Hi, I have just found the culprit
The AD7760 eval board in charge of the ADC task, is equipped with a little 40 MHz clock witch seems to generate a beat note with my 10Mhz noise measurements setup. I have discovered the origin of the problem after disconnecting the LNA from the measurement chain and visualizing the output of the LNA with a scope. The beat note was clearly visible, 10mV peak-peak . After shutting down every 10Mhz sources in my lab (a huge task) and altering the frequency of my phase noise measurement setup noise floor, I noticed that the low frequency noise was in fact a beat note following the ref frequency adjustments. ( The ref OCXO was replaced with a 33521A to alter amplitude and frequency at will) . As soon as I fire up the 7.5V power supply of the eval AD7760 (the eval card is not connected to anything). the LNA give a steady 10mV beat note as the 40 MHz clock seems mixing up somewhere a few herz apart from the regular source, polluting the close in phase noise measurements . So after weeks of questioning, I was a bit disappointed to find out that the AD eval board, the least suspected item was at the origin of the problem. Now, I have to address it . I have noticed the presence of smb connectors on the eval card, so I may substitute the internal clock, and a shield enclosure may cure that little beast. At last DC power supply wires may also be substituted by shielded cable just in case. I am relieved Regards. Loïc -----Message d'origine----- De : time-nuts [mailto:time-nuts-boun...@febo.com] De la part de Bruce Griffiths Envoyé : lundi 22 décembre 2014 20:30 À : Discussion of precise time and frequency measurement Objet : Re: [time-nuts] LNA and Alias On Monday, December 22, 2014 09:26:15 AM Loïc wrote: > Bruce Griffiths <bruce.griffiths@...> writes: > > Are you sure that the setup is aligned to minimise AM response? > > > > Are you using cross-correlation? > > Bruce > > > > On Monday, 22 December 2014 5:08 PM, Loïc Moreau <loic.moreau <at> > > eai..fr> wrote: > > Hi all, > > > > My phase noise measurements system give erroneous results in close in > > phase noise measurements, I got > > > humps in the 1Hz-1000Hz area as high à 20 dB more than expected. > > > > The setup is using a mixer to compare reference and DUT witch drive an LNA > > http://www.ko4bb.com/~bruce/LowNoiseMixerPreamp.html , the output is > > driving an AD7760 ADC and an op > > > amp circuity is connected to the VFC reference to achieve quadrature. The > > results are analyzed with an > > > homebrew FFT charting software > > > > After struggling with different configurations, switched different > > LNA, > > ADC , sound card. A scope > > > connected to the LNA output indicate steady 20MHz residuals just before > > the ADC ( around 10 mv peak-peak). > > > In fact , it seems that the mixer 20Mhz residuals ( DUT + REF ) are > > entering the ADC and so theses alias give > > > erroneous results in the 1Hz-100Hz area, displaying unexpected > > artifacts. In order to fix the problem > > > I will probably include an analog filter just before the ADC input > > (same > > as LNA input 1nF 80µH), but I > > > want to know if some more sophisticated measures should be undertaken as > > an 5th Order Lowpass Filter. > > > I have not found many clue about alias problems in phase noise > > measurements literature so I may have > > > missed something ? > > > > Any advice > > > > Loïc > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@... > > To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/time-> > nuts > > > and follow the instructions there. > > Hi, > > I have a simple setup to measure the noise floor, the mixer is a SYM-2 and > a divider is feeding RF and LO inputs ports for that purpose. > > a pi network give the 90° phase difference. > Have you measured the phase shift when connected to the mixer? I usually use a narrowband 90 degree hybrid. > > The test is running with an HP 10811 giving around 7dBm, but i have > the same results with any sources, especially a 33521A witch can be adjusted > in level and frequency. > > > I intend to use cross-correlation later when i will be confident with > my the setup, for now i run a simple FFT. > > I have no clue about AM noise problem, i suppose that a sufficient > input level on LO will put that problem aside. > The phase shift between the RF and LO ports needs to be adjusted to minimise the response to AM in the source. To do this an AM modulator with low incidental PM is required. The phase is adjusted to minimise the sideband amplitude in the mixer output. > > by the way, I was a bit surprised that nobody pay any attention to RF > leakage from multiplier as the low frequency level is order or > magnitude lower than 20Mhz product present at the input. > Sufficient attenuation is usually achieved by using a well balanced mixer and by the mixer IF port termination and the low pass filter network between the mixer IF port and the low noise preamp. Using a JFET preamp can be useful in minimising any RF rectification effects due to input stage nonlinearity at RF. Bruce > > regards > Loïc > > > > > _______________________________________________ > 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. _______________________________________________ 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.