JP3489993B2 - Digital microwave transmitter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital microwave transmitter, and more particularly to a digital microwave transmitter for transmitting an IF signal from a transmitter for microwave radio transmission. The present invention relates to a high-frequency unit that outputs a microwave signal. 2. Description of the Related Art In a digital microwave transmitter, a transmission output usually needs to be set within ± 50% of a specified value. Further, when there are two or more specified channels, it is necessary to output an output signal after switching the channels and after the frequency is sufficiently stabilized so as not to interfere with other channels when the channels are switched. Also at this time, the transmission output must not greatly exceed the specified value. FIG. 3 shows the prior art. In FIG. 3, a first intermediate frequency (IF) signal, usually 130 MHz, is input to an IF input terminal 1, the level of which is adjusted by an AGC gain variable element 2, and then the oscillation frequency of a first local oscillator 4 in a frequency mixer 3. It is mixed with 1,370 MHz, and the 1,500 MHz component of the sum is output from the filter 22. This output is further passed through a frequency mixer 5 to a second
It is mixed with the oscillation frequency from the local oscillator 6, and the sum component is taken out from the filter 23. This output is amplified by a power amplifier 7 and output from an output terminal 9 via a directional coupler 8. The microwave branched by the directional coupler 8 is converted into a DC voltage by the detector 10, and the gain of the AGC gain variable element 2 is adjusted by the AGC gain control circuit 17 so that the output level of the output terminal 9 becomes a specified level. Adjust. In a QAM system or the like for digital microwave radio transmission, the circuit needs to operate with good linearity, and a back-off of 10 dB is usually taken from the saturation power. When the channel frequency is changed, that is, when the operator operates the channel changeover switch, the control circuit 30 of the microcomputer or the hard logic first sends a changeover signal to the output power control signal input terminal 11, so that the power supply changeover circuit In 12, the voltage of the power amplifier 7 is turned off, and the output from the output terminal 9 disappears. Next, frequency-divided data of the PLL circuit 14 driven in phase synchronization with the reference crystal oscillator 15 is input to the input terminal 13 by a signal from the control circuit 30.
To change the channel frequency. When the frequency of the second local oscillator 6 is set to a new channel frequency, a PLL lock detect signal 16 is input from the PLL circuit 14 to the control circuit 30. As a result, the control circuit 30 sends a signal to the power supply switching circuit 12, the power amplifier 7 is controlled, and an output signal of the changed channel frequency is obtained at the output terminal 9. In the above-mentioned prior art, the output of the power amplifier 7 is eliminated before the channel switching, so that the AGC control circuit 17 adjusts the gain of the AGC variable gain element 2 to the maximum. Will do. Furthermore, since a modulation unit that inputs a signal from an encoder and modulates the IF signal and a high-frequency unit that receives an IF signal and outputs a microwave signal according to the present invention are connected by an IF cable of 0 to 200 m, The AGC needs to perform gain control of 30 dB or more in order to compensate for the amount of gain variation due to the temperature of the circuit including the power amplifier and the amount of attenuation by the cable from 0 to 20 dB. For this reason, immediately after the voltage is applied to the power amplifier after the channel switching, the saturation power exceeding the specified output is output, and the saturation causes the unnecessary wave component to interfere with the adjacent channel. SUMMARY OF THE INVENTION In order to eliminate this drawback, the present invention switches the AGC voltage in conjunction with the channel switching, and shuts down the high-frequency power amplifier during a specified level or below a specified level. Is applied to the AGC. FIG. 1 shows an embodiment of the present invention. 3 is different from FIG. 3 in that a detector output voltage is connected to an AGC gain control circuit 17 via an AGC voltage switching circuit 19, and the other terminal of the AGC voltage switching circuit 19 is connected to a resistor 2
0, 21. This voltage is set to a voltage value that sets the gain of the AGC gain variable element 2 to a standard gain or less. The switching control of the AGC voltage switching circuit 19 is controlled by the control circuit to control the high-frequency power amplifier. During the disconnection, the voltage is determined by the resistors 20 and 21. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The control sequence will be described with reference to FIG. When the control circuit 30 controls the channel switching, first, the output power control signal (connected to the terminal 11) is set to Hi.
After switching from “L” to “Lo”, the channel switching data (terminal 1
3) is output. This data rewriting is performed by three signals of a clock, data, and a strobe signal. When rewriting of channel switching data is completed, PLL
The lock detect signal changes from Hi to Lo.
When the control is completed and the frequency becomes a specified frequency, the frequency changes from Lo to Hi. AGC switching fall timing is AGC and PL
Although determined by the time constant of L, the start-up is performed after the output power control signal becomes Hi. The AGC voltages when AGC switching was performed and when AGC switching was not performed were compared. If not performed, the change in the AGC voltage is large, and a large output is transmitted immediately after the output power control is switched. It is considered that if the AGC voltage is completely reduced and the IF level is minimized, the power amplifier does not need to be switched. However, even if the IF level is not used, the first local oscillator 1,370 MHz 5 Double harmonic 6,850 MHz
Is output as a spurious signal, and the switching circuit is required. According to the present invention, a stable transmission output can be obtained even at the time of channel switching. Depending on the time constant of the AGC and the PLL circuit and the switching timing, the transmission output of 5 dB increases. In this case, the intermodulation distortion between the digital signal modulated waves rises by 10 dB and interferes with the adjacent channel.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing the overall configuration of the present invention. FIG. 2 is a timing chart of each control signal voltage. FIG. 3 is a block diagram showing the entire configuration of a conventional technique. [Description of References] 1: IF input terminal, 2: AGC gain variable element, 3, 5:
Frequency mixer, 4: first local oscillator, 6: second local voltage controlled oscillator, 7: microwave power amplifier, 8: directional coupler, 9: microwave output terminal, 10: detector, 11:
Output power control signal input terminal, 12: power supply switching circuit of microwave power amplifier, 13: channel frequency dividing data input terminal, 14: frequency stabilization PLL circuit of second local oscillator, 15: reference crystal oscillator, 16: PLL lock Detect signal, 17: AGC gain control circuit, 18: AGC switching signal input terminal, 19: AGC voltage switching circuit, 20, 2
1: resistor, 30: control circuit.

Claims (1)

(57) [Claims 1] A transmission device which receives an IF signal, converts the frequency to a specified microwave frequency, and outputs a specified power after power amplification, means for detecting a transmission output level. It has a means for automatic gain control to control the gain based on the detection result, and if the specified microwave frequency is not reached, in a device having a function to cut off the transmission output, if the transmission output is cut off, A device comprising means for interlockingly switching the gain of the control signal of automatic gain to a voltage close to obtaining a normal level or a voltage obtaining a level lower than the normal level.