Electronic Design

Switched-Capacitor-Based Bandpass Filter Boasts (Nearly) Rectangular Shape

The filter described below is useful for detecting a signal
that’s closely surrounded by spurs, even when the task
is complicated by the frequency of the signal of interest
wandering by a few percent
. For example, in telecom, a pilot
signal can originate from sources scattered around the world.
Assume a 10.0-kHz signal of choice. Due to variations in oscillators,
temperature conditions, and aging, a frequency deviation
of ±200 Hz can be expected.

Complicate the situation with a spur at 11.0 kHz. To reject
the 11.0-kHz spur, you need a filter with very sharp rolloff, but
to accommodate the wandering 10.0-kHz signal, the passband
of the filter should be flat from 9.8 kHz to 10.2 kHz. You can
jump to the conclusion that you need a DSP-based digital filter.
But don’t.

A switched-capacitor bandpass filter like the LTC1068 from
Linear Technology can create the very sharp rolloff from 9.0
kHz to 11.0 kHz (Fig. 1). The two markers show attenuation of
2.9 dB at 10.1 kHz and 46.7 dB at 10.6 kHz. But the pass section
is very narrow at about ±100 Hz at 3 dB.

The filter center is 50 times the center-frequency clock and,
according to the datasheet, can differ by ±0.9% from the calculated
value. If we allow the filter pass area to scan the region of
9.7 kHz to 10.3 kHz, the worst conditions (90 Hz of the filter
uncertainty and 200 Hz of signal variation) will be covered.
The 11.0-kHz spur is still attenuated by over 50 dB. One caveat:
a detection delay of few milliseconds will occur (Fig. 1, again).

An improved filter design includes a triangle-wave generator
that produces a voltage of “V_filter_center” ±5% (Fig. 2). This
voltage is applied to the input of the voltage-to-frequency converter
(VFC), which in turn moves the center frequency of the
very narrow bandpass filter back and forth. The filter’s sharp
rolloff creates almost vertical attenuation.

Figure 3 shows one version of a triangle-wave generator.
This generator outputs 2.500 V ±0.125 V. The voltage must
match the VFC coefficient to the desired frequency (for the
LTC1068-50, 2.5 V produces 500 kHz). Then the filter will
pass any signal of 10.0 kHz ±500 Hz to the rms-to-dc converter,
greatly attenuating any frequency outside that range. For this
particular design, the error budget is determined by the accuracy
of the resistors and the power source.

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