3.2.9 Receiver Automatic Gain Control Issues

Chapter 3.2.9 Receiver Automatic Gain Control Issues

Radio Frequency Integrated Circuit Design Second Edition Book
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Radio Frequency Integrated Circuit Design Second Edition Book

  • 62 System Level Architecture and Design Considerationsof this distortion product must be less than -95 dBm. Using (2.66), we note that P2 - P1 = (-20 - (-95)) = 75 dB. Therefore the IIP2 must be greater than -20 dBm + 75 dB = 55 dBm. 3.2.9  Receiver Automatic Gain Control IssuesADCs require relatively constant input amplitude to function properly. They are not equipped to deal with large ranges of input amplitude, and therefore it is one of the most important jobs of the radio to provide them with a constant signal amplitude. On the receive side, as a bare minimum the radio must provide an AGC range at least equal to the dynamic range of the radio. In addition, radio gain will vary with temperature, voltage supply variations, and process. Usually at least an additional 20 dB of gain control is required to overcome these variations. Normally it is pos-sible to use stepped AGC with discrete gain settings (usually spaced by about 3 dB, but maybe less as needed), but some more sophisticated radios may require con-tinuous AGC. It is important to decide where the AGC level will be set and in what order different gain control blocks will be adjusted. At the minimum detectable level, the receiver is set to the maximum gain setting. As the input power starts to rise, it is better to reduce the gain as far towards the back of the radio as possible to have the lowest effect on the noise figure. At high power levels, it is better to adjust the gain at the front of the radio thus reducing the linearity requirements of parts further down the chain. On the transmit side, AGC is often simpler than on the receive side. Even simple transmitters must have some AGC to compensate for process, temperature, and supply voltage variations. Typically, a simple stepped AGC of about 20 dB is used to make sure that the required power can always be delivered to the antenna. More sophisticated radios will also use power control in the transmitter to back transmit power off if the receiving radio is close by, thus reducing the required maximum received power on the other side of the link. Figure 3.17  Illustration of a second-order product causing distortion.