Hi Getting close to carrier with a notch filter involves a bit of calibration of the notch. It’s not imposible to do, but it is a needed step. The generator you use to do the measurement has to be pretty clean to get adequate data at low offsets.
Bob > On Oct 2, 2016, at 3:56 AM, Adrian Rus <adrian....@broadhurst.ro> wrote: > > 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 > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
