Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Generating sawtooth or triangular output
Reexamination Certificate
1998-11-19
2001-08-21
Le, Dinh T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Generating sawtooth or triangular output
C327S131000, C327S137000
Reexamination Certificate
active
06278301
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to an improved waveform generator and more particularly to a pulse width modulation conversion circuit with an output capacitor having a digitally controlled charge/discharge rate that permits shifting of the generator's base frequency for limiting the amplitude of higher order harmonics near commercial RF bands.
BACKGROUND OF THE INVENTION
Without limiting the scope of the invention, its background is described in connection with techniques and methods used to reduce signals within the Radio Frequency (“RF”) range. It should be understood, however, that the principles disclosed may have application in a wide array of signal transfer systems including audio and microwave applications using bands regulated and defined by the Federal Communications Commission (“FCC”).
Today a wide array of products and methods exist for transmitting and receiving broadcast audio and video signals. For example, in a typical Frequency Modulation (“FM”) application, a signal generator is used to create a carrier containing the video or audio content. The carrier, in turn, is transmitted inside an envelope corresponding to the FM station's modulation frequency as prescribed by the FCC which is picked up by a receiver tuned to the modulation frequency. Similar principles are employed in other commercial RF bands such as VHF, UHF and AM.
Modern electronic design applications will often combine both digital and analog signals on a single system platform. An example is an acoustic soundboard wherein the pure acoustic waveform receives digital enhancement prior to being transferred to a rendering system such as a speaker or audio recorder. Precise engineering is required to ensure that the higher order harmonics of the fundamental base frequency are small enough to limit interference with frequencies within reserved bands such as commercial FM or AM radio.
A primary goal in digital audio processing is to make efficient use of the available signal-to-noise ratio and bandwidth over a given transmission channel. For this reason, various modulation techniques have been developed to make RF waveforms travel over long distances. Modulation refers to a way of distorting a pure waveform prior to transmission along a communications channel. The communications channel may be physical (such as copper wiring or fiber optic cabling) or air (such as radio, television or satellite).
SUMMARY OF THE INVENTION
Various noise filtering and suppression techniques are commonly employed by those skilled in the art to limit interference within a given band of frequencies. Typically, a filter or combination of filters, e.g. high-pass, low-pass or band-pass, are used to suppress or dampen unwanted harmonics outside the pass range of frequencies. Based on the application, the filters can be implemented in discrete components or as an Integrated Circuit (“IC”) .
Spectrum spreading is a technique which can be used to disguise a signal by shifting the base frequency of the carrier signal over a range of values. A primary objective is to dampen the amplitude of the higher order harmonics while maintaining the integrity and strength of the base frequency. The energy under the carrier envelope of the spread is equal to the energy passed in the unmodulated waveform.
Tradeoffs in performance, cost and space are the critical factors that dictate a specific implementation. A discrete component solution may provide the sharpest roll-off at the cut-off frequency but may not be practical in high volume applications or where board space is limited. On the other hand, second and third order filters are available but their cost may increase significantly the total system price. What is needed is a cost effective modulation scheme that can be used to dampen energy associated with higher order harmonics.
Accordingly, the present invention is a pulse width modulation conversion circuit having an improved waveform generator with digitally controlled spectrum spreading. A ramp generator has a circuit for controlling the charge and discharge times of a capacitor that effects the generator's base frequency. The charge and discharge currents are modulated in a way that effects the slope of the waveform with a fixed threshold determining the base frequency. By modifying the magnitude of the charge and discharge current at the either peaks or valleys of the triangle, the slope of the waveform is likewise altered resulting in a base frequency shift. The carrier energy remains the same but spread over a range of frequencies inside an energy envelope with RMS values less than that of the base. A set of pass transistors add or subtract currents in proportion to a reference source of current which sets the amount of shift about the base frequency.
In one embodiment, the reference current source is fed into a current divider, which delivers a portion of the reference into a summing block. The sum of the reference and portion currents are fed into a charge/discharge circuit which drives an output capacitor in relation to an input voltage reference at the charge/discharge circuit.
A threshold detector circuit provides a feedback loop from the charge/discharge circuit and is used to control the charge and discharge of the capacitor. The loop controls whether the capacitor is charging or discharging as well as the magnitude of charge/discharge current. The threshold detector is gated to provide a change from charge to discharge of the capacitor.
Switch logic can be coupled to the current divider to set the current ratio. The switch logic, in turn, can be coupled to a control block that provides digital control of the modulator either through hard wired means, programmable memory means or other similar means of program flow control. The energy of the higher order harmonics is spread across a range of frequencies thus dampening their amplitude.
REFERENCES:
patent: 4384305 (1983-05-01), Sonnenberger
patent: 5629644 (1997-05-01), Chevallier
Chen Wayne T.
Cotton Dave
Jones, III Roy Clifton
Brady III Wade James
Le Dinh T.
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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