Page 386 - Complete Wireless Design

Communications System Design

Communications System Design 385

output, and the highest RF signal expected. This will be the range, in dB, that
the AGC must lower the gain of the receiver to maintain the nominal output
signal level—even as the RF signal amplitude increases drastically.
The complete RF and IF gain for the entire receiver, all the way from its
front-end input for the antenna to the output of the last IF, will typically be in
the neighborhood of 125 dB. A minimum value for such gain would be 90 dB,
but in some specialized applications of down-conversion directly into a modem,
the gain may be as little as 18 dB. The receiver’s gain figure includes gains,
which are supplied by the amplifiers, and losses, which are caused by filters,
switches, pads, and mixers. Both antenna and receiver gain are finally com-
bined to confirm that there will be a signal of sufficient amplitude at the
receiver’s output to drive the detector or modem at the lowest signal levels
expected, and with sufficient fade margin.
Reciprocal mixing is a common problem in receiver design that places the
noise sidebands of the local oscillator into the IF of the receiver, and is caused
by the heterodyning of out-of-band interfering signals and the LO noise. It sig-
nificantly decreases the receiver’s SNR if this interfering signal—or the LO
noise—is not attenuated. This mixing action takes place within the receiver’s
mixer, with the interfering signal usually being quite close to the desired signal:

f f or f f f
i N i N IF
where f interfering signal, which is close to the desired signal
i
f frequency of the LO noise, which is close to the frequency of the
N
LO
f frequency of the receiver’s IF
IF
Frequency plan. The receiver’s internal frequency plan can make or break a
design. The proper RF and IF bandwidth and the exact LO and IF frequencies
must all be selected carefully, or serious interference and mixer-generated spu-
rious responses can greatly decrease the expected receiver specifications, some-
times rendering an entire design almost useless (see Chap. 7, “Mixer Design”).
In order to catch most mixer-generated spurs, such as f 3f , f 5f , f
RF LO RF LO RF
3f , f 5f , it will be necessary to display these spurious output fre-
LO RF LO
quencies, and their amplitudes, by employing the proper RF system simulation
software (see Sec. 10.10, “Wireless Design Software”). If it is found through
simulation that the specifications cannot be met with the current design, then
more IF filtering, new LO and IF frequencies, and/or a new mixer topology may
be required. This is because we do not want any strong spurious frequencies,
due to the many nRF ± nLO mixer products, to fall within the bandwidth of the
receiver’s IF. Such spurs would cause the BER to be degraded in a digital
receiver, and in an analog receiver, even the reception of undesired signals and
interference would be possible within our desired channel.
High-side injection occurs when the LO frequency is greater than the RF
frequency in a conversion stage, while low-side injection occurs when the
LO frequency is actually lower than the incoming RF frequency in a mixer
stage. The choice of whether to operate a superheterodyne receiver with a

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