Below what I see when I switch my wifi router on/off (confirmed, I can send morse keying the router on/off
).
The router is ~4m off the plastic box with the 10V Vref and 34401A.
Notebook 1m off the box, wifi on all the time.
The measurement taken on a standard LM399AH wiring (dead bug 78L15+OP07+LT5400), the heater blocked 4u7 tantalum, the zener with 100nF ceramic close to its 4 pin socket plus 4k7/3u3_foil low pass towards opamp. 78L15 blocked and output of the OP07 blocked 25ohm/100nF_foil.
34401A with 1m long twisted cable rolled to a small 15cm dia bundle.
An stm32 MCU on the RS232 with the HC-05 sending data to my notebook.
The router on/off difference is always aprox -1.5ppm as depicted.
Interestingly - when the wifi router is ON the Vref output is "more" stable and quiet.
PS: y axis is ppm (filtered), blue is the temperature inside the Vref box (not related to the issue).
PPS: a clip-on ferrite bead put on:
a. RS232 cable - no change
b. 34401A close to its inputs - no change
c. close to the Vref output - the on/off ppm difference halves (-0.7ppm)