Output Waveform Control Circuit

The present invention relates to an output waveform control circuit of a time division multiple access (TDMA) system, and more particularly to the control of the output waveform of the power amplifier of a transmitter in a TDMA system.

In a TDMA system, in order to prevent the expansion of the spectrum, resulting from the burst switching of the output carrier of a transmitter, from adversely affecting any other transmission channel or the reception channel of the transmitter's own station, the leading and trailing edges of the output signal should be so controlled as to form gentle slopes.

A conventional output waveform control circuit which will be described later referring to FIG. 1 of the accompanying drawings, is disclosed in U.S. Pat. No. 4,706,262 issued to K. Ohta, Nov. 10, 1987 and assigned to the present assignee. In the disclosed circuit, an envelope generating circuit generates a signal whose leading and trailing edges are gently sloped. This signal is then compared by a comparator circuit with a detection signal to provide through a power amplifier an error signal. Next, the error signal supplied by the comparator circuit is applied via a driver circuit to an input level control circuit as a control signal. Responsive to the control signal, the error signal from the power amplifier is so controlled as to become similar to the signal waveform from the envelope detector circuit. In addition, a switching circuit on-off switches the output of a modulator to provide through a frequency converter the switched output to the input level control circuit. The on-off switching at the switching circuit causes the frequency spectrum of the modulator output to be restricted to allow the ON/OFF ratio of the output to be increased.

Since the conventional waveform control circuit smoothes only the leading and trailing edges of the modulator output, the output waveform may still be distorted. This can be due to manufacturing variations of the power amplifier. The distortions on the output waveform cause spurious signals which adversely affect other transmission channels of neighboring base stations.

Furthermore, the rising and falling time of an output in the conventional system may exceed a time period of, for example, 28 microseconds, the standard required for an output with an ON/OFF ratio of 70 dB, under the Groupe Special Mobile (GSM) pan-European mobile telephone system due for launch in July 1991.

Another drawback of the prior art circuit is that the output of the modulator is only switched on and off by the switching circuit and the resulting waveform has steep rising and falling edges. Such a waveform cannot satisfy the GSM standard requiring an output spectrum to have spurious signals of less than 60 dB at f.sub.o .+-.400 KHz and of less than 70 dB at f.sub.o .+-.600 KHz, where f.sub.o is a center frequency.

It is, therefore, an object of the present invention to provide an output waveform control circuit capable of providing an output waveform having gently sloped leading and trailing edges, even when using a class C amplifier or the like whose output characteristic is non-linear.

It is another object of the present invention to provide an output waveform control circuit cleared of the above stated disadvantages and having an output waveform conforming to GSM standards.

According to the present invention, there is provided an output waveform control circuit capable of providing an output waveform having gently sloped leading and trailing edges, having a second driver circuit for controlling the source voltage of a power amplifier in accordance with an error signal from a comparator circuit simultaneously with the input level of the power amplifier even if it is of class C or the like having a non-linear output characteristic.

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic block diagram of an output waveform control circuit of the prior art;

FIG. 2 is a waveform diagram of the output signal from the output waveform control circuit shown in FIG. 1

FIG. 3 is a schematic block diagram of an output waveform control circuit according to the invention;

FIG. 4 is a waveform diagram of the output from the output waveform control circuit shown in FIG. 3;

FIG. 5 is a schematic circuit diagram illustrating the driver circuit of the power amplifier shown in FIG. 3;

FIG. 6 is a schematic circuit diagram illustrating the power amplifier shown in FIG. 3; and

FIG. 7 is a schematic circuit diagram illustrating the input level control circuit shown in FIG. 3.

In FIG. 1, the conventional output waveform control includes a gain control circuit 15 to which a high frequency input signal is supplied via terminal 5. The signal is in turn supplied to a high power amplifier 16, where the signal is amplified and sent to output terminal 21. The amplifier output S.sub.o is detected by an envelope detector 18, and compared by a comparator circuit 19 with a reference waveform which is generated by an envelope generator circuit 20 in response to a control signal fed to terminal 12. From the comparison, the comparator circuit 19 provides an error signal, with which the amplification factor of the gain control circuit 15 is controlled. The gain control circuit 15, high power amplifier 16, envelope detector 18 and the comparator circuit 19 constitute an amplitude control unit 30.

Also included in this output waveform control circuit is a modulator circuit 13 which modulates an intermediate frequency (IF) signal fed from an input terminal 10 with a modulating signal fed from another input terminal 11. The output signal thereby modulated is switched on and off by the switching circuit 132 and then converted into a radio frequency (RF) carrier wave by a frequency converter 14 whose output is supplied to the gain control circuit 15 via the terminal 5.

As mentioned earlier, however, this output waveform control circuit smoothes only the leading and trailing edges of the output of the modulator 13. Therefore, as seen in FIG. 2 the output would be distorted due to manufacturing variations of the power amplifier 16. These distortions cause spurious signals which adversely affect other transmission channels of neighboring base stations in the TDMA system. Another drawback of this system is that it does not meet certain standards set by the Groupe Special Mobile (GSM). Firstly, the rising and falling time in this conventional system may exceed a time period of, for example, 28 microseconds, the maximum standard required for an output with an ON/OFF ratio of 70 dB. Furthermore, the output of the modulator 13 is only switched on and off by the switching circuit 132 and the resulting waveform has steep rising and falling edges. Such a waveform cannot satisfy the above-mentioned GSM standards for the spurious signal levels at f.sub.o .+-.400 KHz and f.sub.o .+-.600 KHz.

A preferred embodiment of the present invention shown in FIG. 3, has an input signal terminal 1 through which a modulated radio signal is provided to an input level control circuit 3. The input level control circuit 3 controls the level of the input radio frequency signal in response to a level controlsignal from a driving circuit 5. A power amplifier 4, which receives the output of the input level control circuit 3 as its input, has its output level controlled in accordance with the source voltage controlled by a driving circuit 6. A detector circuit 7 detects the envelope of the output signal of the power amplifier 4 to provide a detected signal to comparator circuit 8. The comparator circuit 8, responsive to the difference between the detected signal and a signal generated by a control signal generator 9 which has gently sloped leading and trailing edges, provides an error signal. Based on the error signal, the driver circuit 5 provides as a first driver circuit the level control signal to the input level control circuit 3. In addition to the level control on the basis of the error signal, the driver circuit 6 controls as a second driver circuit the source voltage of power amplifier 4 to thereby control the output level of power amplifier 4. The amplifier 4 provides it output of a waveform shown in FIG. 4 to an output terminal as the output of the output waveform control circuit.

As can be seen from FIG. 4 the waveform of the output from the waveform control circuit shown in FIG. 3 has gentler leading and trailing edges than that shown in FIG. 2. Since the level of the modulation high-frequency signal is controlled by both the input level control circuit 3 and the power amplifier 4, the output signal provided at the terminal 2 has an ON/OFF ratio and a spurious characteristic sufficient to conform to the GSM standards. Even if the power amplifier has manufacturing variations, no distortions will occur in the output signal, because distortion due to the variations is eliminated by controlling the source voltage of power amplifier 4 and thus controlling the output level of same.

Referring to FIG. 5 the driver circuit 6 includes an NPN transistor 104 and a PNP transistor 105 connected together as shown to form a Darlington circuit. The input terminal 101 connected to the output of comparator 8 (FIG. 3) is connected to the base of transistor 104, and the collector of transistor 104 is connected to the base of PNP transistor 105. The power supply terminal 103 is connected to the emitter of transistor 105 whose collector is connected at point A which is also connected to the emitter of NPN transistor 104. A capacitor 106 is connected between point A and ground, point A being connected to output terminal 102, which in turn is connected to a source voltage terminal 302 (FIG. 4) for the amplifier 4.

In operation, an input signal is applied to input terminal 101 simultaneously with a bias voltage supplied at terminal 103, to turn on transistors 104 and 105. The connection of these transistors is such that the NPN and PNP transistors of amplification factors hf.sub.1 and hf.sub.2, respectively, constitute a Darlington circuit having a current amplification factor of hf.sub.1 .multidot.hf.sub.2. The bypass capacitor 106 is provided to bypass undesired high-frequency noise and to prevent undesired oscillation. Thus, a large current can be controlled by the Darlington circuit in conjunction with an operational amplifier included in the comparator circuit 8. A large driving current is thus derived from the output terminal 102 and provided to the source voltage terminal 302 (FIG. 7).

Referring to FIG. 6, the input level control circuit comprises an input terminal 201 connected to the input terminal 1 (FIG. 3) and to the anode of a diode 206 through a capacitor 205. The cathode of the diode 206 is then connected to the anode of the diode 207, which in turn is connected to the anode of the diode 208 whose cathode is connected to the output terminal 203 via a capacitor 210. The bias voltage is applied through a terminal 202 and an inductor 204 from the driving circuit 5 (FIG. 3) to the cascading diodes. A second inductor 209 is also connected between the cathode of diode 208 and the capacitor 210.

In operation, an input signal is applied to terminal 201 and then through capacitor 205 to the anode of diode 206. The capacitor 205 is for eliminating the DC components and associated noise. The bias voltage is applied to the anode of diode 206 through the terminal 202 and the inductor 204, which is for preventing any high frequency signals from reaching the terminal 202. If the bias voltage is greater than a predetermined voltage, the input signal is allowed to pass through the cascading diodes 206, 207 and 208. The capacitor 210 blocks any DC components, allowing them to pass to ground through the inductor 209, which is for preventing any high frequency signals from grounding. The required output signal is then present at the output 203 and provided to the power amplifier 4 (FIG. 3).

Referring to FIG. 7, the power amplifier includes a series of cascading amplifiers 304, 306 and 308. The output of level control circuit 3 (FIG. 3) is connected via an input terminal 301 and a capacitor 303 to the input of amplifier 304. The output of amplifier 304 is then connected to the input of amplifier 306 via a capacitor 305. The output of amplifier 306 is connected to the input of amplifier 308 via a capacitor 307. The output of amplifier 308 is connected via a capacitor 309 to the output terminal 310 connected to the output terminal 2 (FIG. 3). The source voltage terminal 302 is connected to amplifier 304.

In operation, the input signal from terminal 301 is applied to the amplifier 304, with the source voltage supplied from terminal 302. The capacitor 303 and succeeding capacitors 305, 307 and 309, block undesired high frequency noise from the incoming signal. Thus, by controlling the source voltage simultaneously with the input signal level, the output characteristic appearing at terminal 310 is prevented from varying abruptly. Therefore, the output waveform of the power amplifier 4 is so controlled as to have gently sloped leading and trailing edges as shown in FIG. 4.

As hitherto described, the simultaneous control of the input signal level and the gain of a power amplifier 4 allows the power amplifier, even if it is of class C or the like having a non-linear output characteristic, to provide an output waveform having gently sloped leading and trailing edges. Thus, the present invention obviates the problem of spurious signals which ma otherwise be unavoidable and adversely affect other transmission channels of neighboring base stations in a TDMA system.