Cheap 10 MHz GPS "reference"

A while ago I was able to buy a Philips/Fuke PM6669 frequency-counter on eBay. This counter included the PM9607 option (Mathematically Temperature-Controlled Crystal Oscillator). Counters of this quality are usually very accurate, even if they have not been calibrated for a long time. But because the owner couldn't tell me anything about it, I had to check it myself. With the help of the service manual this should not be too difficult. The calibration is done with a 10MHz signal on the input and then pressing a button combination, the rest would then happen automatically. Piece of cake.
But still there was a problem, I don't have a good 10MHz signal source. There are GPSDO (GPS Disiplined Oscilator) for sale on the internet. They are not too expensive (150-200 euros), but I thought the price was too high to just only one adjust counter, so I had to come up with something else

On Youtube I came across a video of Andreas Spiess, the guy with the the Swiss accent. In this video he discusses the OXCO and the GPSDO, but also another very interesting alternative. This alternative is showed after 15:44 minutes. There he deals with a very cheap GPS receiver module based on a Ublox. You can find them in all shapes and sizes on Aliexpress. The module has a 1 pps output and this signal is always present, even if there is no GPS lock. Of course with a GPS fix the signal is more stable. Because mine didn't have a decent output, I soldered an SMA connector on it, but if you look carefully on Ali you can also find modules that already have this one.
With a FTDI adapter and the Ublox envaluation software "u-center" (also available on the Ublox website) this 1 ppm signal can be set to any frequency below 24 MHz, so also on the 10 MHz we require. Andreas explains in his Youtube video how to set up the software.

I have no idea how accurate this signal is, so if anyone can compare this with a real GPSDO signal I'd be very curious to see the results. For my simple DIY projects my counter with the MTCXO is accurate enough and I it set this way "as close as possible".

A very strange square wave

Curious as I was I wanted to see what the 10MHz square wave would look like and I presented the signal to my oscilloscope. Instead of a neat square wave I saw a very strange pattern because a lot of jitter. The square wave seemed to have been chopped into irregular pieces. What was going on here, I had no explanation for it.
Fortunately, I came across a video on Youtube that told me exactly what was going on and how it came about.
I'm not going to repeat his entire argument here, but a small hint: 24 divided by 10 is not an integer. If you want to know exactly what's going on, watch the video.
As far as I have been able to observe, my counter has no problems with the irregular square wave / jitter, but this can of course be a problem with other counters.

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