[time-nuts] LNA and Alias

[Bruce Griffiths]

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
> > 
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> 
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> 
> 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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