Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
2000-04-07
2001-12-18
Pascal, Robert (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C330S12400D, C330S20700P
Reexamination Certificate
active
06331801
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to the art of AM radio broadcasting and, more particularly, to an RF power amplifier system having an improved power supply.
2. Description of the Prior Art
The U.S. Patent to H. I. Swanson U.S. Pat. No. 4,580,111 discloses an amplitude modulator for use in AM radio broadcasting wherein the modulator serves to generate amplitude modulated signals by selectively turning on and off a plurality of RF amplifier modules to produce amplitude modulation. The amplifier modules include big step or unit step modules that each, when on, provide an amplified voltage based on the operating voltage supplied to the amplifier. This is a DC voltage obtained from rectifying a three phase AC input voltage. This big step or operating voltage may be referred to as a B+ voltage. Some of the amplifier modules in such a system are referred to as binary or fractional step amplifier modules, each when turned on providing an amplified output signal having a value which is a fraction of the unit or big step voltage.
In prior systems, the operating voltage for the binary amplifier modules was obtained from a binary power supply which utilized a separate single phase secondary winding tapped from the main three phase power transformer with suitable rectifying circuitry to create either a B+/2 or B+/4 operating voltage for the binary amplifier module. Some difficulty with such power supplies has been encountered. There is no adjustment of the DC voltage provided by the binary amplifier power supply and typically there is a tolerance of + or −5%. This has created an error in the fractional step level when compared with a full or big step. If a big step is on the order of 250V, then ½ step is expected to be on the order of 125V. A 5% error may result in a deviation of + or −10V from this anticipated ½ step level. It is desirable to reduce this error. External correction methods have been employed to adjust the size of the binary step relative to the big step. The methods employed have only allowed coarse adjustments and have not been able to remove all the error introduced by the windings of the power supply.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a DC to DC power supply for an RF power amplifier system wherein the supply does not employ additional transformer windings on the main three phase power supply transformer but, instead, provides a transformerless supply.
It is a still further object of the present invention to provide such a DC to DC power supply that employs a switching circuit wherein the switching is accomplished in synchronism with the AM carrier frequency.
It is a still further object of the present invention to provide such a supply wherein the output voltage tracks the input voltage so that variations in the value of the amplitude of the input voltage will be tracked so that the output voltage is always at essentially the same percentage of the input voltage.
It is another object of the present invention to provide such a DC to DC power supply which tracks the input voltage to maintain a desired ratio of input to output voltage while at the same time passing a proper ratio of AC line ripple.
In accordance with the present invention, an RF power amplifier system is provided having a DC voltage source for providing an input DC voltage. An RF source provides an RF carrier signal. An RF amplifier serves to amplify the carrier signal. A DC to DC power supply provides operating voltage to the RF amplifier wherein the operating voltage exhibits a desired percentage of the input voltage. The DC to DC supply includes a low pass filter having a capacitor. A switching device is periodically turned on by turn on pulses for periodically applying the input voltage to the filter which then provides an output voltage across the capacitor. The output voltage serves as the operating voltage. A pulse generator provides the turn on pulses at a frequency synchronized to the frequency of the carrier signal. The turn on pulses exhibit a duty cycle that varies as a function of the value of a control signal. A comparator compares the input voltage with the output voltage and provides the control signal with a value that varies as a function of the comparison.
REFERENCES:
patent: 4580111 (1986-04-01), Swanson
patent: 5838193 (1998-11-01), Myers et al.
patent: 6016075 (2000-01-01), Hamo
patent: 6107875 (1998-11-01), Pullen et al.
Luu Ky Thoai
Schmitt Philip Charles
Choe Henry
Harris Corporation
Pascal Robert
Tarolli, Sundheim, Cove, Tummino & Szabo L.L.P.
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