Page 280 - Complete Wireless Design

Filter Design

Filter Design 279

In designing a lumped filter, especially at higher frequencies, the highest-Q
inductors (typically air) should be used to lessen insertion loss and the subse-
quent rounding of the passband edges. The capacitors, as well, must be chosen
carefully, since the filter’s characteristics of bandpass, center frequency, and
return loss will change if these capacitors have a poor tolerance value or tem-
perature characteristic; the filters may appear fine when a small production
run is tested at room temperature, but may become unacceptable when oper-
ated over temperature variations or over large production runs. Another
important specification of a filter is its ultimate attenuation characteristic,
which depends on the number of filter sections, the Q of the coils, the series
resonances of the LC components, the distributed reactances of the circuit
itself, and the circuit layout and shielding requirements.
Most filters should be designed with the minimum number of poles required
to attenuate the undesired frequencies in order to lessen costs, insertion loss,
group delay variations, and physical space demands. And rarely should we
cascade separately designed filters without an amplifier placed between them.
If we do, there may be undesired interactions, causing unpredictable filter
responses.

6.1.2 Types of lumped filters
There are certain topologies of lumped filters adopted in wireless circuit
design today, and these are normally of the all-pole variety. The following
topologies are the most common:

1. A minimum L bandpass filter (Fig. 6.11) begins with a tuned tank at its
input, and is a great choice for bandpasses of 30 percent or higher. It is one
of the more popular filter topologies because it does not require an exces-
sive amount of different component values. For instance, a fifth-order type
needs just three different values of inductors and three different values of
capacitors in this 10-component filter. Component values, however, can
vary wildly: Some capacitors may have a value of 1270 pF, while another
may have a 66-pF value.
2. A minimum C bandpass filter (Fig. 6.12) is virtually the same as the mini-
mum L above, but starts with a series circuit instead of a shunt tank.
3. A top C-coupled bandpass filter (Fig. 6.13) is adopted for bandwidths of 30
percent or less, and is popular because only one inductor value is required,
and that value can be chosen by the designer. In addition, the capacitors that
are in shunt with the inductors are normally quite close in value to each
other, which can mean that only a single value of shunt capacitor is required
as well. A negative attribute is that 16 components are normally necessary
for a fifth-order top C-coupled bandpass filter, instead of 10 (unless the end
capacitors are at such a high value that they can be removed entirely). The
top C-coupled filter is also unsymmetrical: The upper cutoff frequency does
not have as sharp a skirt as the lower cutoff frequency.

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