234931794Integrated_VGA_Aids_Precise_Gain_Control| Application Note
INTEGRATED VGA
DESI
Integrated VGA Aids Precise Gain Control
g
PHILLIP HALFORD EAMON NASH
An integrated VGA circuit with fixed-gain amplification and variable attenuation is useful for power leveling in both receivers and transmitters, even at high intermediate frequencies.
ain control is necessary in mobile-communications systems because of the wide range of transmitted and received signals. A base station for Global System for Mobile Communications (GSM), for example, must receive and demodulate signals from about 15 dBm to less than 104 dBm, even when in-band blocking signals can be as large as 13 dBm. The goal of the automatic-gain-control (AGC)
Applications Engineer Analog Devices, 804 Woburn St. MS122, Wilmington, MA 01887; (781) 937-1834, e-mail: phillip.halford@analog.com, and Applications Engineer Analog Devices, 804 Woburn St. MS122, Wilmington, MA 01887; (781) 937-1239, e-mail: eamon.nash@analog.com.
in these systems is to provide a relatively constant input level to the analog-to-digital converter (ADC). Although gain control can be implemented at RF, intermediate frequency (IF), or baseband frequencies, most gain control operates at IF. While a precise rela-
tionship between gain control voltage and gain is always desirable, in receiver (Rx) circuitry applications the ability to deliver a constant signal level to the subsequent stages is generally more critical. Because of this, an analog or digital level detector is an important component for setting precise gain levels in mobile-communications systems. Although mobile-communications
IQ demodulator
Amplifier Mixer LNA VGA RSSI/ Phase detect Combiner/ splitter High-power RF VGA amplifier VGA Mixer O 90 O 90
ADC
ADC
VCO/synth DAC
DAC Power detect and control IQ modulator
1. Gain control is instrumental in receivers and transmitters, in order to maintain precise power levels.
MICROWAVES & RF
88
MARCH 2002
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