In this chapter we will be mostly concerned with the sections of the receiver that come before the detector, sections that are common to nearly all receivers: AM, FM, television, cell phones, etc. Basic specifications for any kind of radio receiver are gain, dynamic range, sensitivity and selectivity, i.e., does a weak signal at the selected frequency produce a sufficiently strong and uncorrupted output (audio, video, or data) and does this output remain satisfactory in the presence of strong signals at nearby frequencies?

Sensitivity is determined by the noise power contributed by the receiver itself. Usually this is specified as an equivalent noise power at the antenna terminals. Selectivity is determined by a bandpass-limiting filter and might be specified as “3 dB down at 2 kHz from center frequency and 20 dB down at 10 kHz from center frequency.” (Receiver manufacturers usually do not specify the exact bandpass shape.)

Amplification

Let us consider how much amplification is needed in ordinary AM receivers. One milliwatt of audio power into a typical earphone produces a sound level some 100 dB above the threshold of hearing. A barely discernable audio signal can therefore be produced by −100 dBm (100 dB below 1 mW or 10–13 watts). Let us specify that a receiver, for comfortable earphone listening, must provide 50 dB more than this threshold of hearing, or 10–8 watts.