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87
III. Measurement of small signals
This Section illustrates operation method of accurate measurement of small signals using the
S5105D/E/Fmicrowave analyzer by taking the measurement of a signal of -80dBm as an example.
1
Reduce resolution bandwidth to measure small signal
The noise generated inside the microwave analyzer determines the ability of the microwave analyzer to
measure small signals. There are several methods to change the measurement settings to improve the
measurement sensitivity of the microwave analyzer.
The resolution bandwidth affects the internal noise base of the microwave analyzer, but has no effect on
the level of the measured continuous wave signal. If the input signal is very close to the noise base and
the resolution bandwidth is reduced, the signal can be extracted from the noise.
The relationship between the amount of noise reduction and resolution bandwidth can be expressed by
the following formula:
L
10log BW
1
BW
2
Where:
L—the amount of change in noise amplitude in dB.
BW
1
, BW
2
- Different resolution bandwidths in Hz.
So when the resolution bandwidth is reduced by 10 times, the noise base is reduced by 10 dB.
Steps for measuring small signals by reducing the resolution bandwidth are:
a)
Reset the microwave analyzer:
Press
【
Reset
】
, [Default State] .
b)
Set the signal generator and connect to the microwave analyzer:
Set the signal generator frequency to 1 GHz and the amplitude to -80 dBm. Connect the RF output of the
signal generator to the RF input of the microwave analyzer and turn on the RF output of the signal
generator.
c)
Set the center frequency, span and reference level of the microwave analyzer:
Press
【
Frep
】
, [Center Freq] , 1 [GHz];
Press [Span], 2 [MHz];
Press
【
Amplitude
】
, [Ref Level] , -40 [dBm]
Press [Peak] to place the marker on the peak of the large signal.
The measurement results at this time are shown in Figure 5-16.
