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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 digram below:
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.
Simplified Spectrum Analyzer Block Diagram
7.4.1 Setup:
The following setup parameters are available:
Center Frequency (MHz) and Mode Forward and Reverse
The center frequency selection sets the frequency of the center of the scale to the chosen value - normally where the signal to be monitored would be located.
In this way the main signal is in the center of the display and the frequencies either side can be monitored.
Vertical scale (dB)
Sets the logarithmic scale to 5dB or 10dB per division
Resolution Bandwidth (kHz, MHz)
Selectable filter sizes (125kHz, 330kHz, 1 MHz)
Resolution bandwidth is the bandwidth of the IF filter, which determines the selectivity of the spectrum analyzer. A wide resolution bandwidth is required for
wide sweeps, while a narrow filter is used for narrow sweeps.
The narrower the RBW setting, the better the frequency resolution; however, narrower filter settings require more measurement time
Wider filters are used when the display needs to be updated rapidly, or when wide modulation bandwidths are to be displayed.
Attenuation (dB)
To ensure that the input stages of the analyzer are not overloaded, an RF attenuator is used. If input stages are overloaded, spurious signals may be
generated within the instrument resulting in possible false readings. However, if too much attenuation is inserted, additional gain is required in the later stages
(IF gain) and the background noise level is increased, and this can sometimes mask lower level signals. Thus, a careful choice of the relevant gain levels
within the spectrum analyzer is needed to obtain optimum performance
Dwell Time (ms)
The speed at which the analyzer scans the frequency span is important. The faster it scans the frequency range, the faster the measurement can be made.
However, the scan rate or dwell time of the analyzer is limited by IF filter (RBW) and the video filter that may also be used to average the reading. These filters
must have time to respond. Otherwise signals will be missed and the measurements are rendered useless. Nevertheless, it is still essential to keep the scan
rate as high as is reasonably feasible to ensure that measurements are made as quickly as possible.
Spectrum Analysis - Analog Signal
CX350 e-Manual D07-00-037 RevC01 Page 32 of 81