AGC design and PLL comparison

AGC design and PLL comparison

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Automatic gain control (AGC) is used in a receiver to vary the gain to increase the dynamic range (DR) of the system. AGC also helps deliver a constant amplitude signal to the detectors with different radio frequency (RF) signal amplitude inputs to the receiver. AGC can be implemented in the RF section, the intermediate frequency (IF) section, in both the RF and IF portions of the receiver, or in the digital signal processing (DSP) circuits. Digital AGCs can be used in conjunction with RF and IF AGCs. Most often, the gain control device is placed in the IF section of the receiver, but placement depends on the portion of the receiver that limits the DR. The detection of the signal level is usually carried out in the IF section before the analog-to-digital converter or analog detection circuits. Often the detection occurs in the DSP circuitry and is fed back to the analog gain control block. The phaselocked loop (PLL) is analyzed and compared with the AGC analysis, since both processes incorporate feedback techniques that can be evaluated using control system theory. The similarities and differences are discussed in the analysis. The PLL analysis is used only for tracking conditions and not for capturing the frequency or when the PLL is unlocked.

Chapter Contents:

  • 4.1 AGC design
  • 4.2 AGC amplifier curve
  • 4.3 Linearizers
  • 4.4 Detector
  • 4.5 Loop filter
  • 4.6 Threshold level
  • 4.7 Integrator
  • 4.8 Control theory analysis
  • 4.8.1 AGC design example
  • 4.9 Modulation frequency distortion
  • 4.10 Comparison of the PLL and AGC using feedback analysis techniques
  • 4.11 Basic PLL
  • 4.12 Control-system analysis
  • 4.13 Detector
  • 4.14 Loop filter
  • 4.15 Loop gain constant
  • 4.16 Integrator
  • 4.17 Conversion gain constant
  • 4.18 Control theory analysis
  • 4.19 Similarities between the AGC and the PLL
  • 4.20 Feedback systems, oscillations, and stability
  • 4.21 Summary
  • 4.22 Problems
  • Further reading

Inspec keywords: automatic gain control; phase locked loops; digital signal processing chips; radio receivers

Other keywords: analog detection circuits; radio frequency signal amplitude inputs; analog-to-digital converter; AGC analysis; DSP circuits; control system theory; PLL analysis; phase locked loop; signal level detection; gain control device; digital AGC; AGC design; analog gain control block; RF section; digital signal processing; DSP circuitry; automatic gain control; IF section; constant amplitude signal; intermediate frequency; feedback technique; PLL comparison

Subjects: Digital signal processing chips; Digital signal processing chips; Phase and gain control; Modulators, demodulators, discriminators and mixers; Radio links and equipment; Control applications in radio and radar

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