PHASE MODULATOR APPARATUS FOR GENERATING A BURST PHASE-MODULATED WAVE

Abstract: The phase modulator apparatus for generating a burst phase-modulated wave comprises a carrier generator (1) for gener­ ating a carrier wave, a phase modulator (3) for phase-­ modulating the carrier wave with an input information signal, an amplifier (4) for amplifying the output of the phase modulator (3),and a power source (8) for commonly supplying a source voltage to the carrier generator (1), the phase modulator (3) and the amplifier (4). A first switch (5) is responsive to a burst con­ trol signal for controlling the supply of the source voltage to the amplifier (4), a load is coupled to the power source (8), and a second switch (6), connected to the load (7) in series, is responsive to the burst control signal for operating in a manner mutually com­ plementary to the first switch (5). This phase modulator apparatus eliminates frequency variations of the oscillator caused by fluctuations in power source voltage during burst mode operation.

Description: The present invention relates to a phase modulator apparatus, and more particularly to a phase modulator apparatus for generating a burst phase-modulated wave, which is used in a burst signal communication system such as a TDMA (Time Division Multiple Access) communication system and an MD-TDM (Multi Directional-Time Division Multiplex) communication system. EP-A-56277 teaches a TDMA system having a control circuit which controls a delivery timing of a carrier signal from a local oscillator to a modulator in order to eliminate spurious components. A conventional phase modulator apparatus of this kind includes a carrier generator for generating a carrier wave, a phase modulator for phase-modulating the carrier wave with an input information signal, an amplifier for amplifying the output of this phase modulator, a power source for supplying a source voltage to the oscillator, the phase modulator and the amplifier, and a switch for turning on and off in a time-division burst mode the source voltage supplied to said amplifier, thereby providing a phase-modulated burst wave from the output of the amplifier. In such an apparatus, when the source voltage is turned on or off in a time-division burst mode, the current supplied from the power source varies, thereby giving rise to fluctuations in voltage drop due to minute resistances present on the power supply line. As a result, the carrier frequency of the oscillator is varied to adversely affect the modulating performance. SUMMARY OF THE INVENTION An object of the invention, therefore, is to provide a phase modulator apparatus capable of eliminating frequency variations of the oscillator, caused by fluctuations in power source voltage during burst mode operation. This object is solved with the features of the claims. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram illustrating the structure of a phase modulator apparatus embodying the present invention; FIGS. 2A TO 2D constitute a time chart for explaining the operating of the phase modulator apparatus of FIG. 1; FIG. 3 illustrates a specific example of circuitry for the switches in the embodiment of FIG. 1; and FIG. 4 is a block diagram illustrating an example of transmitter-receiver using the phase modulator apparatus according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 showing a phase modulator apparatus of the present invention, reference numeral 1 represents a carrier generator for generating a carrier wave; 2 and 4, amplifiers; 3, a phase modulator; 5 and 6, switches; 7, a dummy load; 8, a common power source and 9, a driver. Now, the operation of this phase modulator apparatus will be described by referring to Figs. 2A to 2D showing waveforms at various points in the apparatus of Fig. 1. The carrier wave from the generator 1 is amplified by the amplifier 2 and supplied to the phase modulator 3, which phase-modulates the amplified carrier wave with an information signal, i.e., a digitized speech signal a shown in Fig. 2A. The phase-modulated carrier wave is amplified by the amplifier 4, which is selectively actuated by the switch 5. The switch 5 selectively supplies a current c (Fig. 2C) from the power source 8 to the amplifier 4 in response to a burst control signal b (Fig. 2B) polarity-inverted by the driver 9. The burst control signal b has, as shown in Fig. 2B, a time slot T₁, which is allocated for one station and also repeated every one frame. The frame includes other time slots (not shown) for other stations. Thus, the apparatus of each station, i.e., the amplifier 4 provides an appropriate burst phase-modulated carrier wave. The switch 6 featuring the present invention is connected in parallel to the switch 5 and operates in a manner complementary to the switch 5. This is, upon turning on and off the switch 5, the switch 6 is turned off and on, respectively. When the switch 6 is turned on, the current d shown in Fig. 2D flows through the load 7 and the switch 6. If the resistance of the load 7 is adjusted so that the current d flowing through the switch 6 becomes equal to the current c flowing through the switch 5, the output voltage of the power source 8 does not vary despite the turning-on/off of the switch 5. Therefore, the oscillation frequency of the carrier generator 1 remains unaffected. The load 7 may be provided between the output of the switch 6 and the ground. A specific example of circuitry for the switches 5 and 6 is shown in Fig. 3. According to this example, the switch 5 consists of a PNP type transistor Tr₁, a resistor R₁ connected between the base and the emitter, and a resistor R₃ connected between the base terminal and the switch input. The switch 6 consists of an NPN type transistor Tr₂ and a resistor R₂ connected between the base and the switch input. The collector of the transistor Tr₁ is connected to the amplifier 4 (Fig. 1) and its emitter is connected to the power source 8 (Fig. 1). The collector of the transistor Tr₂ is connected to the power source 8 through a resistor R L , as the load 7, and its emitter is grounded. These transistors implement the complementary ON/OFF-operations in response to the drive control signal which is the output of the driver 9. FIG. 4 is a block diagram showing an example of transmitter-receiver for use in a TDMA satellite communication system, to which is applied the present invention. As illustrated, this transmitter-receiver consists of an indoor unit 100 and an outdoor unit 200. A phase modulator apparatus 10 according to the invention, used in the indoor unit 100, includes a phase shift-keyed (PSK) phase modulator for instance, and supplies such a burst phase-modulated signal as what was referred to above in response to an information signal a from a baseband circut 11 and a burst control signal b from a control signal generator 14. This burst phase-modulated signal is mixed by a mixer 16 with an output from a synthesizer 17, filtered by a filter 19, amplified by an intermediate frequency (IF) amplifier 21 and supplied to a multiplexer 24 of the outdoor unit 200 by way of a multiplexer 22 and a single cable line 23. In the outdoor unit 200, the output signal from a multiplexer 24 is converted in frequency into the radio frequency (RF) band by a local oscillator 26 and an up converter 27 and, after being amplified by a power amplifier 29, transmitted by way of a duplexer 30 and an antenna 31 to a satellite. Meanwhile, a signal from the satellite is received by the antenna 31 of the outdoor unit 200 and, after going through the duplexer 30 and a low noise (LN) amplifier 28, is converted in frequency by the local oscillator 26 and a down converter 25. The converted signal is entered into the multiplexer 22 of the indoor unit 100 via the multiplexer 24 and the cable line 23. The output from the multiplexer 22, after going through an IF amplifier 20 and a filter 18, is mixed by a mixer 15 with an output from the synthesizer 17 and, after being demodulated by a PSK demodulator 12, is supplied to the baseband circuit 11. Herein, a controller 13 controls the frequencies of the synthesizer 17. In summary, according to the present invention, there is provided, in addition to a first switch for selectively actuating the amplifier connected to the output of the phase modulator, a second switch for operating in a relationship complementary to the first switch to supply a current to a dummy load which is connected in series to this second switch. The resistance of the dummy load is selected so that the currents flowing through the first and second switches become equal to each other. As a result, the current flowing from the power source is kept constant to make it possible to eliminate the frequency variations of the carrier generator attributable to source voltage fluctuations during burst mode operation.

Claims: A phase modulator apparatus for generating a burst phase-modulated wave in response to an information signal and a burst control signal, comprising a carrier generator (1) for generating a carrier wave, a phase modulator (3) for phase-modulating said carrier wave with said information signal, an amplifier (4) for amplifying the output of said phase modulator, a power source (8) for commonly supplying a source voltage to said carrier generator, said phase modulator and said amplifier, and a first switch (5) responsive to said burst control signal for controlling the supply of said source voltage to said amplifier, characterized by a load (7) coupled to said power source, and a second switch (6) connected to said load in series and responsive to said burst control signal for operating in a manner mutually complementary to said first switch, wherein said load (7) has such a resistance that currents flowing through said first switch (5) and said second switch (6) become equal to each other. A phase modulator apparatus as claimed in Claim 1, wherein said load (7) is a resistor. A phase modulator apparatus for generating a burst phase-modulated wave, as claimed in Claim 1 or 2, wherein said load (7) is connected between the output of said power source (8) and the input of said second switch (6). A phase modulator apparatus as claimed in Claim 1 or 2, wherein said load (7) is connected between the output of said second switch (6) and a potential point.

Inventors: OHMAGARI, SHINICHI C/O NEC CORPORATION
Applicants: NEC CORPORATION
Application year: 1987
Main classification: H04L27
IPC codes: H04L27, H03L1, H04J3