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34 Chapter 1
Making Basic Measurements
Measuring Low Level Signals
Measuring Low Level Signals
The ability of the analyzer to measure low level signals is limited by the
noise generated inside the analyzer. A signal may be masked by the
noise floor so that it is not visible. This sensitivity to low level signals is
affected by the measurement setup.
The analyzer input attenuator and bandwidth settings affect the
sensitivity by changing the signal-to-noise ratio. The attenuator affects
the level of a signal passing through the instrument, whereas the
bandwidth affects the level of internal noise without affecting the
signal. In the first two examples in this section, the attenuator and
bandwidth settings are adjusted to view low level signals.
If, after adjusting the attenuation and resolution bandwidth, a signal is
still near the noise, visibility can be improved by using the video
bandwidth and video averaging functions, as demonstrated in the third
and fourth examples.
Measuring Low Level Signals Example 1:
If a signal is very close to the noise floor, reducing input attenuation
brings the signal out of the noise. Reducing the attenuation to 0 dB
maximizes signal power in the analyzer.
CAUTION The total power of all input signals at the analyzer input must not
exceed the maximum power level for the analyzer.
1. Connect a signal generator to the analyzer input.
2. Set the signal generator frequency to 300 MHz with an amplitude of
−80 dBm.
3. On the analyze, perform a factory preset by pressing Preset,
Factory Preset (if present).
4. Set the center frequency of the analyzer to 300 MHz by pressing
FREQUENCY, Center Freq, 300, MHz.
5. Set the span to 5 MHz by pressing SPAN, Span, 5, MHz.
6. Set the resolution bandwidth to spectrum analyzer coupling by
pressing
BW/Avg, Res BW (SA).
7. Set the Y-Axis Units to dBm by pressing AMPLITUDE, More,
Y-Axis Units,
dBm.
8. Set the reference level to −40 dBm by pressing AMPLITUDE, Ref Level,
–40,
dBm.
9. Place the signal at center frequency by pressing Peak Search,
Marker→, Mkr→CF.