Transmitter Architectures: Present Options

We identified in Chapter 1 a number of modulation techniques, including GMSK for
GSM, π/4DQPSK for IS54 TDMA, 8 PSK for EDGE, and 16-level QAM for CDMA2000
1 × EV. To provide some design standardization, modulation is usually achieved
through a vector IQ modulator. This can manage all modulation types, and it separates
the modulation process from the phase lock loop synthesizer—the function used to
generate specific channel frequencies.

As in the case of the superhet receiver, the traditional transmitter architecture has
consisted of one or more upconversion stages from the frequency generating/modulating
stage to the final PA. This approach allows a large part of the signal processing,
amplification, filtering, and modulation to be performed at lower, cost-effective, highly
integrated stages (see Figure 2.8). The design approach is low risk with a reasonably
high performance.
The disadvantage of this approach is that a large number of unwanted frequencies are
produced, including images and spurii, necessitating a correspondingly large number of
filters. If multiband/multimode is the objective, the number of components can rise
rapidly. Image reject mixers can help but will not remove the filters entirely. A typical
configuration might use an on-chip IF filter and high local oscillator injection. With careful
circuit design, it may be possible, where transmit and receive do not happen simultaneously,
to reduce the number of filters by commoning the transmit and receive IFs. 48