Circuits and methods for reducing interference from switched mode circuits

[0001] 1. Field of the Invention

[0002] The present invention relates in general to switched mode electronic circuits and in particular to circuits and methods for reducing interference from switched mode circuits.

[0003] 2. Description of the Related Art

[0004] Class D audio power amplifiers (APAS) have been used for many years in systems, such wireline telephony, where high bandwidth is not critical. More recently however, new fabrication techniques, and in particular, new techniques for fabricating power transistors, have made integrated Class D APAs possible. This has extended their potential applications to lower-power, higher-bandwidth systems, including battery-powered portable music players and wireless communications devices.

[0005] One major advantage of Class D amplifiers is their efficiency. Generally, an audio signal is converted into a relatively high frequency stream of pulses varying in width with the amplitude of the audio signal. This pulse width modulated (PWM) signal is used to switch a set of power output transistors between cutoff and saturation which results in efficiencies above 90%. In contrast, the typical Class AB push-pull amplifier, using output transistors whose conduction varies linearly during each half-cycle, has an efficiency of around 60%. The increased efficiency of Class D amplifiers in turn reduces power consumption and consequently lowers heat dissipation and improves battery life.

[0006] Similarly, switched mode power supplies have found wide acceptance in the design of compact electronic appliances. Among other things, switched mode power supplies advantageously use smaller transformers and are therefore typically more compact and lighter weight. This is in addition to the increased efficiency realized over linear power supplies. Moreover, the total number of components can be reduced to, for example, a power MOSFET die and a PWM controller die packaged together in a single package.

[0007] Given the importance of improved battery-life, reduced heat dissipation, and component size minimization in the design and construction of portable electronic appliances, improved switched mode techniques will have numerous practical advantages. The possible applications for these techniques are numerous, although Class D APAs and switched mode power supplies are two primary areas which should be considered.

[0008] According to the principles of the present invention, a system is disclosed which includes a radio receiver and switched mode circuitry operating at a selected switching frequency. Circuitry is included for setting the switching frequency of the switched mode circuitry 114/115 as a function of a frequency of a signal being received by the radio receiver.

[0009] The inventive concepts address one of the major disadvantages of conventional switched mode devices, namely, interference (noise) caused by the switching mechanism itself. This interference is of particular concern in systems employing radio receivers and similar interference sensitive circuitry. In accordance with the inventive principles, the switching frequency is shifted as a function of the radio frequency being received such that the switching frequency and its harmonics fall outside the frequency band of the received signal. Advantageously, these principles can be applied to different types of switched circuitry, including pulse width modulated power supplies and class D amplifiers.

[0010] For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0011] FIG. 1 is a diagram of one channel of a digital radio embodying the principles of the present invention;

[0012] FIG. 2 is a diagram of a Class D pulse width modulated (PWM) amplifier suitable for use as audio power amplifier in the system of FIG. 1;

[0013] FIG. 3 is a diagram of a switched mode power supply for purposes of illustrating the inventive concepts.

[0014] The principles of the present invention and their advantages are best understood by referring to the illustrated embodiment depicted in FIGS. 1-3 of the drawings, in which like numbers designate like parts.

[0015] FIG. 1 is a functional block diagram of one channel of a digital radio 100 embodying the principles of the present invention. Digital radio 100 includes an analog section or front-end 101 which receives radio frequency (RF) signals from an associated antenna 102. Analog front-end 101 is preferably a conventional RF down-converter including a low noise amplifier (LNA) 103 for setting the system noise figure, a bandpass filter 104 and mixer 105 driven by an analog local oscillator 106. The mixed-down analog signal is then converted into digital form by analog to digital converter 107.

[0016] The digitized data output from A/D converter 107 is passed to digital processing section 108. A pair of mixers 109a,b generate in-phase (I) and quadrature (Q) signals from a corresponding pair of clock phases from crystal oscillator 110. The I and Q signals are next passed through bandpass filters 111a and 111b and on to digital baseband processor 112. The processed digital signal is then re-converted to analog (audio) form by D/A converter 113.

[0017] According to the principles of the present invention, a switched mode (Class D) audio power amplifier (APA) 114, discussed in detail below, is used to drive an external set of speakers or a headset. Preferably, at least some of the components of digital radio 100 are powered by a switched mode power supply (SMPS) 114. Power supply 114 will also be discussed further below.

[0018] One of the disadvantages of using conventional switched mode devices is the interference (radiated and conducted) generated by the switching mechanism. This problem is of particular concern in compact electronic appliances which include a radio and similar audio circuits. For example, if the switching frequency is nominally at 350 kHz, harmonics will be generated at 700 kHz, 1050 kHz and 1400 kHz, all of which fall within the AM broadcast band. In order to insure that these signals do not interfere with radio reception, as well as preventing injection of noise into the system at other points, shielding and circuit isolation could be used. However, these alternatives are not practical in low cost and/or compact electronic appliances.

[0019] According to the inventive concepts, if radio 100 is receiving a signal near one of the harmonics of the switching frequency, the switching frequency is moved such that the resulting switching noise will not interfere with received signal. Assume that two possible switching signals A and B, used in either APA 114 or SMPS 115, or both, have base frequencies of 350 kHz and 380 kHz, respectively. (More than two signals can be used to provide a greater resolution). The corresponding harmonics are then:

	A (kHz)	B (kHz)
	700	760
	1050	1140
	1400	1520

[0020] One of the signals A and B is then selected as a function of the frequency of the received signal. In this example, where an AM radio is being assumed, the selection could be made as follows:

	Receive Freq.	Switching
	(kHz)	Signal
	Under 730	B
	930-910	A
	910-1100	B
	1110-1280	A
	1290-1460	B
	Above 1460	A

[0021] As a result, the interference created by the switching signal and its harmonics are moved above or below the reception band, where their effect on noise performance is minimized.

[0022] In a digitally controlled system, the selection of the reception band is performed by a microcontroller or microprocessor which can accordingly also instruct the PWM control circuitry to change frequency. In the case of an analog oscillator, the PWM control circuitry can count the frequency of the local oscillator and choose the PWM frequency accordingly. The different switching frequencies can be generated using either an oscillator with multiple crystals or by frequency division.

[0023] FIG. 2 is a simplified functional block diagram of a Class D pulse width modulated (PWM) amplifier 200 suitable for use as APA 114 in one channel of system 100. It should be noted that a while a basic full-bridge amplifier is shown, other circuit designs may be used to practice the inventive concepts, including half-bridge Class D amplifiers.

[0024] In the full-bridge approach, four power MOSFETs 201a,d are used to drive the differential output from a single voltage supply Vdd under the control of gates and drivers 202a,b. In this embodiment, only one transistor of the upper transistor pair and one transistor of the lower transistor pair of MOSFETs is on and conducting in saturation while the other MOSFET in each pair is completely turned-off.

[0025] The gates/drivers 202a,b are controlled by a PWM modulated signal generated by digital PWM controller 204 which receives the analog audio signal Audio In, along with a high speed clock and a lower frequency clock, discussed below. PWM controller 204 also receives feedback from the outputs of the MOSFET pairs. PWM signal generation techniques are discussed in coassigned U.S. Pat. No. 5,815,102 to Melanson, entitled Delta Sigma PWM DAC to Reduce Switching and incorporated herein by reference. The result is a PWM signal having pulse widths proportional to the input signal amplitude. At the output, a low pass filter 203 is used to recover the amplified audio input signal.

[0026] According to the present inventive concepts, the frequency of low frequency clock (square wave) can be adjusted, as described above, such that the PWM switching signal driving the output MOSFETs (through gates/drivers 202) is shifted out of the reception band.

[0027] The inventive concepts provide at least two ways to generate a variable frequency square wave. (The options are generally indicated in the figures by dashed lines.) According to one embodiment, a crystal oscillator 206 selectively operates from one of a plurality of crystals 207 of differing resonance frequencies. A microcontroller 208, selects the crystal, and therefore the frequency, as a function of the selected receive frequency or frequency band. As indicated above, in a digital controlled radio, the receive frequency is known from the tuner selection and in-an analog system from counting the LO. The primary advantage with this embodiment is that all the divide ratios remain the same.

[0028] According to the second embodiment, a programmable frequency divider 209 is used to generate multiple clock frequencies for driving ramp generator 205. Divider 209 could for example start with a base frequency of 512 fs, where fs is the sampling frequency used in the A/D conversion process, and divide by 64 to obtain a frequency of 8 fs. The resulting 64 time slots make it possible to generate PWM pulse widths from 0 to 64 periods wide. Similar, if the divide ratio is changed, for example, to 72, then 72 time slots are available modifying the switching frequency in the ratio of 8:9. Preferably, divider 209 is programmable with the divide ratio selected by microcontroller 208 as a function of the received frequency.

[0029] These concepts can also be applied to switched mode power supplies, such as SMPS 115 in system 100. A simplified functional diagram of a switched mode power supply 300 is shown in FIG. 3 for purposes of illustrating the inventive concepts. It should be noted that while the illustrated embodiment employs an analog ramp generator and analog comparator, that a digital PWM controller similar to that discussed above can also be instead used in SMPS 115.

[0030] SMPS 300 is based on a power MOSFET or semiconductor switch 301 driving an inductor 302 and output impedance 303. Inductor (core) 302 generally filters current ripple while a capacitor 304 is included for filtering voltage ripple. Free-wheeling diode 305 ensures that current is always flowing into inductor 302. A feedback loop is represented by differential error amplifier 306 which compares a feedback signal from the circuit output against a reference voltage Vref.

[0031] The output from error amplifier 306 is passed to the non-inverting input of modulator 307, the inverting input of which receives a triangle or sawtooth wave from ramp generator 308. As discussed above, the frequency of the square wave input into ramp generator 308 is varied depending on the frequency band of the received signal. Consequently, SWPS 300 also includes a crystal oscillator 309 controlled by a microcontroller 310. As indicated above, the inventive principles provide at least two ways in which the switching frequency can be changed. In one option, a plurality of crystals 311 of different resonance frequencies are provided, in which case all the divide ratios remain the same. In the second option, a programmable frequency divider 312 is used to generate multiple frequencies by dividing down a base frequency, as described above.

[0032] Although the invention has been described with reference to a specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

[0033] It is therefore, contemplated that the claims will cover any such modifications or embodiments that fall within the true scope of the invention.