even just the metal housing that's supposedly screening the innards from such external influences. ::) >:( :-// :wtf: |O

 Initially, I thought it may have been instability due to lack of either a 2.2nF or smaller or a 3.3uF or larger capacitor between the anode and cathode. I'd already included a 1nF cap to start with so swapped it for a 10uF ceramic but this made no difference whatsoever even after fitting a 330 ohm resistor and 150uH inductor in series with the connection to the negative reference connection socket which had originally just simply been a ground reference point for the meter connection.

 Since there is something like 70mV pk-pk of ripple and noise on the 5 volt rail (none of it being the 1.2MHz switching of the buck converter used to supply this rail - it's all TTL loading noise with components right up to 200MHz arising out of the use of veroboard construction), I figured I might be able to minimise its effect by rewiring the power feed to a quieter pick off point (the additionally filtered feed to the OCXO and the tuning voltage buffer amp) but this also made no real difference. The only other thing I've left to try (more out of desperation) is to filter the REF terminal of the TL 431 with a couple of ceramic caps going to the anode and cathode. Hopefully, this will kill off the HF noise ingress that's presumably overloading the high gain opamp part of this bandgap reference and generating this unwanted DC voltage shift (or it may make it even more unstable).

 My very first experience of this theremin effect was, unsurprisingly, during the solderless breadboard testing phase after having tested with the 13MHz square wave output OCXO with no such issue, when I started trying out a newly acquired batch of 10MHz sine wave OCXOs. I'd only had to place my hand near the wire link from the 10MHz OCXO's output to the 74HC14 inverting buffer input to detune it. Being a temporary breadboard layout, I could accept that as simply the consequence of such construction.

 However, when I experienced my second theremin fault, it was with the initial final build on veroboard mounted inside a nice screened box just when I'd thought I'd see the last of these theremin faults. Basically, it involved the separately buffered EFC voltage which would shift depending on the length of the connecting wire going to my DMM. Initially, placing a ferrite clamp around the wire was enough to eliminate the 10 mV or so shift in the reading. The actual EFC itself wasn't being effected, just the buffered feed to the external meter test point. The more compact solution was simply to place a 10uF ceramic across the output after the 100 ohm stopper resistor I'd already included to eliminate such instability issues - clearly the stopper resistor on its own had not been enough.

 Out of curiosity, I retested this buffered EFC meter test point using my newfound extra resolution courtesy of that lithium coin cell trick to see whether this was also being effected by external connections (there seemingly hadn't been any such effect other than for my TL431 reference creating errors in the the EFC readings). Close examination, at the tenth of a millivolt resolution that my trustworthy coin cell allows, did reveal a small effect of around one twentieth of a millivolt change by placing and removing a hand on the case or by plugging the cable into the 10MHz output socket. So, there seems to still be some issue of theremin effect on the buffered EFC voltage meter test point. The actual EFC voltage is not necessarily effected in this case but it might be, it's difficult to say at this stage since the effect of a twentieth of a millivolt error is very difficult to detect on the output frequency of the GPSDO - the 1 millivolt error with the 3.000v offset reference otoh, is of rather more significance since even a 0.2mV change in tuning voltage can produce a detectable shift in frequency against the FY6600's 10MHz test reference.

 Anyway, that's the story explaining my slow response. As for your thinking that I'm using a 5MHz OCXO in my FY6600, close examination of the circuit diagram on my part suggests you had misread the "SMAf" just to the left of the OCXO box as "5MHz". I'll admit to it not being too clear but that is what it actually reads when you examine it very closely. I apologise for the confusion but that's one of the hazards of posting a picture of a hand drawn circuit diagram. The image does contain ample detail such that if you scale it to match the screen pixels, it will become obvious as to what I'd actually written on the diagram.

 Like Arthur Dent, I'm using a 10MHz OCXO to drive a 3N502 chip sat right where the original 50MHz XO chip used to be on the main board. This programmed to multiply the 10MHz up to the required 50MHz of the original XO chip it replaces, exactly as per Arthur's OXCO mod.

 Where I've differed from Arthur's mod is with regard to the way the external 10MHz 'lab reference' is used (and the powering of the OCXO modules). I've used a separate 12v half amp smpsu board which maintains power to the OCXO whilst the FY6600 is plugged into a live wall socket regardless of whether it is switched on or off on the rear panel switch whilst Arthur relies on a beefed up replacement smpsu to power his OCXO from the +12v rail only whilst the whole generator is switched on.

 He uses a simple CO switch to select between the extl socket or the internal OCXO module whereas I've elected to use a frequency injection locking module to automatically lock to an external 'Lab Reference' by the action of it being plugged into the Extl socket alone without the need to manually switch between one or the other clock frequency source. The only switch in mine is to choose through or terminated operation and is purely optional - it can be left out if deemed an unnecessary luxury where your 10MHz lab reference is distributed via a star network of feeder cables.

 You mentioned 48MHz which isn't, AFAIK, a clock frequency used anywhere inside of the FY6600. It is however, the frequency of the XO or TCXO used by the various models of u-Blox GPS modules (the M8N uses a 48MHz TCXO for example) so I'm wondering if you're getting confused with GPSDO jitter and 'sawtooth' issues in this case. BTW, I'm assuming you mean PLL in those places where you've used 'PPL'  ;)

 As for the FPGA used in these signal generators, as far as I'm concerned, that's just a 'magic black box' I'm never going to live long enough to learn how to program and therefore, no point in wasting time on its gory details when there's more than enough elsewhere in the guts of these FY6*00 generators to be put right without discovering yet more things to fix in the FPGA itself. I know my limitations, thank you very much!  ;)

 As for FeelElec making a much better version for a modest price hike, the problem is exactly where do you draw the line between quality and end price? In hindsight, it's easy enough to see where a one dollar cost increase could have provided a ten dollar price hike that their target demographic would have happily swallowed, or even a two dollar cost increase for a twenty dollar price hike. The problem is at what point do you, as a manufacturer, decide to call a halt on such 'creeping featurism' and fix your costing and pricing limits?

 Perhaps if FeelElec where to properly assess the feedback from these EEVBlog fora, they might produce a more upmarket version without any of the gross bean counteritus defects, which plagues even the latest FY6900 model, at a still affordable price. You never know but, going on their record so far, it doesn't seem all that likely. :(

JBG

Labrat101:
ok
So I misread SMAf  as being 5Mhz .. Ok  SO What is a SMAf  ???.

 Also I think you need to use Tantalum caps on both sides of your 1117 .regulator .
 Ceramic capacitors do have there limits .
Also as you said your SMPS is remote and these do put out Harmonics . long wire . will pick up
stray frequencies . . 200Mhz is a harmonic ..
Sorry. it was mentioned in one of the spec sheets for the 3n502 that Curtain multiples can cause
problems .
That's why I went for 20Mhz x 2.5  as per one of the manufactures stated this ..

My friendly supplier has found me 2.  CTS 20MHZ 0.02PPb OCXO 3 volt. New old stock 2015 still sealed in the original packing .  ;D, should be arriving in the next 48hrs.. :D
Spent 4 hours building a dedicated power supply for these beasts which require 3.300 volts
to hold stable during warm up. as per spec sheet says can be up to 3.8 watts.
used all Tantalum caps and tested .. second attempt got it to less than 1% .
BTW you really need to treat your self to a good 8+ digit meter . there are some good
S/Hand out there going really reasonable price . I found a few years back GW Instek GDM 8246.
fully calibrated $120 .I have been very happy with it  its accurate to +_ 1 uv .
But I am sure there may be better. came from a local tech collage sale.

I will let you know how my project works  >:D  .. The Jitter maybe my hand shaking  :-DD




Johnny B Good:
 Good question re the "SMAf". It's simply an SMA female socket for the short male to male 15cm SMA patch lead to plug into which links to the OCXO input SMAf socket on the injection locking module. I had originally intended to simply solder an SMAm flylead to the OCXO board but changed my mind when I realised it would be neater to simply solder an SMAf PCB mount socket to the OCXO board (I just happened to have a suitable SMAm plug ended patch cable to hand).

 As for that sketch of the 117 pin outs, I'd added that to the diagram when I'd been planning on adding the injection locking circuitry to the existing OCXO board. It became redundant when I decided to build the injection locking circuit as a separate module, powered directly from a handy 5v connection on the main board (I only had the single 12v rail available on the OCXO board itself, hence the pin out sketch for a 117-5 LDO that the original plan had called for).

 Using a 20MHz OCXO with a 3n502 multiplying it by 2.5 might allow the resulting jitter to be minimised but I don't think the minimal 20 or 40 ps of jitter makes any difference to the FPGA's performance in this application. In any case, if you wish to add an external 10MHz reference socket, it will add a little more complexity in the form of another 3n502 to multiply your 10MHz reference to the 20MHz needed to drive that on-board 3n502's clock input line. However, in this case of a very precisely fixed 10MHz frequency, you could probably double it up with a simple 'XOR gate clock doubler' tuned to give a reasonably even mark/space ratio output to reduce costs (see below).

 Still, despite all that, it could work out a cheaper solution if those 20MHz OCXOs are considerably cheaper than the more in demand 10MHz (with price premium) variety. Mind you, those 3n502 clock multiplier chips aren't particularly cheap items of inventory (circa five quid a pop - about 7 or 8 dollars each at the UK/USA exchange rate) so it might still be a 'close run thing' cost-wise.

 I'm planning on rebuilding my GPSDO 'dead bug' style on a copper clad ground plane board using one of my 12v 10MHz sine wave output OCXOs to eliminate most of the "TTL power rail hash" that's presently generated by two 3n502s, an Old Skool 74193 modulo N counter (divide by 13 stage) and a 7427 triple 3 input nor gate (to even up the resulting 8:5 ratio 2MHz to a more squarish 7:6 ratio) clock input signal to drive the final five times multiplier stage which drives the 74HC14 inverting buffer which drives the LPF converting the rather jittery 10MHz square wave into a low jitter sine wave at +11.5dBm.

 Since I'd like to hang onto the current 7 to 24 volt PSU requirement, I'll have to obtain a 5 to 12v boost converter module which will nicely eat up the 200 to 300mW savings afforded by the elimination of the four highest energy consuming of the existing total of eight ICs in the current design.

 The new layout on ground plane construction and simplified circuitry of this MK II version using a proper 10MHz sine wave output OCXO should eliminate most, if not all, of the sins of veroboard construction compounded by needlessly complex circuitry. My noise and ripple concerns over the use of a 5v buck converter to supply the 5 volt rail proved to be baseless - the minuscule level of its 1.2MHz switching operation and associated switching transients had been completely submerged by the ocean of noise generated on the power rails by the processing logic in the FFT plots I'd made of the noise and ripple on the 5v rail.

 Although the use of a boost converter means I'll have to deal with higher levels of noise and ripple on the +12v rail, I don't think this will be too difficult to tame with LPF components mounted onto an effective ground plane. The OCXO will be its only 'consumer' and I'm confident that the ripple on the 12v heater supply won't matter and the built in filtering of the LDO used to power the 14mA consumption of the oscillator itself will deal rather effectively with any 12v supply ripple anyway.

 A noteworthy side benefit of employing a 12v OCXO in this case being that I get a temperature stable reference voltage from the OCXO for free - no need for a separate TL431 voltage reference in this case to generate a more stable 3.0000v (or 4.0000v) meter test point reference connection to monitor the EFC to a resolution of 100uV with a cheap MESTEK DM91A 9999 Counts Digital Multimeter I'd purchased last July for a mere 13 quid and some pennies - I notice they've gone up in price by over two quid since then  :) .

________________________________________________________________________________________________
[EDIT 2020-05-25]

CORRECTION!!!  That price increase is actually for a Chinese supplier :wtf: The original UK based supplier has actually dropped his price down to a mere £10.58 delivered postage free to UK addresses within just 5 to 7 days of placing the order. Contrary to what I stated above, the price has actually dropped by just over two quid. :)  :palm:

 For a 9999 counts DMM, that's an absolute bargain for us Brits (almost a fiver cheaper than the slow boat from china option). Mine reads 0.4% higher than the 25 yo ex BT Fluke MMF 1A FIG 84/1 I inherited as a parting gift from my former employer way back in '92 and due its next calibration March '93. :-DD

 Although it's way past its next calibration check, it compares closely to a UNI-T UT58A. Since it remained largely unused and kept stored in an office filing cabinet for all of that time (and Fluke being Fluke), I don't doubt that it's as good a reference against which to compare that Chinese cheapy as anything else I have to hand. I did open the Mestek up to locate its calibration trimmer so I could align it to my Fluke but it simply doesn't have one!

 That's both a "Good" and a "Bad" thing - good because there's no damned cheap trimpot to introduce errors and bad because there's no simple trim tool means to recalibrate it. Presumably, it's just a question of soldering the correct value of trim resistor across one the two potentiometer arms of the high precision bandgap voltage reference circuit (wherever the heck that lives!).

 However, if consistency and stability is of over-riding importance, mine seems to possess these in abundance regardless of temperature or the state of its two AAA cell battery pack voltage right down to the low voltage warning level after some 400 plus hours of continuous use ( monitoring voltage with the auto-switch off disabled - end point cell voltage, afaicr was circa 1.1v not the industry standard 0.8 to 1 volt per cell).

 I highly recommend this as a cost effective 9999 counts DMM that's both stable and consistent and very cheap to run. The only foible being that it jumps from a plus or minus 0.3mV reading straight to zero on a slowly reducing in magnitude test voltage whether manually or automatically set to its millivolt range  :wtf:. It's a strange behaviour but at greater voltage magnitudes, it functions perfectly ok. Since I rarely want to check for voltages in the +0.2mV to -0.2mV range, I can live with this idiosyncratic behaviour. In any case, there are ways and means to get around this issue if needed. ;)

 The auto-shutoff feature can be disabled, unlike that damned UNI-T which shuts off without any warning beeps just as you're trying to record your next reading :palm: The time-out is 15 minutes with a triple warning beep a minute before it actually switches itself off with a rapid series of beeps. When the auto-shutoff is disabled, it still emits these warning beeps which can be regarded as either (or even both!) an 'annoyance' or a useful reminder feature (take your pick - glass half empty/full PoV sort of thing). Personally, I find it slightly irritating but can see the benefit of not disabling the associated warning beeps.

[END_EDIT]
=============================================================================================

 I'd like a 6.5 digit (or better) lab bench meter but at ten times the price of that 9999 counts handheld DMM for a 49999 counts bench meter, let alone the several hundred quid for a 6.5 digit model, I'll stick with my Chinese cheapy and a precision offset reference voltage source for now, thank you very much! I'll hang fire on such an expensive acquisition until I can find sufficient justification for much higher precision. Just keep in mind that I'm making do with an FY6600 signal generator and you'll understand my reluctance to splash all that cash on a "Voltmeter". ;)

 A major consideration in my choosing to use the "Five Volt" 13MHz OCXO along with all the additional complexity of converting its output to a precision 10MHz sine wave was the elimination of a +12v supply rail (besides which, I'd already proved it could be used to generate a precise 10MHz reference source - in essence, it was "Because I could." :)). The exercise did provide a valuable lesson in how not to build a GPSDO so it hadn't been a complete waste of my time. ::)

 At the time when I was first trying to verify what voltage this 13MHz OCXO had been designed to run off (I was unable to track down any data sheets for this specific model of OCXO), I didn't have a handy variable voltage lab bench supply to test with and all the evidence had suggested a very real possibility of it being a five volt part that wouldn't take at all kindly to a dose of 12 volts on its Vcc pin.

 Since it seemed to function just fine (admittedly with a rather protracted 7 or 8 minute warm up delay), I decided to 'play safe' and assume, until I could gather sufficient evidence to the contrary, that it was actually a 5 volt part and avoid the risk of releasing its 'magic smoke' before I'd gotten any use out of it (4 quid might be chump change but it's still 4 quid spent on an item that I could still experiment with using just a 5 volt supply).

 Now that I have seven in total of its 12 volt 10MHz brethren, I can now well afford to, literally blow this 4 quid investment away in a voltage probing test accident which I now have some reason to think is a risk worth taking since I have a new found suspicion that it might actually be a 12 volt part after all. Obviously, this is a test scheduled for after I've rebuilt my MK II version GPSDO. ;)

 The initial warm up current demand (280mA in the case of both the 13MHz and the 10 MHz CQE OCXOs - just one reason amongst others for my suspicion that the 13MHz unit might be a 12 volt unit after all) was the reason why I opted for a 50MHz TCXO as my very first XO upgrade (just 28mA off the 5v supply - exactly the same as the relays and my 5 volted 40mm square 12v add on cooling fan!).

 The TCXO had been a vast improvement over the original XO chip but I was never going to be able to calibrate it to stay within better than +/-30ppb of 50MHz so when I lucked out during my search for datasheets for the 13MHz OCXO by finding a cheap source of its 10MHz brethren and got hold of a life time's supply (seven in all), I changed my mind with regard to Arthur Dent's seemingly OTT OCXO upgrade and followed him down the rabbit hole. ::)

 Since I'd merely modified the original smpsu board rather than replace it with a better one, as Arthur had done with sufficient current rating on the +12v rail to power an OCXO upgrade, I had no choice but to shoehorn the innards of a Linksys 12v half amp smpsu wallwart into the box to satisfy the additional power requirement.

 The upside of this arrangement being that I could wire it direct to the C6 mains inlet socket, bypassing the rear panel on/off switch meaning that whilst it remained plugged into an outlet, I could switch the generator off, leaving the OCXO undisturbed, ready for immediate use the next time it was switched back on - no warm delay and only some 200ppt of warm up drift over the next hour or so's run time rather than the several minutes it would otherwise require to get within a few ppb of frequency.

 Warmed up to operating temperature inside of some foam rubber insulation, these OCXOs only draw some 800mW or so of power which equates to some 1.3 watts at the mains socket after taking into account the losses in the 12v smpsu board. Relying on using the front panel on/off standby button only saves around 2 watts at most, reducing the consumption from around the 7 or 8 watt mark to around the 5 or 6 watt mark so the split mains supplies provided a handy energy saving feature without unduly stressing the OCXO with thermal retrace issues to compromise its accuracy and ageing drift rate.

 BTW, that requirement for supplying precisely 3.300 volts to those 20MHz OCXO modules may not be quite so critical as the datasheet states. The at temperature current demand is likely to be in the ballpark of 300mA at the specified 3.3v peaking at some 1150mA during the warmup phase.

 I suspect that as long as the supply voltage can be held above the three volt mark until the heater current drops down to half an amp, allowing the supply to come back onto its 3.3 volt setting, the worst you're likely to experience is a slightly longer warm up time. That maximum power consumption figure of 3.8 watts is remarkably similar to the 3.36W of those CQE 10MHz OCXOs of mine which on test were still able to reach operating temperature with a 5.2 volt supply and reach their intended frequency despite the sine output voltage being a little on the low side.

 The main symptom had simply been an even longer delay before the oscillator started showing signs of activity compared to the 10 to 15 seconds delay with the 13MHz unit when powered from the same 5.2v supply before eventually getting to temperature after some 8 minutes or so versus the more typical 3 minutes warmup of the 10MHz units when powered off a 12 v rail. Indeed, the 12v wasn't critical, I sometimes ran my breadboard builds off a 9v wallwart feeding the OCXO directly whilst feeding the 5 and 3.3v LDOs on the plug in breadboard's psu board, the only effect of this reduced voltage being an additional minute or so of warmup time.

 Of course, with 3.3v OCXOs, the exact voltage level does tend to be rather more critical than for a 5 volt version which, in its turn is a little more critical than the 12v versions. but as long as the voltage can recover to its stated requirement of 3.3v at a heater current level say some 50% greater than its final at temperature current demand, apart from a protracted warm up delay, it's likely to be just fine once it has warmed up.

 The main concern with these low voltage high current OCXOs being the greater risk of an LDO overheating and failing catastrophically even when fed off a 5v rail, with the risk of overvolting your precious OCXO with a fatal voltage surge. You might want to create a crowbar protection module using an LT431 set to trigger an SCR or triac at the 3.4v threshold to place in parallel with the OCXO (a fuse is optional - personally speaking, I'd rather blow the offending LDO to smithereens for its treachery than offer it any token gesture of protection with a fuse (Transistor :- a ten dollar device designed to protect a ten cent fuse)  :-DD ).

 Well, I think that's covered the points you raised and I hope my observations have been of some use.

JBG

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod