Page 406 - Complete Wireless Design

Communications System Design

Communications System Design 405

of attenuation to prevent the high-power transmit frequency and power ampli-
fier (PA) noise from negatively affecting the very sensitive receiver, while also
attenuating transmitter harmonics and some of the receiver’s image frequency,
and at the same time not adding excessive group delay variations.
The LNA will set most of the receiver’s NF, and thus sensitivity, and must
be high in gain and low in internal noise generation. However, just deciding to
design an LNA with the lowest NF and the highest gain will result in a receiv-
er with poor intermodulation performance, since the mixer’s third-order inter-
cept point (IP3) will be reduced by the gain of the LNA (this would be true even
if the LNA itself had an infinite IP3). This makes the first mixer, and thus the
entire receiver, very intolerant of strong input signals—the mixer is basically
predetermining the entire receiver’s IP3. As the first mixer is such a critical
element in receiver IP3 performance, DBM diode mixers are usually employed
for this purpose, since they have a far higher IP3 than most active mixers. The
DBM thus permits more gain in the LNA stage, which lowers the effect of
noise contributions of the following stages—including the high-NF first mixer
stage. Obviously then, LNA gain and first mixer IP3 must be selected with
care to maximize receiver IP3 and minimize NF so that we may obtain a highly
linear and sensitive receiver. Most sensitive receivers will have an image fil-
ter placed just before this mixer, which will further assist the diplexer’s fil-
tering action, and will almost entirely eliminate the LNA’s own
self-generated noise within the undesired image band from adversely affect-
ing the receiver’s SNR.
The wideband amplifier that follows the first mixer has a high reverse isola-
tion to prevent the large mixer-generated sum frequency from reflecting off the
reflective stopbands of the IF BPF and reentering the first mixer, which would
increase IMD. The IF BPF rejects all signals that are not on channel, which is
an important task, as the gain of the IF strip is quite high (usually at least 90
dB). In some radios, rejection of the second image noise may be necessary, and
the IF BPF provides this capability as well. The attenuators into/out of the sec-
ond mixer stage help to increase the return loss of the conversion stage, there-
by decreasing IMD. This is especially vital at the IF port, where reflections off
of the filter’s stopbands will be quite severe. In this FM receiver case, a limiter
sets the IF amplitude into a discriminator, with IF then flowing into sig-
OUT OUT
nal-processing integrated circuits and to an output speaker.
On the transmitter side, the modulated IF is placed into IF BPF at the IF
in port, converted up in frequency by the first mixer, amplified and filtered,
then mixed up by the second mixer to RF. A wideband, high-isolation ampli-
fier at the first mixer’s output port is used to dampen reflections back into
the mixer, with attenuators performing a similar function. The second mixer’s
TX BPF suppresses wideband transmitter noise, as well as harmonics and
mixer products. These two frequency-conversion stages are necessary so that
we may economically filter out the close-in sum (or difference) frequency, as
well as the inevitable LO feedthrough, from exiting the transmitter in
strength. The driver amplifies the signal to an input level that is acceptable

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