Your adev graph look very much like when I tried to gps discipline mine. What happens if you disconnect it from GPS?
I'm going to try that next. Also the SR620 configured with the configuration from the PRS10 manual seems to get me 5 digits time interval from the picosecond range when I tried it with fine configuration. It was bouncing back and forth slowly between .050000 Hz and -.050000 Hz. It took it maybe 15 minutes to go back and forth. I used this setup:
Set-up for an SR620
Described here is the set-up for an SR620 Time Interval Counter to make precision frequency
measurements. For a detailed description for the operation of the SR620, refer to the
instrument’s operation and service manual.
Four input connections:
The 10MHz reference frequency is connected to both the rear panel 10MHz input and to the
“A” (START) input. (Place the tee on the rear panel input.)
Connect the 10MHz from the DUT to the “B” (STOP) input.
Connect the 1kHz TTL square wave from “REF” output to gate “EXT” input BNC.
Four input setups:
From the front panel “CONFIG” menu, use “SET” to choose the “cAL” menu, then use
“SELECT” to select the “cLoc SourcE”. Use the arrow keys to set the clock source to
“rEAr”. This will allow the SR620 to use the 10MHz reference frequency which has been
applied to the rear panel 10MHz input as the timebase for all measurements.
Set the “EXT” gate input “LOGIC” to POS, “TERM” to 50Ω, and “LEVEL” to +1.0V. The
“TRIG” LED will go on when the “GATE/ARM” is setup properly.
Both “A” (START) and “B” (STOP) are AC coupled and terminated into 50Ω. The
“SLOPE” is set to “+”, and the “LEVEL” is turned full counter-clockwise to “AUTO” and
the “UHF” LED should be off. The “TRIG” LEDs will be on when the 10MHz sources are
present.
“Coarse” Frequency Measurements
You should verify that the DUT is very close (within 0.1Hz) to 10MHz. To measure the
frequency, set “MODE” to “FREQ”, set “SOURCE” to “B”, set the “GATE/ARM” to 1s, and
set the “SAMPLE SIZE” to 1. Hold the “START” button down for a few seconds to start
continuous measurements. Set the display to “MEAN” to display the frequency of the
10MHz output from the DUT.
“Fine” Frequency MeasurementsIf the 10MHz from the DUT is within 0.1Hz of 10MHz, you may use the “fine” frequency
measurement technique to make measurements to a few parts in 10 to the 12th in a one second interval.
As explained above, the frequency offset between the reference and the DUT is inferred by
time-interval measurements between their zero-crossings.
To carry out these measurements: Set the “MODE” to “TIME”, select the “SOURCE” (of
START) to “A”, set the “GATE/ARM” mode to “+TIME” and “EXT”, and set the
“SAMPLE SIZE” to 1000. With the external gate triggered by the SR620’s 1kHz reference
output, the unit will display a new “MEAN” every second. If the DUT is adjusted so that the
mean of the time interval measurements changes by less than 10ps per second, then the DUT
is within 1 part of 10 to the 11th of the reference frequency.
Bill