High Dynamic Range Bandpass Filters for Communications
Design Note 37
Richard Markell
Introduction
Octave or decade wide bandpass filters are non-trivial
to design. Wide bandpass filtering requires the designer use a highpass filter at the input in series with
a lowpass filter to achieve the desired specifications.
This becomes evident if one examines the transfer
functions of the state-variable-filter configuration, be
it switched capacitor or active RC. Either option limits
severely the achievable dynamic range if a wideband
bandpass filter is designed using the bandpass output.
Wideband bandpass filters occupy a niche in the communications area of signal processing. The wideband
bandpass function is required in receiver IF applications
which traditionally use the crystal filter and/or active
RC’s. Sonar applications also demand steep, wide, low
noise bandpass filters to allow analysis of “chunks” of
the frequency spectrum, one at a time or in parallel.
Recent improvements in switched capacitor filter technology allow designers the luxury of using switched
capacitor filters in these applications. True clock tuning
allows variable bandwidth filters to be implemented
with only a few parts.
10.00
0.0
–10.00
–20.00
GAIN (dB)
–30.00
–40.00
–50.00
–60.00
–70.00
–80.00
–90.00
1k
10k
FREQUENCY (Hz)
100k 200k
DN037 F01
Figure 1. Frequency Response. VIN = 2.2VRMS,
Output Buffer = LT1122
08/90/37_conv
This note details the design of a wideband (10kHz100kHz) bandpass filter using a single LTC®1064 plus an
LTC1064-4. The LTC1064 is a quad universal switched
capacitor filter while the LTC1064-4 is the same silicon
with integrated resistors configured to implement an 8th
order elliptic lowpass filter. Both filters have low noise
and can operate to frequencies beyond 100kHz. The
combination bandpass filter has a set of tough design
specifications: total integrated noise in the passband
less than 350μV, passband ripple less than 0.4dB or
±0.2dB, and steep rolloffs at the band edges (–70dB
at 5kHz, and –70dB at 200kHz).
Design
The design ideology combined an LTC1064-4 elliptic
lowpass filter with a highpass LTC1064 designed using
Linear Technology’s FilterCAD® filter design software.
The LTC1064-4 provides an attenuation of greater than
70dB at 2 times cutoff, while the highpass filter was
designed to be almost the mirror image of the lowpass
filter. Figure 2 shows the schematic of the composite
bandpass filter. Note the 74LS90 is used as a divide
by 5 whose output clocks the LTC1064 used as the
10kHz highpass filter. In addition to providing both
5MHz and 1MHz clocks, this circuit allows the clocks
to be synchronous. This is essential for well behaved
operation of the sampled data filters.
Test Results
Figure 1 shows the overall frequency response of the
bandpass filter. Note the steep slopes at the transition
regions of the filter. The measured noise in the passband
of the filter was 320μVRMS. Passband ripple is shown
in Figure 3. Figure 3 shows excellent ripple specifications of less than ±0.2dB. It should be noted that the
values of the resistors marked with an asterisk were
modified slightly from the values given by FilterCAD.
Otherwise the passband ripple measured better than
±0.4dB. Additional note should be made of the 50pF and
L, LT, LTC, LTM, Linear Technology, the Linear logo and FilterCAD are registered
trademarks of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
46.4k
CLOCK IN
5MHz
487k
43.2k
12.4k
50pF**
1
14
2
13
12
3
16.5k
VIN
1
24
2
23
16.9k
74LS90
10k
12.7k
+5V
80.6k
11.85k*
11.5k
4
21
5
20
V–
10
6
9
7
8
0.1μF
19
6
11
5
0.1μF
22
3
4
LTC1064
V+
7
18
8
17
9
16
1MHz CLK
14pF
0.1μF
17.65k*
10k
18.47k*
LTC1064-4
22.1k
10
15
11
14
53.6k
1
14
2
13
3
12
4
11
5
10
6
9
7
8
10.5k
13
12
10k
30pF**
V+
46.4k
V–
0.1μF
V+
0.1μF 20k
475k
NOTES:
SCHOTTKY DIODES = 1N5817
RESISTORS 1% UNLESS NOTED
LTC1064, LTC1064-4 AND LT1122
DO NOT SHARE POWER LINES
VS = ±8V
*AND** NON-FCAD VALUES
V+ 0.1μF
20k
24pF
CONNECTION
UNDERNEATH
DEVICE
–
+
VOUT
LT1122
0.1μF
V–
DN037 F02
Figure 2. Schematic Diagram 10kHz-100kHz Bandpass Filter
Conclusions
A 16th order low noise high quality bandpass filter can
be designed with only four active components and a
handful of resistors. The filter meets specifications that
can only be approached with the most sophisticated
hybrid filter solutions. Further, the filter was primarily designed with the FilterCAD program, so that the
designer may change parameters with the push of the
“Enter” key. The filter is another in the amazing set
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of achievements available to the system designer by
using the new switched capacitor filters from Linear
Technology.
1.0000
0.8000
0.6000
0.4000
GAIN (dB)
30pF capacitors shown with double asterisks. These
capacitors serve as RC filters on two of the highpass
outputs of the LTC1064 to rolloff the response of the
HPF (well past the passband of the overall filter) to limit
noise aliasing back to the filter’s passband. Note that
these capacitors may limit the overall tunability of the
filter or they may need a range changing switch.
0.2000
0.0
–0.2000
–0.4000
–0.6000
–0.8000
–1.0000
100k
8k 10k
200k
FREQUENCY (Hz)
DN037 F03
Figure 3. Passband Frequency Response.
VIN = 2.2VRMS, Output Buffer = LT1122
For applications help,
call (408) 432-1900
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© LINEAR TECHNOLOGY CORPORATION 1990