[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
> 
> _______________________________________________
> 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.


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.
 

regards 
Loïc
  





More information about the time-nuts mailing list