Andrew, I eagerly await the chance to acquire a PCB to assemble this project, which seems to keep growing in cost and complexity with time. All I want is 10 MHz, no other functions required. Perhaps it will be easy for me to see how to get that.
It seems to me that you're attempting to build your very first DIY GPSDO and trying to overcome your fear of starting a project with a seemingly high level of complexity that you're worried you might not be able to master without making a rather greater investment in both time and money than you can afford.
In view of your stated requirement for a basic 10MHz reference, may I suggest that you start with the most basic of GPSDO projects as outlined by Gyro in this topic thread here:-
https://www.eevblog.com/forum/projects/my-u-blox-lea-6t-based-gpsdo-(very-scruffy-initial-breadboard-stage)/msg929133/#msg929133 As he mentions, it was inspired by James Miller's simple GPSDO (a small factoid I'd forgotten when I first described my own initial GPSDO projects as being inspired by this version, namely the circuit diagram he'd attached in the final thread posting).
I'd substituted the 74HC4046 with a 74HC86 quad XOR IC on account I didn't have a 74HC4046 handy but I did have a few 7486s to hand to exactly emulate the PC1 portion of the HC4046 being utilised. Also I'd left out the dual RRO 5v cmos opamp and fed the output of the LPF direct to the the VFC pin on my 13MHz square wave output OCXO which I was feeding into a TTL based divide by 1.3 circuit to generate a 10MHz square wave locked to the OCXO.
Whilst a 13MHz square wave OCXO can be used to make a 10MHz GPSDO reference source, it really has little to recommend it other than, in my case, it being a cheap way to get an OCXO that can directly drive logic gates without the need to include a voltage level shifter (as I discovered, much to my chagrin, when I first started using a 10MHz sinewave version of these AE CQE OCXOs -ex Symmetricon kit as I later found out).
I was using a ublox M8N (my very first GPS module purchase and, rarity of rarities, a genuine one to boot!) in place of the LEA6T used by Gyro. I built the whole thing on a solderless breadboard, experimenting with various enhancements before transferring it onto vero stripboard and boxing it all up into a rather petite 110x110x50mm extruded aluminium enclosure which completed my MK I GPSDO which I powered from a cheap 12v plugtop psu (slightly modified to kill off the "Y" cap induced 90vac mains leakage 'touch voltage' that typically afflicts such smps based wallwarts).
I'd made the choice of external psu power deliberately to facilitate rapid swap out of a failed psu to minimise the down time involved in disassembling it to troubleshoot an integrated mains psu, plus, this would also eliminate a significant source of internal heating.
As a further measure to minimise internal waste heat, I'd used a low noise 7 to 24v input 5v output buck converter (designed to power the avionics in drones powered from 3 to 6s lipo battery packs) rather than the classic 7805 which would otherwise have required the additional complication of heatsinking to the aluminium case. This gave me another degree of freedom regarding my wallwart output voltage options since it can be powered from supply voltages ranging from 7 right up to a maximum of 24 volts and still only draw (OCXO at set temperature) 1.7 to 1.8 watts in the case of the MK I (the MK II only needs 1.3 to 1.4 watts over this range after the OCXO has come up to temperature).
The main downside with using a navigation class GPS receiver module in such a basic James Miller based design is the 30 to 50ns phase wander due to ionospheric effects (space weather). Long term, with impractically long PLL time constants, it can match the much more expensive single and multiband timing modules. This where a cheap microcontroller and properly optimised firmware can win out but there are a whole bunch of temperature related issues that have to be dealt with which the basic James Miller design can neatly 'sweep under the carpet' as demonstrated in this interesting comparison here:-
http://www.leapsecond.com/pages/gpsdo/ The significant improvements over the MK I GPSDO I'd made in the MK II were the use of a 10MHz (sine wave output) OCXO (same AE CQE brand) allowing elimination of all the noisy and power hungry TTL 'Magic' required to create a 10MHz clock from a 13MHz OCXO and a GPS Rx module upgrade to a ublox M8T which can be 'surveyed in' to put it into 'overdetermined mode' allowing it to retain a locked PPS output even when only a single valid SV signal is available (the navigation types such as the M8N require a minimum of four good SV signals to maintain a valid locked PPS output - not a problem when feeding it from a cheap active puck antenna mounted where it has a clear all round view of most of the horizon).
With the MK II, I only see around a 6 to 7ns Pk-Pk phase wobble on a time scale of a minute or three which makes syntonising my LPRO 101 to the GPS atomic time reference much easier. I've connected a ten turn helipot to the C field input, padded out by a factor of ten with a pair of 22k resistors either side to provide a much finer adjustment of the Rubidium's frequency than is available from its internal ten turn trimpot.
Even with such fine control, it only takes a degree or two of rotation to discern its effect after allowing the minimum of two or three hours required to identify the drift against the GPSDO's background phase wobbles. The toughest part of this syntonising process is resisting the urge to fiddle with the helipot before enough time has passed by to properly identify how much, if any, drift has occurred. In this, patience is most definitely a virtue.
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/smiley.gif)
Since encasing my fan cooled LPRO in a 2cm thick layer of polystyrene foam about three weeks ago to minimise changes to the internal thermal gradients due to variations in room temperature, I'm now routinely seeing some 30 to 60ns net daily phase shifts between it and the GPSDO as recorded by the infinite persistence displayed after a 24 hour run using an SDS1202X-E to compare the output waveforms.
This shows that it's possible to syntonise a rubidium oscillator to better than 10E-10 with a very basic GPSDO. If you're not planning on doing anything more with your rubidium oscillator than simply mount a large passive heatsink to its baseplate to prevent it overheating as most video bloggers appear to have done and leave it at the mercy of room temperature variations, a simple GPSDO will more than suffice to syntonise it to around the 10E-9 mark. Even the more simplified version of Gyro's that I described should be good enough for the job.
The component parts won't go to waste since the expensive bits will be needed anyway when you eventually do come round to adding a microcontroller into the mix and the experience gained will ultimately prove rather useful.
I've attached four images which might prove inspirational (if inspiration was needed).
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/smiley.gif)