Page 391 - Complete Wireless Design

Communications System Design

390 Chapter Nine

or 3F ±2F ). Other undesired output signals, such as harmonics of the desired
2 1
RF carrier signal and the feedthrough of the LO and IF, can all cause inter-
ference. Transmitted noise (especially in a multipoint environment) will raise
the noise floor of the receiver at the other end of the link, lowering its SNR,
which will decrease the distance the communication link can reach; so any
Class A or Class AB power amplifier should be specifically designed to output
a minimal amount of additive wideband noise. Depending on the frequency,
power, band, modulation, and service, certain frequency-stability require-
ments are mandated by law, or are simply required for proper demodulation
at the receiver and/or to prevent adjacent channel interference.

9.2.2 Transmitter design
The generic linear double-conversion transmitter of Fig. 9.3 could just as easily
be a single-conversion unit. The choice as to whether to employ single or double
conversion is based on the transmit RF frequency and the much lower input fre-
quency. At higher RF operation, double conversion is required to properly sup-
press, through low-cost IF filtering, the close-in sum or difference frequencies of
the input signal mixing with the LO, as well as the LO feedthrough, while also
suppressing spurious mixer responses caused by the limitations of a real-life IF
filter’s limited realizable percentage of bandwidth. In other words, if we desire
a high transmitter RF output frequency with only a single conversion stage—
yet we have a low input frequency that must be converted to this much higher
carrier frequency—we would need a filter with an impossibly narrow bandwidth
and ultrasteep skirts, along with the escalating problem of the inevitably high
group delay variations that would severely distort the output signal.
Taking the first element of Fig. 9.3, the antenna, we see that it is at DC
ground through the inductor to protect the RF output filter (RF BPF) and the
solid-state power amplifier (SSPA) against static buildup discharge damage.
The RF BPF suppresses much of the transmitter-generated harmonics, wide-
band noise, IMD products, and out-of-band conversion frequencies. As an
added consideration in FM service, most of the SSPAs will run saturated for
maximum efficiency, which will create large harmonic output levels; these har-
monics of the fundamental must be sufficiently attenuated by this last RF out-
put filter. In fact, since the output filter is typically reflective in the stopbands,
most of these undesired harmonic frequencies are actually reflected back into
the SSPA, which will then create significantly higher-than-expected harmonic
output from the transmitter. This annoying effect will necessitate an increase
in the rated attenuation of the RF output filter by approximately 15 to 20 dB.
If the SSPA is to be operated at less than saturation for digital or SSB voice
communications, it must be designed to maintain the desired output power
with low distortion levels for the RF signal. This means we may have to run
the power amplifier at up to 10 dB (or more) under its maximum output power
rating. To put it another way, the SSPA has been “backed-off” in power by up
to 10 dB in order to maintain the required linear operation that a particular
modulation technique demands for decreased spectral regrowth (a form of

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