[time-nuts] notch filter for close in phase noise measurement

[Adrian Rus]

Hello list,
For those of you interested in phase noise measurement without using fancy/dedicated gear, here you are the way I have got. Disclaimer: as far as I am concerned, all phase noise measurements use a technique to get rid of carrier: quadrature mixing, interferometric [more on that, later] and notch filters.

The simplest way use notch filters, and the simplest notch filter can be arranged with just 3 elements:
- one return loss bridge
- one quartz crystal
- one resistor
Hook the crystal on DUT port, the oscillator to be measured on IN port, the SA [spectrum analyzer] on OUT port and the resistor on REF port. The resistor have to be determined by trial and error to equal the series resistence of the crystal at series resonance. From some -50dB up, can hook a potentiometer in parallel to the resistor[s] and fine tune for the deepest notch.
It is easy to get notches as deep as -85-90dB. The filter is useful in close in measurements not closer than 100-200Hz from carrier. Yes, between the notch and SA you should insert a 40-60dB amplifier. The amplifier will not degrade the flicker noise [as there is practical no carrier - see Rubiola papers], but will set the noise floor.
The series resonance freq have to be selected from multiple crystals; I have experienced series resonance in 10MHz crystals ranging from -300Hz to +100Hz against 10MHz sharp, and have selected a crystal resonating at +25Hz at room temperature. For exact fit you can either tune the oscillator @+25Hz, or better, thermostat the crystal; thermostating the crystal will also tune the notch to the desired freq.
My selected crystal was equilibrated by a series resistance of 14.7ohm. Please note, the series resistance of other 11 crystals I have tested range from 14ohm to tens of ohm.
Regards,
Adrian