[time-nuts] LNA and Alias

[Loïc Moreau]

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