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Fig. 3.10 Measuring Small Signals by Using Track Average
3.4 How to Identify Signal with Closely Spaced Frequency
3.4.1 Description of resolution bandwidth
Signal resolution is determined by the bandwidth of IF filter of the Spectrum Analyzer, i.e. the resolution bandwidth
(RBW) When a signal passing through the IF filter, the Spectrum Analyzer uses the signal to sweep the band pass
shape of the IF filter. When the Spectrum Analyzer receives two signals with the same amplitude and closely spaced
frequency, the top of a band pass filter waveform swept by one signal will cover the other signal, and as a result, the
two signals may look like one signal. If the two signals have different amplitudes but the frequency is still close, then
the small signal may be covered up by the response of large signal.
3.4.2 Measurement for distinguish two signals with same amplitude
In general, in order to distinguish two signals of same amplitude, the resolution bandwidth must be less than or
equal to the frequency spacing of two signals. Taking distinguishing signals of same amplitude with a spacing of
100 kHz as an example, the user should select the resolution bandwidth of the Spectrum Analyzer as less than or
equal to 100 kHz.
Signal generator
Spectrum analyzer
T connector
Signal generator
Fig. 3.11 Test instrument connection diagram of two input signals
a)
Reset the spectrum analyzer