56 N9069C Noise Figure Measurement Guide
Measurement Related Tasks and Concepts
Setting the Measurement System
LO Harmonics (with specific DSB information)
Many mixers have product pairs associated with harmonics of the LO.
Depending on the mixer, these could be at a sufficient level to distort the
measured noise figure results. To avoid this insert an input filter between the
noise source and the DUT. A highpass filter may also be needed in this location
if signal leakage is a problem.
There is no general guidance on what filtering is needed. Each case needs
individual consideration:
1.
Decide the frequency ranges that have to be covered; Input, LO, and
Output.
2.
Calculate the frequency range that the LO harmonic modes will cover.
3.
If LO harmonic related products are a problem, choose a filter to go
between the noise source and the DUT, that will pass the wanted input
band and stop the LO harmonic modes. If the frequency ranges are wide,
the measurement may have to be split into frequency ranges with different
filters for each.
4.
Consider the LO frequency (and harmonics). Is a filter needed to protect
the analyzer input being desensitized by LO leakage in the 0 to 4.6 GHz
range?
5.
Choose a filter, if necessary, to go between the DUT and the analyzer.
The analyzer can handle DSB mixer measurements when using a
Downconverter
, or when the
System Downconverter
is
On
.
Choosing and Setting Up the Local Oscillator
Selecting a Local Oscillator for Extended Frequency measurements
with the X-Series
Because of reciprocal mixing, noise components in the LO are converted into
the IF band applied to the analyzer. This converted LO noise causes the
measured noise figure to be higher than the noise figure of the mixer.
If the mixer is to be used with a particular LO in its final application, its noise
figure should be measured with the same LO. The measurement then gives the
noise figure for the combination of the extended frequency device and the LO
in the final system.
For testing of extended frequency measurements, the LO must have a low
noise floor over frequencies equal to the LO ± IF. It is also important that the
LO has low broad-band noise because any noise at the IF frequency will pass
through to the IF and distort the results.
Summary of Contents for CXA N9000B
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