Page 382 - Complete Wireless Design

Communications System Design

Communications System Design 381

to select a band of frequencies, while rejecting out-of-band signals, thereby
minimizing IMD products. However, this filter must not be so tight that inser-
tion loss (IL), noise figure (NF), and group delay variations (GDV) are increased
excessively. BPF1 will also help reject the first LO frequency from radiating its
CW signal back through BPF2 and the LNA. The LNA, because of its reverse
isolation, can significantly attenuate this reradiation by itself, selecting the
proper mixer with an adequate amount of port isolation. BPF1 will further
assist BPF2 in the attenuation of image frequencies and image noise located at
(2 LO) RF, or LO IF. (Normally image noise at a second or third mixer
stage can be ignored because of the predominant noise contribution of these
first stages of the receiver.) In fact, the frequency of the IF is chosen to permit
a receiver to reject this first RF image frequency without requiring an exces-
sively expensive, complex, and tight image filter before the first down-mixer,
MIXER1. The next stage, the LNA, will set most of the receiver’s noise figure
and IMD performance. The LNA normally gives us approximately 20 dB of
front-end RF gain, with a low NF of less than 2 dB. BPF2 attenuates any har-
monics created by the nonlinearity of the LNA, as well as the image noise
caused by the LNA itself (since no amplifier is noiseless). BPF2 will further
reject some of the out-of-band signals and LO feedthrough.
In receivers with a single filter in the receiver’s front-end, the BPF1 would
match the antenna to the input of the LNA, decrease the amplitude of the out-
of-band signals from overloading this amplifier, and provide a certain amount of
image filtering. But considering that filter insertion loss before the LNA is
directly translated into increased NF, an image filter (BPF2) is usually placed
between the LNA and the first mixer to decrease IL at the BPF1 preselector. The
two-filter method decreases the NF, while assisting the preselector in rejecting
undesired signals as well as any image signal and the ever-present image noise.
As stated above, but well worth repeating: Low noise figure for a receiver
means utilizing a low NF and high-gain amplifier in the receiver’s front end,
since any losses before the receiver’s LNA will correlate dB for dB in an
increased NF; 3 dB of filter loss before the LNA equals 3 dB more NF, which
translates into a less sensitive receiver. Unfortunately, there is a slight compro-
mise that must be made in LNA design: For the highest front-end gain, the LNA
must be matched to the receiver’s front-end filters. But this matching can
increase the noise, since an optimum NF match will rarely coincide with a high
return loss, or low mismatch loss, match. So, considering that the first stage of
a receiver is required to be designed for the lowest NF and the highest gain, we
will normally match for the best NF that the amplifier can provide, while pro-
viding acceptable gain.
We can see the importance of LNA gain by glancing at the formula below, as
the second stage of the receiver will only add to the overall noise figure of the
receiver by the second stage’s NF divided by this first stage of gain, or:

second-stage NF
NF
OVERALL first-stage gain

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