Implementation of a 900 MHz transmitter system using highly integrated ASIC

A highly integrated single-chip inphase and quadrature (I/Q) modulator is implemented for low power mobile applications (AMPS, ADC, GSM). A quadrature carrier is generated internally without having an external phase shift network. Baseband signals can modulate these carriers directly in the 900 MHz range. A variable gain amplifier with good linearity performance is incorporated to meet tough digital cellular requirements for linear modulation. The device contains an on-chip frequency divider, a crystal oscillator, phase detector, and VCO, which combined to form a PLL for analog FM modulation (pure AMPS system) and frequency synthesizing. It also has three additional dividers for generating clock signal for a micro-controller, DSP chip, and system timing unit. All the functions of the device are controlled by a 3-wire data bus, which can be connected to the system microcontroller in the receiver. The I/Q modulator can achieve 40 dBc carrier and sideband suppression when single sideband modulation is applied