Page 393 - Complete Wireless Design

Communications System Design

392 Chapter Nine

IMD) and to maintain a good BER. The SSPA must also be exceptionally sta-
ble, and not begin oscillating nor decrease power with the wide impedance
variations encountered in a mobile or portable operation. The impedance vari-
ations are caused by the antenna being constantly presented with many con-
ducting structures that are passing nearby within the mobile environment. As
well, the driver amplifier must be able to supply the necessary input signal
amplitude to the SSPA, and without significant distortion levels.
The TX BPF must be tight enough to attenuate the LO feedthrough, the unde-
sired sum or difference frequency, and other mixer products—but not contribute
significantly to the signal’s group delay variations nor amplitude ripple. The
wideband amplifier is not always required, but will permit all the undesired mix-
er products to pass through to the TX BPF, where they are reflected back toward
the mixer stage. The wideband amplifier, because of its high isolation, will atten-
uate most of these reflected signals so that they do not create increased IMD lev-
els within the mixer. Many transmitter designs may suppress reflections from
the TX BPF’s stopbands by use of a diplexer or attenuator pads, or may simply
dispense with all of the above and place the TX BPF directly at the output of the
mixer port. This choice will depend on the output power from the mixer; the less
output power, the lower in amplitude the reflections, and the less the require-
ment for their suppression. (These mixer products reflect back into the diode ring
of a passive mixer stage, causing mixer diode imbalance; with the resultant
increase in spurious outputs and decreased third-order intercept points.)
The second mixer itself can be a double-balanced type, of a level 10 (10 dBm
LO power) or higher, to suppress IMD. Typically, the IF input port to the mix-
er should never have a signal higher than 10 to 15 dB less than that at the LO
port, or excessive intermodulation products will result. As an example, if the
LO port is at 10dBm, then the IF input must be at or lower than 0 dBm. The
3-dB pad located at the LO port helps to present a 50-ohm impedance to the
sensitive LO BPF. This LO BPF suppresses LO harmonics, which lowers mixer
IMD, as well as wideband noise, which improves the mixer’s NF. The synthe-
sizer amplifier, SYNTH AMP, buffers the output of the synthesizer, minimiz-
ing VSWR, as well as increasing the synthesizer’s output power to the mixer’s
nominal level to maintain its rated noise figure, IMD, and conversion loss
specs. The synthesizer stage itself should have high stability and low phase
noise so as not to degrade the downstream receiver’s SNR and BER. The 3-dB
pad at the input to the second mixer may be used to maintain a more even 50-
ohm input impedance for the IF BPF, which will shift in characteristics if not
presented with its design impedance. The IF BPF should be tight enough to
reject the first mixer’s products, its sum (or difference) frequency, the LO
feedthrough, and excess noise (since increased bandwidth also increases input
noise), yet loose enough to minimize group delay variations and sideband cut-
ting. Selection of the proper filter topology, each with its own positive and neg-
ative attributes, is vital (see Chap. 6, “Filter Design”).
The wideband IF amplifier stage, IF AMP, not only does amplification
chores, but will also have high reverse isolation to prevent the mixer products

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