Another potential issue is that the setup should be adjusted for minimum AM sensitivity.Because of diode mismatch and phase mismatch between the internal transformers this doesn't occur when the mixer dc output is zero. To find the correct point a dc offset needs to be introduced at the input of the PLL integrator. The offset is adjusted to minimise AM sensitivity. In order to do this an AM modulator with very little incidental PM is required.This is tricky to implement although it has been done using an unbalanced Mach-Zehnder interferometer plus a pure Phase modulator (easy to do since AM detectors aren't phase sensitive): Microwave Sources of Pure Phase and Amplitude-‐Modulated Signals E. N. Ivanov
This adjustment is important as the residual AM of most RF sources isn't negligible.The technique works uses standard components and techniques (3 db hybrids, mixers, voltage controlled phase shifters, lock in amplifiers) and works at far lower frequencies than microwave. At lower frequencies simpler AM modulators may suffice but you would need to show that any incidental PM they produce is insignificant in that it wont have a significant effect on the depth of the AM null at the mixer/phase detector output.Typically a modulation frequency in the audio band would be used. Bruce From: Oleg Skydan <olegsky...@gmail.com> To: Discussion of precise time and frequency measurement <time-nuts@febo.com> Sent: Thursday, 31 March 2016 9:28 PM Subject: Re: [time-nuts] Oleg' s PN test Re: A new member & PN test set -------------------------------------------------- From: "Bruce Griffiths" <bruce.griffi...@xtra.co.nz> > You actually need to measure the filter > response. OK. It is here (the frequency span is 2..102MHz, the amplitude axis is 10dB/div): http://skydan.in.ua/PNTestSet/PN_LPF1.jpg Sorry, the network analyzer is a bit older than I am :), but it is still in a good condition. At 100MHz we still have more than 40dB attenuation. The inductor I used is low Q axial choke, so I do not expect multiple resonances at higher frequencies - there should be large losses and inductor will look much more like a resistor (at least until we go too high). The capacitors in the pi-LPF are 0805 SMD good quality ones. But, we all like perfect things :), so I tried to make the LPF a bit better adding the BLM31AJ601SN ferrite bead in series with the inductor. Here is the result: http://skydan.in.ua/PNTestSet/PN_LPF2.jpg I like it :) I also did another test checking DC shift at the AD797 output when the mixer was fed with two signal generators (there should be no DC - only different combinations of RF/LO signals). I recalculated all signal levels to LNA input point. Before installing the bead the DC shift was: <150MHz less than 150uV 150..250MHz less than 400uV 250..500MHz less than 1mV After installing the bead: <260MHz less than 80uV There is one big peak near the 300MHz where the DC goes up to 900uV, and several smaller peaks (up to 250uV) higer, up to 500MHz. When the two mixer ports are fed with the same signal (inphase) the DC voltage at LNA input is 130..150mV. With no RF at the mixer ports DC unbalance is 20uV (all voltages recalculated to LNA input). The filter can be made even better by cascading several pi sections using different capacitors and inductors/beads. But as far as I understand, with the current filter even at 500MHz it will not move too far from the quadrature, and at 60MHz everything is definitely OK. All the best! Oleg _______________________________________________ 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.