[time-nuts] LNA and Alias

[Loïc Moreau]

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-bounces at 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 at ...> 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 at ...
> > 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
> 
> 
> 
> 
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