I rather doubt the tuning rate would be much of an issue with these ocxos (casual observation during the initial lock in phase, after the gps receiver module had achieved a lock with the minimum of 4 good gps svs suggested a positive tuning rate of ~3Hz/volt). The required tuning voltage for 10MHz ranged from a high of 4.49 down to a low of 2.34 volts (this last being the one I used in the MK II gpsdo which has drifted down by just 15mV over the past two or three years).
A recent test using a bench meter on its Hi-Z setting (10G ohms) suggests an infinite dc impedance on the EFC pin. Most likely simply something like a 10MR/100nF LPF connection to the the reverse biased varactor tuning diode which consequently accounts for the positive tuning rate (as is typical of virtually every electronically tuned XO regardless of whether they're just a tunable xo (Siglent 1000 and 2000 AWG series), a tcxo or an ocxo).
Also noteworthy is the use of input clamp diodes to limit the EFC tuning voltage between ground and Vcc. I remember testing that 5 volt powered 13MHz ocxo using a 9v battery (with a 10K current limiting resistor) to see if it would tune past the 5 volt limit. It didn't so I concluded it must have had clamping diodes. This test btw had been inspired by the Vectron datasheet's mention of a 10v upper tuning range voltage limit.
I don't recall repeating a similar test with the 12 volt powered 10MHz units (most likely because the worst case requirement was safely below the 5 volt limit of the PLL's output giving me no urgent cause to check this out). However, having now mentioned this, I'll repeat this 9v tuning test on the 4.49v ocxo some time soon to check out my clamping diodes hypothesis
The point I was trying to make was that if you happen to chance upon a seller offering these ocxos at a sensible price (sub 30 dollars), they'd be worth taking a punt on (assuming the seller has a decent 95 + % rating of course).
Also worth mentioning is the fact that the oscillator output must be unbuffered since my injection locking to an external 10MHz source in the one I'd used to upgrade my cheap Feeltech FY6600 only worked by injecting into its output pin, that plus the fact that I had to add a 100 ohm resistor across the output of the MK II's ocxo to eliminate a puzzling jitter on the resulting square wave output from the 74HC14 no matter how I trimmed the mid voltage bias on the gate's input pin (I'd connected directly via a 1nF cap without any load in order to get the maximum voltage swing into the 74HC14).
I guess the oscillator had been designed with a 50 ohm loading in mind (possibly including a 75 ohm loading requirement) to prevent clipping induced jitter effects such as the one I'd experienced where it had taken the form of a change in duty cycle every alternate cycle. The 100 ohm had been chosen as a compromise between highest possible voltage swing without this peculiar jitter effect. At least one other EEVBlog member had also reported a similar peculiarity with another brand of sine wave output ocxo which vanished when he tried my solution.
In retrospect, a better choice of resistor would have been 75 ohm. I had made a crude test of the output impedance of these ocxos and got figures ranging from a high of 90 ohms down to a low of 76 ohms, hence my choosing the 100 ohm option as an initial compromise value which seemed to be sufficient to eliminate this weird jitter behavior on the first try.
One other thing worthy of mention is the useable voltage range these will function with. If you don't need the thermally stabilised voltage reference, you can use 6 to 13.2v (tested to a maximum of 14 volts btw) otherwise it's 11.5 to 13.2 volts if you need the 5.128 or so volt reference. Since the heater current is limited to a maximum of 280mA, a lower voltage can ease the initial startup power requirements at the expense of warm up time before it starts bouncing up and down as it over and undershoots the oven target temperature.
In the end it makes no difference to the time they take to reach a sufficiently stable temperature some 20 minutes or so regardless of whether you use 5.5 or 13.2 volts. In your case, you've no such option to change the ocxo's supply voltage but in a DIY gpsdo project this can be an important consideration with regard to the power ratings of a 2.6 to 5.5v input boost to 12v output converter module feeding both the ocxo and a buck converter module powering the 5 volt logic rail all powered from a 2.1A continuous rated 24v max input buck converter supplying it and a float charged at 3.85v protected LiPo cell that happens to have completely discharged to its cut off point.
Using three dc-dc converters like this allows me to power the MK II from a minimum of 4.5v (below the cut off point of a battery bank) to a maximum of 24v (5 to 15 volt 10 watt rated wallwarts and 19 to 22 volt laptop charging bricks for example).
I've no doubt you're wondering why I've gone to so much trouble but the long and short of it is that I was getting totally pissed off by the ocxo's oven controller's boot up behavior in response to even the briefest of supply interruptions (whether those caused by a wallwart swap over or a cable rerouting exercise or the 10 to 20ms interruption when using the type of power bank that can act like a UPS (feeding the gpsdo via a 5 to 12v boost converter).
The problem in this case being due to the oven controller's "Shoot first, ask questions later." startup algorithm where it blindly assumes a literally cold oven startup requiring the maximum heating current to reach operational temperature before it checks whether such an extreme response is really necessary. In this case the result is a massive overshoot, followed by the inevitable under/overshooting that requires another ten minutes before the PLL stands any chance of winning this game of chasing "Will o' the Wisp". The two or three seconds for the gps receiver to regain lock is tolerable enough but the disruption caused by the ocxo's hot startup behavior is anything but.
I modified the 5 to 12v boost converter to 'paper over the' deficiency of a battery bank's brief interruption during any power source changeover or loss of its input power but realised this still didn't solve the problem caused by swapping out to another power source or a supply lead rerouting exercise so opted for the internal back up "battery" solution using one of a couple of LiPo cells I'd acquired for free.
I only needed a mere 5 minutes of autonomy which could have been served with a couple of expensive 200F supercaps so it was a 'no-brainer' to make use of what I already had to hand, landing up with even only a 60% charged LiPo cell providing some 110 minutes of autonomy (now reduced to 90 minutes with the GNSS/RTK multiband timing antenna upgrade. Still an order of magnitude more than I required.
Since I had to add a third dc-dc converter to the power train, I took the opportunity to extend the low voltage input limit from 6.8 down to 4.5 volts to maximise the benefit of all this radical modification effort.
Anyway, there you have it : all you really need to know (and then some!) about these "CQE" ocxos should you chance upon a cost effective source.
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