JPH0117853Y2 - - Google Patents

Description

[Detailed description of the invention] This invention consists of a circuit configuration for automatically maintaining the transmission output and modulation degree at appropriate values in an AM (amplitude modulation) transmitter.

Traditionally, SSB transmitters use ALC (Automatic Output Control), which detects the maximum amplitude of the output signal and controls the amplification level in the front stage of the transmitter, in order to prevent the quality of radio waves from deteriorating due to overloading of the power amplifier. Action is taking place. On the other hand, since AM transmitters have a carrier wave, the output is monitored and automatically controlled by specifying the amplitude of the carrier wave. As a result, the output level of the carrier wave can be maintained at an appropriate value relatively easily, but separate monitoring and adjustment is required to determine whether the degree of modulation is appropriate.

In the AM ALC circuit of the present invention, as illustrated in FIG.
(Unnecessary in the case of post-modulation) In the AM transmitter that sends high frequency power to the antenna 3,
A part of the transmitted output signal is added to the average value detection circuit 10 , and the detected output generated at a predetermined amplitude or more or the output obtained by DC amplification of this is added to the carrier wave control stage 4 to control the carrier wave intensity input to the modulator 1. do.
In addition, a part of the transmitted output signal is added to the peak value detection circuit 20 , and the detected output generated at a predetermined amplitude or more or the output obtained by DC amplification of this is added to the low frequency signal control stage 5 and input to the modulator 1. It controls the modulated low frequency signal strength.

The average value detection circuit 10 connects in parallel with the load resistor 12 of the detection diode 11 a capacitor 13 of relatively small capacity that exhibits low impedance at the carrier frequency and high impedance at the modulation frequency. If only the DC component is taken out through an integrating circuit having a sufficient time constant for , the output voltage is equal to the peak value of the carrier wave, and since the modulation component is 0, it is not output regardless of the degree of modulation. If this output is applied as is to the control circuit, it will only result in a linear feedback and reduce the overall modulation degree, so the operation starting point must be set so that the operation will occur at a predetermined carrier wave intensity or higher. In the figure, adjustment can be made by a potentiometer 16 that applies a reverse bias voltage to the return path on the input side of the detection diode.

The peak value detection circuit 20 is a relatively large capacitor that is connected in parallel with the load resistance 22 of the detection diode 21 and exhibits a sufficiently lower impedance than the load resistance 22 at the modulation frequency. Output. Potentiometer 2 provides a reverse bias voltage on the input return path of the detection diode.
4 to set the operation start point. AM wave is the second
As shown in the figure, when the modulation degree is 0, the peak value is equal to the carrier wave amplitude, and at 100% modulation, the modulation value is twice the carrier wave.As the modulation depth becomes deeper, the peak value also increases. Overmodulation can be prevented by controlling the peak value so that it does not exceed the amplitude equivalent to 100% modulation.

As detailed above, in the present invention, since the overload prevention control is performed by actually measuring the carrier wave amplitude in the transmission output of the AM transmitter and the increase in amplitude due to modulation, extremely reliable effects can be expected. Furthermore, without using special measuring instruments for measurement,
Since the control signal is obtained from the average value detection output and the peak value detection output, the AM ALC circuit of the present invention has a reliable implementation effect and requires little additional cost, so it has extremely great practical value.

[Brief explanation of drawings]

FIG. 1 shows an example of a circuit according to the present invention including a part of the block circuit, and FIG. 2 shows a transmission output waveform diagram. DESCRIPTION OF SYMBOLS 1... Modulator, 2... Power amplifier, 4... Carrier wave control stage, 5... Low frequency signal control stage, 10 ... Average value detection circuit, 20 ... Peak value detection circuit.

Claims (1)

[Scope of utility model registration request] Amplitude modulation of carrier high frequency signal with low frequency signal
In an AM transmitter, the amplitude of the carrier high-frequency signal input to the modulator is limited within the range where the average value detection output of the output signal exceeds a predetermined level, and the modulator is applied within the range where the peak value detection output of the output signal exceeds the required level. The system was configured to limit the amplitude of low-frequency signals input to the
ALC circuit for AM.