Loic,
If you have a single-pole low-pass filter at say 20 kHz you will have a
60 dB damping as you hit 20 MHz. No reason to get really fancy.
Either just do a two-pole low-pass filter (inductance, capacitance) or a
two-stage RC-link, or consider doing a LC-series link between signal and
ground to swallow that 20 MHz, providing a notch.
Do it near the mixer, no need to polute the amplifier with the signal.
You can do another LC-link at the ADC end to make sure you swallow that
20 MHz. Consider adding additional such links for say 10 MHz or other
reasonable likely mixer products.
Cheers,
Magnus
On 12/21/2014 09:39 PM, Loïc Moreau 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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