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Only for training and service purposes
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3. Technical Brief
3.2.1. Transmitter Part
WCDMA Transmitter Block
The QSC62x0 device supports tri-band UMTS transmissions with three separate driver amplifier outputs; in fact,
most Tx active circuits are contained within the device. All three UMTS transmit paths (Figure 1.2.1.1) begin with a
single, shared analog baseband signal from the device’s baseband circuits. The baseband signal is composed of
two differential lines, one in-phase component and one quadrature-phase component. Each component is lowpass
filtered and amplified to levels sufficient for driving the quadrature upconverters. There are two upconverters —
one for low-band signals and one for high band — but only one upconverter is active at a time.
Figure 1.2.1.1 WCDMA Transceiver Architecture
The active upconverter’s output is at the desired RF channel frequency and drives the QSC output stages.
These RF circuits include multiple variable-gain stages that provide transmit AGC control.
A greater than 81 dB gain control range is realized using information from the on-chip Tx power detector combined
with a control signal from the baseband circuits. The wide range of driver amplifier output levels is achieved
while supporting the WCDMA standard’s requirements for ACLR, spurious emissions, Rx-band noise, etc.
The high-band driver amplifier output is followed by a SPDT switch that provides two high-band outputs,
thereby fulfilling the tri-band UMTS requirement. The low-band output port is driven directly by its output amplifier.
All three output ports are single-ended with 50ohm nominal impedance. Each requires a matching network to
interface with its band-pass filter.
Each of the three UMTS Tx output chains are functionally identical: the QSC Tx output drives the PA through
the band-pass filter and a matching network; a directional coupler provides a sample of the PA output signal;
the through path of the coupler is routed to the Tx port of the duplexer; the duplexer antenna port is connected to
the antenna switch; and the switch is connected to the antenna.
The coupler outputs provide a low-level sample of the active transmitter’s Tx power. An on-chip power-detector
circuit provides a Tx power estimate that assists in setting the transmit gains and helps ensure that the maximum
allowed output power is not exceeded. The three UMTS couplers (one for each band) use a daisy-chain configuration
that allows them to share the single, on-chip power detector circuit.
