Page 308 - Complete Wireless Design

Filter Design

Filter Design 307

1
3. R R
1 2 6.28f C C
c 1 2
When cascading low-pass filter sections for added selectivity, we double the
cutoff frequency (f ) in the above equation for each filter we plan to cascade.
C
For instance, if we are cascading two sections, and we desire a 1-kHz 3-dB cut-
off frequency, then design each section to have a 3-dB f of 2 kHz. The final cas-
C
caded output will then have an approximately 1-kHz low-pass cutoff.

An active audio high-pass filter (Fig. 6.54)
1. A 1
v
2. C C 0.022
F
1 2
1
3. R R
1 2 6.28f C C
c 1 2
When cascading high-pass filter sections for added selectivity, we cut in
half the cutoff frequency (f ) in the above equation for each filter we plan to
c
cascade. For instance, if we are cascading two sections, and we desire a 1-kHz
3-dB cutoff frequency, then we design each section to have a 3-dB f of 500
c
Hz. The final cascaded output will then have an approximately 1-kHz high-
pass cutoff.

6.6 Tunable Filters
6.6.1 Introduction
A filter that can change frequency with the application of a control voltage
across varactor diodes can be an invaluable asset in the design of receivers and
transmitters in today’s packed spectrum. Certain ultrawideband transmitter
frequency conversion stages can also benefit by allowing the selective filtering
of excessive local oscillator feedthrough from entering the IF or RF stages.
Single varactors are capable of changing capacitance from 0.63 to 2.67 pF all
the way up to 3.8 to 20 pF (and above) by the placement of a 0- and 20-V con-
trol voltage. This range of capacitances is perfect for tunable filter designs that
are not in excess of the VHF and lower UHF regions.
Alpha Industries manufactures an entire range of low-resistance varactors
that are ideal for operation in a high-Q, variable-frequency bandpass filter.

6.6.2 Tunable filter design
A half-octave range 50-ohm tunable bandpass filter (Fig. 6.56)

1. Design a basic top capacitor-coupled bandpass filter (Fig. 6.57) with any
standard RF filter design program, such as AADE’s low-cost Filter Design
software (available at AADE.com). Select a center frequency for the top

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