AN912 Datasheet, PDF (6/16 Pages) Microchip Technology – Designing LF Talkback for a Magnetic Base Station

AN912
THE LOW PASS FILTER STAGE
The output signal from the decoupling stage consists of
the 125 kHz carrier ripple and the modulated data
signal, if one ignores the dynamic response signal. The
carrier ripple is about 300 mV peak-to-peak. The data
is 4 mV peak-to-peak with 6 dB of gain of the decoupler
and a cutoff frequency at about 10 kHz. The aim of the
low-pass filter stage is to amplify the data signal at
2.5 kHz and to filter out the carrier ripple in the most
effective manner.
The three most common active filter topologies used
are the Chebyshev, Butterworth and Bessel filters. The
Chebyshev filter has the steepest transition from pass
band to stop band, but has ripple in the pass band. The
Butterworth filters have the flattest pass band
response, but does not have such a steep transition as
the Chebyshev. The Bessel filter has a linear phase
response with a smooth transition from pass to stop
band. It seems the Chebyshev filter would best be
suited for this application, but the frequency response
does not tell the whole story.
The data signal is amplitude modulated and the tank
has steep transient response dynamics. As a result, the
filter should have a stable and flat transient response.
The Chebyshev filter has a very sharp frequency cutoff
response, but has the worst transient response of the
three filter topologies. The Chebyshev filter also has an
underdamped step response with overshoot and
ringing. The Butterworth filter has a better transient
response, but still some overshoot. The Bessel filter
has the worst response from a frequency perspective,
but has the best transient response as a result of its
linear phase characteristics. There are of course other
active filter topologies such as elliptical, state variable,
biquad and more, but a Bessel filter has adequate
performance for the application.
The data signal, in this example, has maximum
modulation frequency of 2.5 kHz or a TE of 200 µs. A
Bessel filter, with a cutoff frequency of 1/(2.2TE) =
2.27 kHz, would be ideal from a noise rejection point of
view, but a 2.5 kHz cutoff was chosen to minimize sym-
bol overlap. The target is to design a filter with sufficient
performance using a single operational amplifier in
order to reduce the system cost. A dual operational
amplifier can then be used because the decoupling
stage also uses an amplifier. A third order Bessel filter
can now be implemented with the remaining amplifier.
The filter gain is the final aspect to specifying the
Bessel filter. Using Microchip's FilterLab^® program,
one can get the response for a unity gain – 2.5 kHz, 3d
order Bessel filter. At 125 kHz, the filter has 93 dB of
attenuation and the input ripple amplitude is 300 mV
peak-to-peak. Assuming the filter should have an
output ripple of no more then 1 mV peak-to-peak with
12 dB of headroom for noise, coupled through the
supply line, then one needs at least 62 dB of attenua-
tion. This leaves 31 dB of allowable gain from the third
order filter. For the design, a gain of 20 or 26 dB was
chosen, leaving some additional headroom for ripple
rejection. The 3d order low-pass Bessel filter is shown
in Figure 8 and has a Fc = 2.5 kHz and 26 dB of gain.
Please note that the circuit shown in Figure 8 has a
fairly high output impedance at the data rate, but the
output of the filter will be driving a high-impedance
load, and this is therefore acceptable.
FIGURE 8:
78.7k
Input
3.92k
16.5k
10 nF
150 pF
-
+
Output
4.87k
10 nF
DS00912A-page 6
 2004 Microchip Technology Inc.