[time-nuts] LNA and Alias

Loïc loic.moreau at eai.fr
Mon Dec 22 04:26:15 EST 2014

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
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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.

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. 

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.


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