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Generating Switched-Capacitor-Filter Clocks

Recent advances in semiconductor-process
technology and circuit-design techniques have brought down the
prices of switched-capacitor filters (SCF) by a factor of three. As
an example, 5-pole Butterworth lowpass filters sell for a mere
$0.99 each. Recent switched-capacitor filters offer low-power,
single-supply operation down to 3V.

Implementing an SCF is easy. First, choose the type.
Fixed-response, 5- and 8-pole Butterworth, Bessel, Chebyschev, and
elliptic (Cauer) types are commonly available, as well as the
universal SCF with user-programmable features. Follow the
data-sheet recommendations on power supplies, bypassing, and signal
coupling, and then choose a clock frequency appropriate to the
application.

Some SCFs are self-clocking with the addition of external
components, but most applications use a CMOS-level clock signal to
set the SCF's corner frequency. Otherwise, many acceptable methods
are available for setting the clock signal for an SCF.

Proper Clock Characteristics

The first major concern in clocking an SCF is the clock's duty
cycle. Deviation from the range specified by most manufacturers
(40% to 60%) produces inadequate settling of the switched-capacitor
element, which can cause signal distortion, inaccurate filter
response, and signal attenuation. Most sources inherently produce
the desired 50% duty cycle; if not, that performance can be ensured
by design in most cases.

Another important characteristic is the clock signal's rise and
fall times. These rising and falling edges should move swiftly
through the threshold of the filter's clock input. Failure to do so
can produce jitter in the internal clock signals, which may cause
unpredictable spurs in the filter's output spectrum. For most HCMOS
devices, the output speed and symmetry is sufficient to prevent
this problem.

It's also prudent to verify that the SCF's input threshold is
compatible with the clock's output levels. When operated with a
single supply, a filter designed for dual-supply operation can have
unexpected requirements for these input levels. Be sure to consult
the filter data sheet for this purpose.

Crystal Clock Oscillators

A crystal clock oscillator (XCO) offers one of the easiest ways
to program the corner frequency of a filter. Expect to pay about
U.S.$3.00 for these devices in small quantities, with the price
climbing about 50% for versions at the extreme ends of the
available frequency range (usually 1- to 60-MHz). This approach is
easy if an XCO at the desired frequency is available, but the
drawbacks are space, cost, and delivery. Only a limited number of
frequencies are available, and custom XCOs are out of the question
for all but the highest-volume requirements.

Traditional XCO packages require a pcb footprint 0.5- by
0.75-inch with an installed height of 0.25-inch, but the newer XCO
packages offer dimensions comparable to that of an IC. As an
alternative for µP or µC systems in which the clock
requirements are somewhat arbitrary, you can save cost and real
estate on the board by using the same clock for processor and
SCF.

Digital Dividers

For many SCFs, the required clock frequency either falls outside
the available range of standard XCO values, or is not available
among them. A digital divider can provide a satisfactory clock in
these applications. Three types are commonly available: binary
ripple counters, pre-settable synchronous counters, and
programmable timer-counters.

Binary ripple counters are easiest to apply, but suffer from
inflexibility. They progressively divide the input signal by two,
over multiple stages. The name “ripple counter” stems from the
output behavior: each stage changes state on the rising edge of the
previous stage. (This action is not a problem unless you attempt to
perform combinatorial logic on the outputs.) Ripple counters are
appropriate when the available clock frequency is a binary multiple
(2, 4, 8, 16, etc.) of the desired clock frequency.

When an application meets this binary
criterion, the ripple counter is easy to implement (
Figure 1). A common IC (the 74HC161) divides a
2.0-MHz clock down to 125-kHz. The 74HC161's output levels and
symmetry are ideal for a single-supply filter operating from 5V. If
you require 3V operation, substitute a MAX7405 for the filter, and
(if operating beyond the 74HC161 voltage or frequency range) a
74VHC4040 for the 74HC161.

If the desired clock frequency is an integral multiple of the
available clock, but not a binary factor, you can use a
pre-settable synchronous counter. Usually available as a 4-bit
device, it counts down from a pre-loaded count to zero. Thus, a
single device is capable of dividing by integers in the range of 2
to 31.

For larger integers, simply cascade the
4-bit stages. To maintain a 50% duty cycle, the last stage should
perform a straight binary division. As an example (
Figure 2), two 4-bit 74HC161
counters—dividing first by six, then by eight—divide a
16.384-MHz clock down to 340.6-kHz. The resulting cutoff frequency
is ideal for telecom applications.

The ultimate digital divider is a programmable timer. Originally
designed as a peripheral for 8-bit microprocessors, this device is
still found on many data-acquisition cards for PCs. One of the
original devices, the 8053, was superceded by the 8054 and is still
multi-sourced as the low-power CMOS 80C54. It has three 16-bit,
general-purpose counters with software-programmable operation and
counting intervals. You can configure these three stages to produce
a staggering number of timing combinations. If an 80C54 can't
provide an acceptable configuration for clock source and counter,
it probably can't be done!

A learning curve is associated with 80C54 applications. You must
load counts and configurations as 8-bit words via the multiplexed
address and data buses. Prudent practice dictates that you
prototype the desired configuration before committing to a
printed-circuit board. Because of difficulties in achieving a 50%
duty cycle with the 80C54, its output should be followed by a
divide-by-two stage such as the 74HC74 D-type flip-flop.

Phase-Locked Loops

Generating an SCF clock from a lower operating frequency yields
an added benefit. Because the clock and lower frequency are
synchronized, no “beat frequency” noise is introduced in the signal
path. A time-division-duplex (TDD) system operating at 8-kHz, for
example, can easily support a 320-kHz clock frequency.

SCF applications lack modulation and have predefined operating
points, so the design of phase-locked-loops (PLLs) for SCF clocks
is easier than for other PLL applications. SCF clock frequencies
also allow use of the ubiquitous 4046 divider. This venerable CMOS
IC (CD4046, MC14046, 74HC4046, etc.) is easily configured for a
particular operating point using external resistors and capacitors.
The addition of an external divider completes the PLL.

PLL design is further simplified by the
availability of free design software. In the system mentioned
above, with an 8-kHz clock source, the cutoff frequency is 3.2-kHz.
To obtain the desired 320-kHz SCF clock, simply add an external
divide-by-40 stage in the form of a divide-by-10 followed by a
divide-by-4 (
Figure 3).

RC Oscillators

If the SCF does not have a self-clocking option, an RC
oscillator will usually suffice in systems for which the
cutoff-frequency accuracy is not critical. Avoid the unpredictable
frequency output of single-stage RC oscillators, and avoid the use
of digital gates with hysteresis, because they specify very loose
limits for the hysteresis voltage.

Instead of a single stage, consider a
ring oscillator constructed as an odd number of inverter stages
separated by single-RC lowpass stages. The signal at each digital
input swings from ground to VCC and crosses its
switching threshold rapidly, producing a stable, low-power
oscillator with predictable output (
Figure 4).

The analog comparator offers another
approach. Its high impedance and high gain, in conjunction with an
external hysteresis circuit, provides a high-accuracy oscillator
(
Figure 5). Operation relies on a single RC
time constant, enabling ready adjustment of the output frequency
with a potentiometer. Frequency is a linear function of the
resistance value. Because the positive feedback is ±33%, the
oscillation period is very close to 67% of the time constant formed
by R1 and C1. Thus, the output frequency is closely described as
fOUT = 1/R1C1.

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