Over a longer period of time, I wanted a 10 MHz GPSDO as a standard for the lab. But I was not sure, which one to get and our friend and eevblog member "ZUCCA" offered to send me his HP Z3805A in the mail for testing.
I started with a cheap GPS Antenna but could not get Z3805A to go in to "GPS-LOCK"
Then I tried a much more expensive GPS Timing Antenna and also could not get the Z3805A in to "GPS-LOCK"
Suspecting that something was wrong with the unit from ZUCCA, I ended up, ordering the very same Z3805A unit from the same seller in China. It arrived in a few days and I tried to get this one to go in to "GPS-LOCK" but it did not lock as well.
Then I ordered a new antenna. This time a Trimble Miniature 5V GPS Antenna P/N: 56237-00
This one seems to be much higher quality, including the lab quality SMA connector.
With this Antenna, I got the unit from Zucca in to "GPS-LOCK"
Then I ordered the same Antenna again for my unit and now both are in GPS Lock for a few days already.
But you have to have patients, it takes anywhere from 1 to 4 hours, before the unit will show the GPS-Lock light ON.
Now the question was, how stable is the signal from these two Z3805A units.
I hooked up my two counters and a scope
Placing the counters in RATIO mode "Frequency 1 / Frequency 2" should give me a good reliable reading, even if my counters are not perfectly calibrated.
And these are the results:
Counter 53131A shows: 1.000,000,00 and does not move at all
Counter 53230A shows: 1.000,000,000,001,9 but the last three digits are moving slightly
On the scope, I am measuring the Phase shift between the two waveforms
From yesterday to today, the phase shift was about 30 degree.
Which means, if this phase shift would stay constant, it needs 12 days for a full 360 degree cycle.
So, I think it is safe to assume, that both units are in perfect GPS lock and we can rely on the 10 MHz output.
So, before I am sending this one unit back to Zucca, does anyone here in the forum has some suggestions of what to measure and how to compare the two units for stability? I don't think it happens too often, that we see two GPSDO's next to each other.