A radio frequency (RF) signal includes useful information that is modulated onto a carrier signal. An RF receiver retrieves the useful information from the RF signal. RF receivers are used in a wide variety of applications such as television transmission, cellular telephones, pagers, global positioning systems (GPS), cable modems, cordless phones, satellite radios, and the like. As used herein, an RF signal means an electromagnetic signal having a frequency in a spectrum from about 3 kilohertz (kHz) to hundreds of gigahertz (GHz), regardless of the medium through which such signal is conveyed. Thus an RF signal may be transmitted through air, free space, coaxial cable, fiber optic cable, etc.
In many broadcast RF transmission systems, the frequency spectrum is relatively wide and is divided into separate channels that include different information. A television receiver receives the wide spectrum RF signal, mixes a desired channel to a convenient intermediate frequency (IF) to make it easier to filter, and then converts it to baseband where the information may be processed further. For example, a television receiver may translate a channel in the frequency spectrum of 48 megahertz (MHz) to 870 MHz to an intermediate frequency of 44 MHz.
Often, the RF signal power level in a particular channel is low, and needs to be amplified before being mixed or otherwise processed in the receiver. Thus receivers such as television receivers commonly use a technique known as automatic gain control (AGC). AGC systems use a feedback control loop to adjust the gain of an amplifier based on the input signal power level, so the output signal power level is relatively constant. In order to make a proper gain adjustment, the AGC loop needs a power detector capable of accurately measuring the signal power.
The use of the same reference symbols in different drawings indicates similar or identical items.