This article documents the startup characteristic of the OCXO in a Motorola R2009D comms analyser.
Frequency error from cold start was logged using the FA-3-6G counter to make record of the time for reasonable stabilisation from cold. The R2009D had been calibrated to within 100mHz some days before this test.
This test was performed with power applied to the box, but the front panel switch in standby position.
Above is a plot of the startup.
At powerup, the oscillator is 514Hz high (error 5.14e-5), at 3min error is 2.8e-5, at 5min, error is 6.4e-6, at 20min error is -1.2e-7, improvement beyond this is very slow.
The front panel “Oven ready” LED is not a good indicator that the oscillator is very close to frequency, it comes on when OCXO heater current reduces, but the oscillator has not yet stabilised.
Calibration was carried out with the whole analyser powered and operating for 24h, the oscillator will not reach calibration frequency unless the whole box is operating. Experience is that setting the reference oscillator closer than 100mHz is wasted effort.