JP3260718B2 - Transmission device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transmitter used for microwave communication, and more particularly to a transmitter having a squelch circuit for transmitting a service channel signal even when there is no IF input signal.

[0002]

2. Description of the Related Art A transmitting device used for a relay station of microwave communication or the like transmits a high-speed data signal as a main signal and transmits a service channel different from a main signal used for maintenance or the like.

[0003] The transmission of this service channel is described in, for example, JP-A-62-154921 and JP-A-61-248625 so that voice signals can be transmitted and received between relay stations even when the main signal is interrupted. A squelch function is provided. Then, even if the main signal is interrupted, the service channel is superimposed on the squelch signal and can be transmitted.

FIG. 4 is a block diagram showing such a conventional transmission device with a squelch signal switching circuit. In the figure, an IF input signal is a signal in which transmission data is modulated into an intermediate frequency band by a modulator (not shown), and after input to a variable attenuator (variable ATT) 1, an IF amplifier circuit (IF AMP) 2) and the switching circuit 7 via the detector 3
To enter.

[0005] Here, the detector 3 detects the output level of the detector 3 with a diode or the like and outputs it as a detection voltage. This detection voltage is input to the AGC circuit 4 and negatively fed back to the variable ATT1, thereby forming a so-called AGC loop. The output level of the detector 3 is made constant by this AGC loop.

On the other hand, the detection voltage from the detector 3 is input to a judgment circuit 5, and the judgment circuit 5 compares the detection voltage with a predetermined reference voltage to judge the presence or absence of an IF signal.

When the detection voltage becomes lower than the reference voltage, the determination circuit 5 determines that there is no IF input signal. (Hereafter IF
It is assumed that the presence or absence of a signal is based on the determination result by the determination circuit 5. The switching circuit 7 selects the output signal (IF signal) side of the detector 3 when it is determined that there is an IF input according to the output of the determination circuit 5, and squelches when it is determined that there is no IF input. The squelch signal side from the oscillator (squelch OSC) 8 is selected.

The oscillation frequency of the squelch OSC 8 is
The same level as the center frequency of the IF input signal is selected, and the output level of the squelch OSC 8 is set to the same level as the output level of the detector 3.

The output of the switching circuit 7 is a mixer (MIX)
9 and is frequency-converted to the output of a local oscillator (LOCAL) 10 to output an RF signal of a predetermined frequency. This output signal is input to the variable ATT12.

Here, the service channel (SC) interrupt circuit 11 outputs to the LOCAL 10 a service channel digitally modulated by a service signal such as an audio signal. The LOCAL 10 frequency-multiplexes the service channel signal with the RF signal by interrupting the service channel signal with the output signal from the SC interrupt circuit 11 in response to the local oscillation signal.

Since the conventional transmitting apparatus has such a configuration, even if there is no IF input signal, the switching circuit 7
Is switched to the squelch OSC 11, and the signal of the service channel can be transmitted in a predetermined RF signal band.

Next, the output signal of the mixer 9 is a variable AT
The signal is output to the RF output through T12, RF AMP13, and detector 14.

The detection output of the detector 14 is ALC
The RF level is controlled by a so-called ALC loop, which is input to the circuit 15 and controls the attenuation of the variable ATT 12 so that the RF output level of the detector 14 is constant.

[0014]

In the conventional transmitting apparatus described above, when the IF input signal is cut off,
Since it is necessary to transmit the service channel, it does not have a function to mute the RF transmission signal. Therefore, I
When the switching circuit 7 switches from the squelch signal to the IF signal when the F input signal is input, there is a problem that overshoot occurs in the RF transmission signal.

The above problem will be described with reference to a time chart of an RF output signal at the time of squelch of the transmitting apparatus shown in FIG.

In FIG. 1, when the IF input signal changes from presence to absence at time t1, the switching circuit 7 switches from the IF input to the squelch input. At this time, the switching circuit 7
While switching from the IF input to the squelch input, there is no signal. As a result, the above-mentioned ALC enters a full gain state, and an overshoot occurs in the RF output.

On the other hand, when the IF input signal changes from absent to present at time t2, the switching circuit 7 switches from the squelch input to the I
Switch to F input. In other words, since there is no IF input signal, the AGC loop stands by in the full gain state, so that when the switching circuit 7 switches from squelch input to IF input, an overshoot of the IF signal level due to the AGC loop occurs. This overshoot cannot be absorbed in the level change because the time constant is short even in the next-stage ALC loop, and is output as RF output as it is.

As a countermeasure against these overshoots, it is conceivable to provide an integrating circuit 20 for delaying the judgment output of the judgment circuit 5 as shown in FIG. Here, the integration circuit 20 of FIG. 6 is configured using a resistor 23 and a capacitor 24.

Since the output of the determination circuit 5 is delayed by the integration circuit 20, the switching timing of the switching circuit 7 can be delayed. As a result, as is clear from the time chart shown in FIG. 7, when switching from the absence of the input signal to the presence of the input signal at t2, the overshoot can be reduced to almost negligible level.

However, when there is a change from the presence or absence of the input signal at the time t1, the provision of the integration circuit alone cannot suppress the overshoot of the output signal when switching to the squelch signal selection.

Such overshoot of the RF output signal generates a nonlinear distortion signal because the output level of the RF AMP 13 becomes a saturated output, and this nonlinear distortion signal becomes an interference wave signal with respect to an adjacent channel. In particular, in recent years, with the development of communication means, regulations and demands for out-of-band radiation are strict, and it is necessary to suppress them to a low level.

[0022]

As described above, according to the present invention, a transmitter which does not overshoot any change in input conditions can be realized by simply adding a simple circuit. The purpose is.

In order to solve the above problem, a transmitting apparatus according to the present invention is a transmitting apparatus for microwave communication having a squelch transmitting function and a service channel transmitting function, wherein an AGC loop for inputting an IF signal and controlling the signal to a predetermined output level;
A determination circuit for detecting an output of the AGC loop to determine the presence or absence of the input of the IF signal; and a delay circuit connected to an output of the determination circuit. When the input of the IF signal changes from non-existent to present, a time constant circuit that gives a time constant sufficiently longer than the time for the AGC loop to converge, and the output of the time constant circuit includes the input of the IF signal. A switching circuit that switches the output of the squelch oscillator if the AGC loop output is selected when it is determined that there is no input of the IF signal, and a local oscillation signal obtained by superimposing a service channel on the output of the switching circuit. And an ALC loop for controlling the output of the frequency conversion circuit to a predetermined output level. .

[0024]

Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing a transmitting apparatus according to the present invention.

In this figure, the detailed description of the portions denoted by the same reference numerals as in FIG. 4 is omitted.

The determination circuit 5 receives the detection voltage output of the detector 3, determines the presence or absence of an IF input signal, and outputs a determination output to the switching circuit 7. Then, the switching circuit 7 includes the determination circuit 5
If it is determined that there is an IF input signal, the switching circuit 7 selects the squelch input if it is determined that there is no IF input signal.

Here, the time constant circuit 6 provided between the determination circuit 5 and the switching circuit 7 responds quickly when there is a change from the presence or absence of the IF input signal, and changes from the absence of the IF input signal to the presence. In this case, the circuit is delayed by the time constant of the resistor 17 and the capacitor 18.

Therefore, when there is a change from the presence or absence of the IF input signal, the switching circuit 7 switches quickly from the IF input to the squelch input, so that the ALC circuit 15 has a full gain and the RF output signal does not overshoot.

Further, when the signal changes from the absence of the IF input signal to the presence of the IF input signal, the switching circuit 7 switches from the squelch input to the IF input after the AGC circuit 4 is sufficiently stabilized, so that there is no overshoot.

Therefore, in the transient state of the IF input, R
Overshoot of the F output signal can be prevented.

Here, the configuration of the time constant circuit 6 will be specifically described.

As shown in FIG. 1, the time constant circuit 6
NP transistor 16, resistor 17, capacitor 18
It is composed of And the PNP transistor 16
Is connected to the decision circuit 5, the emitter is connected to the positive power supply, and the collector is connected to the resistor 17, the capacitor 18 and the switching circuit 7. The other of the resistor 17 and the capacitor 18 is connected to GND.

However, the output signal of the decision circuit 5 is HIGH level when there is an IF signal, and LOW level when there is no IF signal, and the signal output to the switching circuit 7 is LOW level when there is an IF signal, and there is no IF signal. At the time of the HIGH level.

When there is a change from the presence or absence of the IF input signal, there is no time constant and there is no delay. On the other hand, when there is a change from the absence of the IF input signal to the presence, the signal is delayed by the time constant (T _D ) of the resistor 17 and the capacitor 18.

Therefore, the switching circuit 7 switches from the IF input signal to the squelch input immediately when the signal changes from the state with the IF input signal to the signal without the IF input signal. The input is switched from the squelch input to the IF input after a delay.

Here, the time constant (T _D ) of the time constant circuit 6
Is set to be sufficiently longer than the time until the AGC loop converges (AGC time constant: T _AGC ) as in the following equation (1).

T _AGC <T _D (1) FIG. 2 is a diagram for explaining a time chart when the switching circuit 7 operates in the configuration of FIG.

In the figure, time t1 represents the time when the IF input signal changes from presence to absence. At time t1, the detector 3 has no IF input and no detection voltage is output. As a result, the determination circuit 5 determines that there is no IF input signal, and the switching circuit 7 switches from the IF input to the squelch input.

At this time, although the time constant circuit 6 has no time constant, there is a switching time of the switching circuit 7, so that R
The F output signal is turned off.

After that, the mode is switched to the squelch input.
Since there is no input to the detector 14 for the time from the F input signal disconnection to the switching to the squelch input, there is a possibility that the ALC circuit responds and the output level overshoot by the ALC circuit 15 occurs.

[0042] Therefore, in the present invention, it is set the time constant (T _{AL C)} (referred to as T _S) switching time of the switching circuit 7 from large enough to have the ALC circuit as shown below (2) I have.

T _S <T _ALC (2) As a result, no overshoot occurs.

Time t2 represents the time when the IF input signal changes from absent to present. An input signal is input to the detector 3 and a detection voltage is output. As a result, the determination circuit 5
When it is determined that there is an IF input signal, the switching circuit 7 switches from the squelch input to the IF input.

In a state where there is no IF input signal, the AGC circuit 4 waits at a full gain. Therefore, when the squelch input is switched to the IF input, an overshoot of the output level by the AGC circuit 4 occurs. However, the signal of the determination circuit 5 arrives at the switching circuit 7 by the time constant of the time constant circuit 6.

Since the time constant of the time constant circuit 6 is set longer than the time during which the output level of the AGC circuit 4 is stabilized as shown in the equation (1), the switching is performed after the determination circuit 5 determines that there is a signal. AG until circuit 7 switches to IF input
Since the output level of the C circuit 4 is stabilized, overshoot of the output level by the AGC circuit 4 does not occur.

As the time constant circuit 6 of the present invention, for example, the configuration shown in FIG. 3 can be applied in addition to the configuration shown in FIG.

In FIG. 3, the anode of the diode 19 is connected to the cathode of the diode 20 and the determination circuit 5,
The anode of the diode 20 is connected to one of the resistors 21, and the other of the resistor 21 is connected to the cathode of the diode 19, the capacitor 22, and the switching circuit 7. The other end of the capacitor 22 is connected to GND. In the circuit of this figure, the occurrence of overshoot can be prevented as in the case of the time constant circuit 6 of FIG.

Further, the time constant circuit 6 converts the output of the determination circuit into a digital signal and operates at high speed without delay when the IF input signal changes from absent to present, instead of performing analog delay processing as described above. When the signal changes from the presence of the IF input signal to the absence of the IF input signal, it is needless to say that digital time processing may be performed so as to provide a predetermined time constant.

[0050]

As described above, according to the present invention,
A signal switching circuit with two different time constants is provided for the case where the IF input is turned off and the case where the IF input is turned off. This has the effect of preventing shoots and not interfering with radio waves of adjacent channels.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a transmission device according to the present invention.

FIG. 2 is a block diagram illustrating a configuration of a time constant circuit of FIG. 1;

FIG. 3 is a time chart illustrating an operation of the switching circuit 7 of FIG. 1;

FIG. 4 is a block diagram illustrating a configuration of a conventional transmission device.

FIG. 5 is a time chart illustrating an operation of the switching circuit 7 of FIG. 1;

FIG. 6 is a block diagram of the integration circuit 20 of FIG. 1;

7 is a time chart showing an operation of the switching circuit 7 using the integration circuit 20 of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Variable ATT 2 IF AMP 3 Detector 4 AGC circuit 5 Judgment circuit 6 Time constant circuit 7 Switching circuit 8 Squelch OSC 9 Mixer 10 LOCAL 11 SC interrupt circuit 12 Variable ATT 13 RF AMP 14 Detector 15 ALC circuit

Claims (4)

(57) [Claims] 1. A transmission apparatus for microwave communication having a squelch transmission function and a service channel transmission function, comprising: an AGC loop for inputting an IF signal and controlling the output signal to a predetermined output level; A judgment circuit for judging the presence / absence of a signal input; connected to an output of the judgment circuit; if the input of the IF signal changes from presence to absence, there is no delay; A time constant circuit that gives a time constant that is sufficiently longer than the time required for the AGC loop to converge when it changes; and selects the AGC loop output when the output of the time constant circuit determines that the IF signal has been input. And the IF
A switching circuit that switches the output of the squelch oscillator when it is determined that there is no signal input; and a frequency conversion circuit that frequency-converts the output of the switching circuit and a local oscillation signal on which a service channel is superimposed and converts the signal to an RF frequency. A transmitting apparatus comprising: an ALC loop for controlling an output of the frequency conversion circuit to a predetermined output level. 2. The transmission apparatus according to claim 1, wherein a time constant of the ALC loop is sufficiently longer than a switching time of the switching circuit. 3. A time constant circuit comprising: a PNP transistor connecting an output of the determination circuit to a base terminal;
2. The transmitting device according to claim 1, comprising a resistor and a capacitor connected in parallel to the collector terminal of the NP transistor and grounded. 4. The time constant circuit is connected to a series circuit of a first diode and a resistor connected to an output of the determination circuit and a parallel connection circuit of a second diode, and is connected to an output of the parallel connection circuit. 2. The transmitting device according to claim 1, comprising a grounded capacitor.