Page 314 - Complete Wireless Design

Source: Complete Wireless Design

Chapter
7

Mixer Design

Mixers are 3-port active or passive devices. They are designed to yield both a
sum and a difference frequency at a single output port when two distinct input
frequencies are inserted into the other two ports. This process is called fre-
quency conversion (or heterodyning), and is found in most communications gear.
It is used so that we may increase or decrease a signal’s frequency. The two sig-
nals inserted into the two input ports will normally be a continuous wave, pro-
duced within the radio by a local oscillator, and the incoming (for a receiver) or
outgoing (for a transmitter) signal. If we want to produce an output frequency
that is lower than the input signal frequency, then it is called down-conversion;
if we want to produce an output signal that is at a higher frequency than the
input signal, it is referred to as up-conversion. Indeed, most AM, SSB, and dig-
ital transmitters require mixers to convert up to a higher frequency for trans-
mission into space, while superheterodyne receivers require a mixer to convert
a received signal to a much lower frequency. This lower received frequency is
then called the intermediate frequency (IF). Receivers must use this lower fre-
quency signal, as it is much easier to efficiently amplify and filter with the IF
stages tuned and optimized for a single, low band of frequencies, and the receiv-
er’s gain and selectivity are thus increased. The frequency conversion process
within the nonlinear mixer produces the intermediate frequency by the hetero-
dyning, or beating, of the input signal with the receiver’s own internal LO. This
mixer circuit can consist of a diode, BJT, or FET that is overdriven or biased to
run within the nonlinear part of its operation. However, the beating of the two
input signals yields not only the signal, the local oscillator, and the sum and dif-
ference frequencies of these two signals, but also many spurious frequencies at
the mixer’s output port. Nonetheless, most of these undesired frequencies will
be filtered out within the receiver’s IF, resulting in the new desired frequency
(consisting of the carrier and any sidebands) now at the difference fre-
quency. This new lower frequency will then be amplified and further filtered as
it passes through the fixed tuned IF strip.

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