Resonator system for an RF power amplifier output circuit

A resonator system is presented that has first and second cavity resonators for use in an RF amplifying system employing an RF amplifier device having an output circuit and an RF signal broadcasting antenna coupled to the output circuit. The resonators are interposed between the amplifying device output terminal and the antenna. The first resonator is comprised of a transmission line being a length of two coaxial conductors and tuned to the 3rd harmonic of the operating frequency (3fo). Each resonator has first and second opposing ends with the first end being an open end and the second end being a shorted end. The open end of the first resonator is connected to the output terminal of the RF amplifying device. The second resonator is connected in series with the first resonator and is tuned to the fundamental operating frequency (fo).

FIELD OF THE INVENTION

This invention is directed to RF broadcast and communication systems and is particularly related to improving the efficiency of RF power amplifiers.

BACKGROUND OF THE INVENTION

It has been known in the art to employ a lumped “LC” parallel-tuned circuit third harmonic resonator in series with the output terminal of a power amplifier device intermediate the output terminal and a broadcasting antenna. The efficiency of an “LC” resonator is not as great as desired, particularly at VHF and higher frequencies due to circuit losses, stray reactances and undesired resonances, hence, such a “LC” resonator is not practical at these higher frequencies. It is desirable, therefore, to provide an improved third harmonic resonator for use in the output circuit of such an RF amplifying system and located between the RF amplifier device and the output circuit which provides an RF signal to the broadcasting antenna.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a succession of two cavity resonators is provided as the output circuit in an RF amplifying system for feeding a broadcasting antenna. The first of two resonators is interposed between the output terminal of the RF amplifier device (Vacuum Tube or Transistor) and the open end of the second, output circuit resonator. The first resonator is tuned to the 3rdharmonic of the operating frequency (3fo). The second resonator is tuned to the fundamental operating frequency (fo). Both resonators are coaxial transmission lines. The first resonator is formed by a length of coaxial conductors having first and second opposing ends with the first end being an open end and the second end being a shorted end. The open first end of the first resonator is connected to the output terminal of the RF amplifier device.

In accordance with a more limited aspect of the present invention, the first resonator is a cavity resonator in that a portion of the length of an inner conductor is spaced from and surrounded by a portion of the length of an outer conductor, creating a cavity between the conductors. Parts of the conductors are slidable relative to each other to achieve a variation in the resonant frequency.

DESCRIPTION OF PREFERRED EMBODIMENTS

This invention may be used to increase the operating efficiency of an RF power amplifier, by reducing the third harmonic current component in the output waveform of the amplifier. This invention provides a high impedance to the third harmonic current component on the output terminal (anode, drain, collector) of the RF power amplifying device which reduces the amount of power wasted in the third harmonic content. This invention also increases the transition slope of the output waveform which improves the switching efficiency of the output device by reducing the switching transition time spent in the active, series “on” resistance, area of the output device.

The embodiment of the invention presented herein utilizes a ¼ wavelength transmission line segment at three times the fundamental operating frequency (3fo) which is shorted at one end and open at the other end creating a resonant circuit at the third harmonic of the power amplifier's operating frequency. The impedance at the open end of this coaxial transmission line segment is very high at the third harmonic of the fundamental operating frequency while simultaneously providing a low, inductive, impedance at the fundamental, operating frequency.

This transmission line segment is placed with the open end located at the output terminal of the amplifying device which places it in series between the output terminal of the amplifying device and the input terminal of the fundamental frequency, ¼ wavelength resonant cavity output circuit of the RF power amplifier. Placing a high impedance at the third harmonic frequency in series with the output device changes the voltage waveform on the output terminal of the power amplifying device from the quasi-sinusoidal waveform shown inFIG. 1to the more square wave like waveform shown inFIG. 2which has faster rise and fall times.FIGS. 3-8show the embodiment of this invention in a VHF power amplifier cavity for an FM transmitter.

Reference is now made toFIGS. 3-8which illustrate, in detail, the electrical and mechanical features of the cavity resonator system employing the present invention. As is well known, a cavity resonator may be defined as any region bounded by conducting walls within which resonant electromagnetic fields may be excited. The first or inner cavity resonator disclosed herein may be referred to as a third harmonic stub resonator and, as shown in the drawings, includes a transmission line which is a length of two coaxial conductors including an inner conductor18of the fundamental output cavity resonator20and an outer conductor22of the 3rdharmonic coaxial cavity resonator23. These conductors are cylindrical in shape with the outer conductor coaxially surrounding and being radially spaced from the inner conductor so as to define the 3rdharmonic cavity24therebetween. The length of the cavity is adjustable and is on the order of ¼ wavelength of the 3rdharmonic frequency and 1/12 wavelength of the fundamental operating frequency.

The structure noted above is located within a metal (aluminum) housing21which provides the outer conductor of the second resonator20. The vertical walls of the housing are spaced from conductor18of the inner resonator20to define a cavity25therebetween.

As shown inFIG. 3, the lower end of the inner resonator cavity23is open with the inner conductor26being connected to the anode or output circuit of the amplifier tube to which the resonator is coupled to. The upper end of the outer conductor22is closed and this is best shown inFIGS. 5 and 6wherein it is seen that the upper end is provided with a metal ring30which extends between the upper end of the outer conductor22and the inner conductor18. The metal ring30is connected, as by welding or the like, to the inner conductor18and to outer conductor22. This then provides the upper closed end of the first or inner 3rdharmonic cavity resonator23.

It should be noted that conductor18of resonator20is electrically connected by shorting ring30to the conductor22of resonator23and both serve as the inner conductor of resonator20. These conductors are surrounded by the walls of housing21to define resonator20with a cavity25therebetween. As best seen inFIG. 7, resonator20may be tuned by moving shorting plate60up and down within the housing. This plate shorts the open end61of the resonator between conductor18and the walls of the housing21.

The 3rdharmonic cavity resonator is tunable by effectively adjusting the length of the cavity24. This is achieved with the structure described below.

The inner conductor18is a two-part device, in that it includes a first segment40of circular cross-section and a second segment42of circular cross-section that coaxially surrounds a portion of the length of the first segment40. The lower end of the second segment42is provided with an annular array of fingers44which make frictional and electrical engagement with the outer surface of segment40. This relationship is such that segment42may be displaced relative to segment40in an axial direction. In a similar manner, the outer conductor22has a first segment50of circular cross-section and a second segment52of circular cross-section that coaxially surrounds a portion of the length of the first segment of the outer conductor. It is to be noted that the upper portion of segment52is provided with a plurality of fingers54that frictionally and electrically engage the outer surface of segment50. This permits segment52to be displaced relative to segment50in an axial direction. Consequently, the cavity24may be expanded or shortened in an axial direction by movement of these segments relative to each other. If the length of the cavity24is shortened, this will increase the resonant frequency of the cavity. Similarly, if the length of the cavity is decreased, this will lower the resonant frequency of the cavity.

Reference is now made to the circuit diagram ofFIG. 8for another representation of this embodiment of the invention. As noted herein, this embodiment of the invention includes two cavity resonators20and23that are electrically connected in series between an amplifying device70and an antenna72. The shorting plate60(FIG. 7) of the resonator20is electrically connected to one end of an output coupling loop64that extends through a suitable opening66in a sidewall of housing21and thence to antenna72by way of a harmonic low pass filter68.

Although the invention has been described in conjunction with a preferred embodiment, it is to be appreciated that various modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims.