Techniques for synchronizing a self oscillating variable...

Details Techniques for synchronizing a self oscillating variable... Techniques for synchronizing a self oscillating variable...

1999-12-23

2001-10-02

Pascal, Robert (Department: 2817)

Amplifiers

Modulator-demodulator-type amplifier

C330S251000

Reexamination Certificate

active

06297693

ABSTRACT:

The frequency spectrum of variable frequency a class D modulator can potentially interfere with many RF signals most notably AM broadcast radio. This limits the use of such modulators to electronic equipment without an AM tuner. However, many customers for audio amplifiers require that the amplifier have an AM receiver. The technique described here overcomes this defect with several techniques which synchronize the class D modulator to an external clock. If the frequency of the external clock is chosen wisely, interference with AM radio signals can be prevented.
BACKGROUND OF THE INVENTION
The performance of class D amplifier can be improved with the use of a self oscillating variable frequency modulator. The most notable advantage is a lower noise floor. The drawback to this approach is that the frequency spectrum of the modulator spans the entire AM band, because the frequency falls as the power output increases—see FIG.
1
. Furthermore, the front end bandpass filters used in tuners effectively convert the frequency modulation of a variable frequency modulator into AM modulation, since the amplitude of the interference varies with frequency (slope detection). All of these issues make the modulator in
FIG. 1
unacceptable for use with an AM radio. The modulator as shown in
FIG. 1
is unacceptable for audio amplification because the frequency transverses the audio range at high powers, and at clipping the control loop opens and the integrator saturates. An improved modulator based on the one in
FIG. 1
which limits the frequency above 100 kHz at full power can be found in my co-pending patent application U.S. Ser. No. 09/342,376, filed Jun. 29, 1999, whose entire disclosure is incorporated by reference.
This invention maintains the benefits of self oscillation, and eliminates AM interference by synchronization to an external clock whose frequency is chosen so that it does not interfere with AM reception. The details of how this frequency is chosen are discussed in my co-pending application (U.S. Ser. No. 09/342,376, filed Jun. 29, 1999).


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