52-0024-0100 EP05-02 Mòdul (Anglès).doc 2 of 4
ANTI-ALIASING FILTER.
The anti-aliasing filter, like on every digital equipment, is intended to allow signal
processing of input signal bandwidths just up to a half the system’s sample rate
(BW
input
<
1
/
2
∙
SR ), to avoid A/D conversion errors. Our system has an input signal
bandwidth of 35KHz, so the anti-aliasing cut-off frequency is slightly above this value.
THE PULSE WIDTH MODULATOR (PDM).
The amplifier runs on a 250KHz switching rate. This frequency is obtained from a
8MHz xtal, whose oscillation frequency is passed through a frequency divider, in order to
bring it back to a 500KHz. This allows to generate the 250KHz sawtooth wave, which will
be the carrier signal used by the PWM modulators.
The sawtooth waveform is obtained by processing this 500KHz square signal
through an integrator circuit, which produces a 250KHz 4V
pp
sawtooth wave.
As the modulator used operates in BD-mode, this module has two PWM modulators,
which compare the same sawtooth carrier with each their own input signal, the modulating
audio+
and
audio-
signals, which in fact are the same but in phase opposition, obtained by
processing the module’s main input signal.
Each of this comparator is bound to a half power stage, and has two output nodes,
Q and Q’, which are the signals used to drive both upper and lower MoSFET’s. The driving
signals have a dead transition time, and operate between a maximum and a minimum pulse
width, in order to always avoid both upper and lower MoSFET’s to be conducting at the
same time in one single branch.
As already told, the power stage is build using a bridged structure, this is, each
branch has two half-bridges, and the module’s loading impedance hangs between their
outputs. This structure allows us to use an asymmetric power supply, which will have to
deliver just a half of the voltage needed if the same power level would have to be obtained
from each half-bridge module. Each half-bridge delivers a powerful PWM-signal, which,
once it is filtered, generates two opposed-phase low frequency AC signals.
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