Well, this is wierd.....or is it?
I was running a 5hr test with my HP3245A, 3458A and R6581T.
Actually, I was testing the 10pF mod on the R6581T that MickleT came up with a day or so ago and posted.
Below is the playback of the 5hrs......notice the sudden 1.5ppm jump that the HP 3245A experienced.....and then drifted back down again!
The workshop aircon was not in it's tight metrology mode, and at 9pm switched off hence the temp stopped regulating and started to drop (winter here).
Coinciding with the 1.5ppm jump the workshop humidity appears to drop a little also, not sure if related.
For info, the PPM Dev. trace is on the R6581T.
My 3245A is modded, it's using an ADR1399 (3mA set) and with about 200hrs on it, and, I'm testing chopper op-amps on the Vref circuit.
The 3245A has also been off for the past few months and just switched back on about 50hrs ago.
I still have the original LM399AH so could go back to it for comparison.
Ideas?
Ian.