Page 387 - Complete Wireless Design

Communications System Design

386 Chapter Nine

high- or a low-side injection will depend on the system design and whether
the final demodulated signal needs to have the sidebands with an inverted
frequency spectrum. In digital communications, this frequency inversion
can be a major consideration, since the output of the IF is typically sent
straight into a modem, which may or may not require inversion. But even if
we do not have a choice as to whether we can frequency-invert or not, we
may still select our high- or low-side injections throughout both the trans-
mitter and receiver stages, and base our high/low preferences on the avail-
ability of oscillators, multipliers, or PLLs, as well as their cost and design
complexity, along with the location of our undesired image frequency and
the undesired mixer spurs.

General receiver calculations. There are several important design formulas to
assist the designer in calculating the receiver’s specifications, and these are
presented below. However, it will be far easier and more accurate to obtain
these important receiver specifications by using the included AppCad program
by Agilent. This software will, within seconds, compute total receiver gain, NF,
SNR, MDS, sensitivity, noise floor, input/output IP3, dynamic range, etc.
1. The total gain required of a receiver can be calculated by finding out what
the lowest expected RF signal level will be after the antenna (into the
receiver), and deciding on what the minimum receiver output signal
requirement is into the modem or detector:
G P P
dB OUT IN
where G required gain of the receiver, dB
dB
P lowest acceptable signal output level of the receiver, dBm
OUT
P lowest expected RF signal level into the front end of the
IN
receiver, after the antenna, dBm.
2. Minimum discernible signal (MDS) is a sensitivity rating for receivers, and
is the lowest signal detectable. This can be at 0 dB above the receiver’s
noise floor, and can be calculated by:

MDS (dBm) 174 dBm 10 log BW NF
10
where BW noise bandwidth of the receiver, or approximately the 6-dB-down
bandwidth (instead of the typical 3-dB bandwidth), and NF receiver’s noise
figure, dB.
3. The third-order intercept point (IP3) is approximately 10 to 15 dB above the
P1dB compression point, and is the location where, if the gain slope of the
receiver could continue, the undesired output third-order frequency prod-
ucts would be at the same amplitude as the output two-tone fundamental
test signals that had been originally placed at the receiver’s input. To com-
pute the total cascaded input IP3 (IP ) of multiple stages of a receiver
TOT
(Fig. 9.2), use the formula:

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