7.3 Spectrum Analysis
The spectrum analyzer, like an oscilloscope, is a basic tool used for observing signals. While an oscilloscope provides a window into
the time domain, the spectrum analyzer provides a window into the frequency domain. Spectrum analysis provides a convenient way
to measure the amplitude of digitally modulated carriers and to troubleshoot ingress in the forward and reverse paths.
The following measurements and capabilities are possible:
Analog Channels
Direct channel tuning and positioning of visual and audio carriers
Visual and audio carrier level and frequency measurement
Survey of system visual and aural carrier levels and frequencies
Depth of modulation measurement
Audio carrier FM deviation measurement
Visual carrier-to-noise measurement
In-channel response measurement
Hum/low-frequency disturbance measurement
Intermodulation distortion measurement (CTB and CSO)
Digital Channels:
(applicable to QAM, QPSK, and OFDM signals)
Average power
Adjacent channel leakage (ACL)
CSO and CTB
Direct channel tuning and positioning of digital signal
Survey of system digital channel levels
System frequency response
To understand the capabilities of the spectrum analyzer and the various elementary settings, it is important to have a basic
understanding of how a spectrum analyzer operates. Referring to the block diagram below:
Simplified Spectrum Analyzer Block Diagram
1. The RF input signal first travels through the attenuator and the low-pass input filter where the attenuator limits the amplitude
of the signal, while the filter eliminates undesirable frequencies.
2. After the input filter, the RF signal is mixed with another signal generated by a Voltage Controlled Oscillator (VCO) to produce
an intermediate frequency. The frequency of the VCO is controlled by a repeating ramp generator, whose voltage also drives
the horizontal axis of the display. As the frequency of the VCO changes, the intermediate frequency sweeps through the
resolution bandwidth filter (IF filter), which is fixed in frequency.
3. A detector then measures the power level of the signal passing through the IF filter, producing a DC voltage that drives the
vertical portion of the display. As the VCO sweeps through its frequency range, a trace is drawn across the screen. This trace
shows the spectral content of the input signal within a selected range of frequencies.
CX380C User Manual RevA00
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