Interesting info Johnny
Incidentally, I sent you a PM.
For anyone looking to use a Ublox M6, 7 or 8 (inc the M8T), there is a UK vendor on eBay selling cheap boards with fake Ublox modules that are pin compatible with a genuine Neo M8T. Removing the old module needs care, but I managed it.
The eBay item is 195486735732 and includes a USB configuration socket and SMA antenna socket. You can use the boards with the original module, just take the module label with a large pinch of salt (the label on mine showed Neo-M8N, U-Center showed it looked like a 7).
While a ZED9T sounds nice, obtaining a board with one ready mounted, plus a dual band “survey” antenna, looks a bit much for my needs. The ZED9T is not pin compatible with the Neo 6/7/8 series.
SJ
I saw your PM but rather than answer it via PM, I think it would be better to reply here since the PM system doesn't support attachments and the reply, though peripheral to the topic thread, will nevertheless be of some interest to anyone thinking of DIYing their own cheap GPSDO setup.
For starters, just in case you missed Gyro's original post, have a read of this one page topic thread (it was the diagram he'd posted at the bottom of that page which had inspired me to create my first prototyped on a breadboard GPSDO from a genuine M8N module I'd bought several months earlier and a 13MHz AEL ocxo that I'd picked up for a mere 4 quid at the blackpool NARSA radioham rally just a couple of months after that)
https://www.eevblog.com/forum/projects/my-u-blox-lea-6t-based-gpsdo-(very-scruffy-initial-breadboard-stage)/msg929133/#msg929133 I didn't have a suitable RRO 5v cmos opamp in my parts collection at the time so I simply joined the LPF out direct to the EFC pin on the ocxo. Surprisngly at the time it worked quite well, considering the use of solderless breadboard to cobble the bits together.
Just this week, I discovered why the lack of the opamp hadn't upset the circuit - it turns out that the 10MHz sine output versions' EFC pin have an input impedance in excess of 10GΩ! I'm assuming (untested as yet) that this was also the case with the original 13MHz sq wave version, which I'd purchased at the rally, that I just happened to be using at the time.
So, it seems in retrospect that I could have gone one better (by design if I'd realised that the ocxo had possessed such a high EFC impedance) in reducing the IC count to just two (but I didn't have a benchmeter with a 10GΩ input option to test this at the time)! The fact that I
had managed to reduce the IC count without compromising the LPF had been rather lost on me (I was only trying this as a temporary bodge just to try and get some (or any!) sort of result to be getting on with. Of course, since I'd like to retain the EFC voltage monitoring feature, I've no intention of removing the RRO opamp from BoM.
Using a 13MHz ocxo to create a 10MHz gpsdo frequency standard is far from ideal even if not too technical a challenge to achieve (see the attached images of the hand drawn cct diagrams for the gory details).
I managed to touch lucky with a UK ebay seller who was selling the 10MHz sinewave versions of these AEL ocxos (supposedly NOS but in reality very carefully recovered pre-aged specimens). They were so cheap (£4.99 each - just a pound more than I'd paid for the 13MHz sample) that I took a punt on three. After testing them I bought another four. Only much later did I discover that they were indeed a high grade product by seeing exactly the same model mounted on the mainboard inside of a rack mount Symmetricom time and frequency standard in a YT teardown video.
As a result of this good fortune, I have a rather jaundiced view of these CTI ocxos so I'd recommend you use the NEC ones you mentioned in your PM and put the CTI one aside as a "consumable/disposable" test component. BTW, what are the specs on these NEC units? Presumably much better than the CTI, surely?
Since the ocxo / docxo / rubidium oscillator is the key component in any type of gpsdo design it's best not to skimp on its quality if you can afford it. The only good thing to be said about the CTI is that it's a cost effective step up from a vcxo. It was no accident as to why the G3RUH unit performed so well against commercial gpsdos since this was down to the deliberate choice of a high grade AXTAL ocxo.
http://www.leapsecond.com/pages/gpsdo/BTW, I'm using the Sparkfun ZED9 T adapter board rather than a bare ZED9 T module. A more expensive option I know but less likely to end up in the "Valuable Parts" drawer unused. The Sparkfun adapter board is much easier to neatly integrate into a DIY GPSDO project (I've already got it mounted onto the mainboard).
[EDIT] I forgot to comment on those cheap Ebay modules you linked to. Aside from the fact that they're fake uBlox modules, there's also the possibility that the groundplane, onto which those patch antennas are stuck, may have active circuit traces carrying high speed signals which can render the attached patch antenna a useless ornament due to the unfiltered LNA chip being driven into saturation.
This had finally proved to be real cause of the mystery failure of the patch antenna on my original genuine M8N module.
If you're buying them to extract the fake modules to solder into a custom GPSDO board, that doesn't matter except to anyone wanting to use them as is. [EDIT] As per usual, I managed to get hold of the wrong end of the stick regarding your purchasing those cheap break out boards to use as carriers for your collection of genuine u-Blox modules.
Hopefully, I've got the right end of the stick
this time.
Either way, the issue regarding a patch antenna groundplane being riddled with circuit traces to batter the unfiltered LNA into submission remains a valid concern.