Page 384 - Complete Wireless Design

Communications System Design

Communications System Design 383

If the receiver is required to operate only within the HF region or below,
then both the LNA and BPF2 could be dropped from the receiver design, as
noise figure is not as important in this region because of massive natural and
artificial noise generation. This will decrease sensitivity, but will give more
IMD immunity in a crowded HF spectrum.
MIXER1 will normally be subjected to high amplitude input signals, so a
high compression point is mandatory to decrease mixer-generated intermodu-
lation distortion. This will generally demand a high-level diode double-bal-
anced mixer (DBM) instead of an active mixer stage in this sensitive location.
And for decreased IMD generation, the RF signal into the DBM should be at
least 10 dB less than that injected into the DBM’s LO port. Thus, a level 10
mixer could accept a maximum of 0 dBm at its RF input port before excessive
IMDs became a problem; 15 to 20 dBm less RF would be even better, and
would be needed for higher-quality, lower-distortion receivers. DBMs have the
added advantage that they suppress even-order LO and RF mixer-generated
harmonic products, as well as the RF and LO frequencies themselves, at the
IF output. Attenuation of these frequencies is far from complete, however.
The diplexer, placed after the MIXER1 stage, will filter and pass the desired
IF but, unlike other filters, it will stop other frequencies from entering the IF’s
bandwidth by absorption rather than reflection. Reflection of the undesired
frequencies—such as LO harmonics, the sum of the RF and LO, and the IMD
products—would cause RF power to be sent back into the mixer, which would
unbalance its diode ring (causing increased IMD), as well as adversely affect-
ing the mixer’s dynamic range and conversion loss. Indeed, many viable
receiver architectures may simply pad the output of the mixer so that these
reflections are attenuated not only as they enter the pad, but also as they are
reflected back into the mixer’s IF port. The pads lower the input/output VSWR
by supplying the mixer with an almost pure 50 j0 ohm termination (pads
placed at the RF port will, however, reproduce thermal noise, which would
have normally been removed by the image filter, at the image frequency). A
wideband, high-isolation amplifier may also be used at the mixer’s output, as
shown for the second IF strip, since this will permit all of the mixing products
to pass through this amplifier, and, after filtering from a normal reflective IF
filter (such as BPF6), the BPF will “bounce” much of the undesired signals
back toward the sensitive IF port of the mixer. However, these reflected sig-
nals will have been significantly attenuated by the reverse isolation of this
wideband amplifier. Nonetheless, the amplifier must have a high P1dB to lin-
early accept the sometimes high-powered out-of-channel signals that can
occur, without producing significant distortion.
The LOs must generate low phase noise in order to mitigate BER and adja-
cent channel selectivity degradation. This can be difficult in phase-locked
loop–based LOs, but is relatively easy in properly designed high-Q crystal
oscillators (see Chap. 4, “Oscillator Design”). All oscillator-generated spuri-
ous signals must also be as low as possible to minimize receiver spurious
responses. The LO amplifiers are broadband types used to buffer the local

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