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
.
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[EDIT 2020-05-25]
CORRECTION!!! That price increase is actually for a Chinese supplier
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.
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.
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
. 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
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]
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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)
).
Well, I think that's covered the points you raised and I hope my observations have been of some use.
JBG