Chapter 9 / 9.2 / 9.2.9

9.2.9 Receiver Performance Criteria

Having examined each functional stage, we now formalize the principal performance metrics by which receivers are specified and compared:

• Noise figure. The noise figure or noise factor (NF) of a radio receiver is a measure of the degree to which the receiver degrades the input SNR and is normally quoted as the ratio of the input and output SNRs. The theoretical lower limit on sensitivity is set by thermal noise power, given by kTB, where k is Boltzmann’s constant, T is system temperature in kelvin, and B is receiver bandwidth. Practical receivers approach this limit within a few decibels depending on front-end design. A low NF (ideally < 3 dB) indicates superior performance. It is dominated by the first active stage—the LNA.
• Sensitivity. Sensitivity is the minimum input level that produces a specified output SNR, typically 10 dB. Representative values: ≈ 50 µV for AM, ≈ 2 µV for FM, ≈ 1 µV for SSB. Modern HF and VHF receivers achieve sensitivities below 0.2 µV (–120 dBm) using LNAs and DSP filtering.
• Selectivity. Selectivity describes the receiver’s ability to discriminate adjacent-channel signals. It is expressed as the output response in dB versus frequency offset or by –6 dB / –60 dB bandwidth ratios. Typical values: 9 kHz (AM), 3 kHz (SSB), 200 kHz (FM). In digital receivers, selectivity corresponds to digital-filter bandwidth.
• Fidelity. Fidelity measures how accurately the receiver reproduces the full range of modulating frequencies. High-fidelity audio systems reproduce 300 Hz–3.4 kHz for voice or 20 Hz–15 kHz for broadcast music. Fidelity depends on both bandwidth and circuit linearity.
• Dynamic range. Dynamic range is the span between the minimum detectable signal (set by sensitivity and noise figure) and the maximum input level that can be tolerated without distortion, blocking, or desensitization. High dynamic range is essential in dense spectrum environments where weak signals may coexist with strong interferers.
• Third-order intermodulation level. The third-order intermodulation level is a key indicator of strong-signal handling. Ideally, third-order products change 3 dB for every 1 dB change in input level (a 3 dB/dB slope). The third-order intercept point (IP3) and blocking dynamic range (BDR) quantify this behavior. Receivers for professional and satellite applications often specify IP3 > +30 dBm and BDR > 100 dB [6].