User`s guide

AM Noise Measurement Fundamentals 11
Agilent E5505A User’s Guide 291
Calibration and Measurement General Guidelines
• The AM detector must be well shielded from external noise especially 60 Hz
noise. The components between the diode detector and the test system
should be packaged in a metal box to prevent RFI interference.
Also, the AM detector should be connected directly to the test system if
possible, to minimize ground loops. If the AM detector and test system must
be separated, semi-rigid cable should be used to keep the shield resistance
to a minimum.
• Although AM noise measurements are less vulnerable than residual
phase-noise measurements to noise induced by vibration and temperature
fluctuation, care should be taken to ensure that all connections are tight
and that all cables are electrically sound.
• The output voltage monitor on the AM detector must be disconnected from
digital voltmeters or other noisy monitoring equipment before noise
measurement data is taken.
• The noise floor of the detector may degrade as power increases above +15
dBm. Noise in the region of the detector is best measured with about +10
dBm of drive level. The noise floor is best measured with about +20 dBm of
drive level.
• An amplifier must be used in cases where the signal level out of the DUT is
too small to drive the AM detector or is inadequate to produce a low enough
measurement noise floor. In this case the amplifier should have the
following characteristics.
• It should have the lowest possible noise figure, and the greatest possible
dynamic range.
• The signal level must be kept as high as possible at all points in the test
setup to avoid noise floor degradation.
• It should have only enough gain to get the required signal levels. Excess gain
leads to amplifiers operating in gain compression, increasing their
likelihood of suppressing the AM noise to be measured.
• The amplifier’s sensitivity to power supply noise and the supply noise itself
must both be minimized.
NOTE
Read This The following general guidelines should be considered when setting up and
making an AM-noise measurement
NOTE
The internal detectors in the N5500A Option 001 and N5507A, along with the 70429A
Option K21 provide this level of protection.
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