Page 363 - Complete Wireless Design

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362 Chapter Eight

Microwave distributed balun (Fig. 8.33). This planar structure is relatively
broadband (30 percent bandwidth when six sections are used), and is simple
to design and implement. With the addition of more sections, wider band-
widths are possible, while maintaining the required 180 degree phase shift
between ports 2 and 3 for balanced operation:

1. A V .
4 P

2. B V .
2 P
where A length of 50-ohm microstrip (calculate width as described in Sec.
1.4, “Transmission Lines”)
B length of 50-ohm microstrip (calculate width as described in
Sec. 1.4)
V velocity of propagation as a fraction of the speed of light (depends
P
on E of PCB; calculate as described in Sec. 1.4) .
r
8.6 Splitters/Combiners
8.6.1 Introduction
A splitter/combiner is a 50-ohm circuit used to “mix” different signals in a
linear manner, leaving them unchanged, with no new signals created. For a
splitter, a signal is placed at the input, and two or more signals, usually of
equal amplitude and phase, are removed from two or more separate ports at
the output. For a combiner, two or more signals of equal phase are placed at the
input, and a single signal is removed from the one output port that is equiv-
alent to their vector sum. A splitter and a combiner are exactly the same cir-
cuit; to turn the splitter into a combiner, the circuit is merely reversed, with
the input becoming the output.
Complex splitter/combiners can be utilized to split or combine the signals of
various RF power amplifiers into a single high-power output.
For two in-phase, same-frequency signals placed at the input to a com-
biner, the insertion loss will be quite small (less than 0.4 dB). However,
if these two signals are at different frequencies, the insertion loss for a
two-way (three total ports) combiner will be 3 dB. With the same circuit
configured as a two-way splitter, the insertion loss will be approximately
3.5 dB—with 3 dB of the loss due to the signal being split into the two out-
put ports.

8.6.2 Splitter and combiner design
50-Ohm LC Power 0 degree splitter/combiner (Fig. 8.34). This LC device will
have a lower insertion loss (3.5 dB) than a resistive splitter, while maintain-
ing a high 20-dB isolation between ports. However, this circuit will have a

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