74 Chapter 3
Making Measurements
Tuned RF Level Measurement
Tuned RF Level Measurement
This procedure describes how to make a Tuned RF Level measurement. In this
measurement example, a signal generator is the UUT and is adjusted to deliver a
test signal. Tuned RF Level measurement is used to measure that signal. The
source power is stepped down and the PSA displays the Tuned RF Level of the
signal.
Unlike RF Power measurements, which measure total power across a wide
frequency band, the Tuned RF Level measurement requires the PSA to be tuned to
the frequency of interest and is capable of measuring low power levels. This is
particularly useful when a step attenuator or a signal generator is tested for its step
accuracy of power output with incremental changes.
NOTE You need a Power Meter with a Sensor Module to make an “absolute” Tuned RF
Level measurement. The Power Meter provides the initial absolute power
reference for the PSA.
Alternatively, you can make a relative Tuned RF Level measurement without a
Power Meter.
Avoid Residual Responses
As the displayed average noise level of a spectrum analyzer becomes very low,
residual responses can be revealed. This is important when measuring signals in
the –130 to –140 dBm range. These residual responses are rare and almost always
occur at multiples of 50 MHz.They are caused by various internal clocks and
mixing products. Residuals are present on all modern spectrum analyzers and will
limit the dynamic range of the analyzer when the residual appears at, or very close
to, the signal to be measured.
To determine if a residual is present, place the spectrum analyzer in spectrum
analysis mode, terminate the input with a 50 ohm load, and use the following
settings:
Frequency: the desired measurement frequency
Span: 1 kHz
RBW: 3 Hz
Ref Level: –60 dBm
Avg Number: 10
If a residual is identified, choose a measurement frequency to avoid the residual. A
spacing of two resolution bandwidths away from the residual will provide 48 dB of
attenuation. For example, if the default 10 Hz RBW is used, make your
measurement ±20 Hz away from a residual.
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