Interference Testing with Handheld Spectrum Analyzers Manual

Interference Testing with Handheld Spectrum Analyzers Manual
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Interference Testing with Handheld Spectrum Analyzers Manual

  • 13Interference affects a radio system when it enters the receiver and corrupts the detector in the receiver. If the amplitude of the interference is very large, the interference can overpower the receiver’s front-end electronics and reduce system performance. Filters are added to the receiver to eliminate interference and noise from entering the system, but any interference that falls within the passband of these filters are combined with the desired signal.Receiver anatomyFigure 7 shows a simple block diagram of the four main functions of a receiver: amplification, down-conversion, filtering, and detection.Receivers and Spectrum AnalyzersThe bandpass filter (BPF) at the input is set wide enough to allow the entire block of frequency channels to pass while rejecting interference outside the operating frequency range. Amplification in a receiver is required for two reasons: first is that power entering the receiver antenna can be as low as -100 to -120 dBm and amplification is necessary to increase the signal power above the required sensitivity of the detector. Second, the receiver electronics will add noise to the signal as the signal moves along the receive path. By adding a low-noise amplifier near the input, the receiver’s signal-to-noise ratio (SNR) can be improved. Amplification is typically spread along the entire receive path but is shown in Figure 7 as a single component for simplicity in the diagram.The down-converter block function is to tune the system to a specific frequency channel and then translate the high frequency radio signal to a lower frequency in order to ease the process of detection. The lower frequency output from the down-converter is selected to match the center frequency of the channel filter. The channel filter improves the selectivity of the system by attempting to reject signals in the adjacent channels and beyond. The detector, often referred to as the demodulator, recovers the transmitted data that may include voice, video, or other forms of data.Figure 7. Block diagram of a receiver system. Dotted line represents the components in a typical spectrum analyzerOutDetectorChannel filterDown-conversionAmpBPFIn