JP2514571Y2 - Transmitter - Google Patents

Description

The present invention relates to a transmitter, and more particularly, to a transmitter having a large AGC range due to a large IF cable loss, such as an IF amplifier of a broadcast field up device.

[Conventional technology]

Conventionally, as a transmitter of this type, as shown in the block diagram of FIG. 2, the transmitter is composed of an IF signal input terminal 4, a signal input terminal 5, an IF modulator 6 and an IF switch 7. Section, an IF amplifier 8, an AGC circuit 8A, a local oscillator 9, a mixer 10, an IF filter 11, a power amplifier 12, and an output terminal 13. Are connected by an IF cable (not shown) with a considerably large cable loss. The baseband signal input from the signal input terminal 5 is transferred to the intermediate frequency modulator 6
The signal is modulated into an IF signal by (hereinafter referred to as an IF modulator) and is input from the modulated IF signal or IF signal input terminal 4.
The IF signal and the IF signal are selected by the IF switch 7. Here, when a baseband signal is not input from the signal input terminal 5, that is, when it is not operated as a field pick-up device for broadcasting, a receiver (not shown) on the other end of the transmission receives an unmodulated transmission signal (hereinafter referred to as a squelch signal). Sent)
It prevents the noise of the local IF amplifier, which will be described later, from being demodulated by the receiver on the other side to become baseband noise or interfere with adjacent channels. Usually, this squelch signal is set to a level almost equal to the reception electric field of the receiver of the other party, or at a minimum level, a level higher than the threshold level of the receiver of the other party by several dB. On the other hand, this IF output is connected to the high frequency section with a long IF cable. The output of the IF signal selected on the high frequency side is amplified to a constant output level by the IF amplifier 8 and the AGC circuit 8A. Furthermore, the signal of the local oscillator 9 and the output signal of the IF amplifier 8 are mixed.
Only the desired signal was mixed by 10, and only the desired signal was passed by the filter 11, amplified by the power amplifier 12, and the transmission power was output from the output terminal 13.

[Problems to be solved by the device]

The conventional transmission device described above is separated into two housings, a high frequency part and a control part, like a field pickup device for broadcasting business. When IF is transmitted by a cable between the high frequency part control part, the loss of IF signal occurs. IF to be about 60 dB
The level AGC range is required to be around 60 dB.

Now, if there is no signal when the input signal of the IF modulator is switched to the IF input terminal 4 by the IF switch 7,
Since the C amplifier has full gain, noise and I
The noise input from the F signal input terminal 4 will be amplified with full gain. Since the filter 11 cuts the image signal, it has a wide band, and this noise component is not attenuated but is amplified by the power amplifier 12 and then transmitted from the output terminal 13. Now, when the adjacent channel is about 18 MHz, there is a drawback that interference noise is generated in the adjacent channel.

On the other hand, when there is an adjacent transmitter / receiver that transmits from the same point and receives at the same point of its own station, there is a drawback that the leakage of the transmitted wave on the receiver side causes image quality deterioration in broadcast image transmission.

[Means for solving the problem]

The transmitter of the present invention comprises an IF modulator that outputs a first IF signal that is modulated by a baseband signal that is input from a first input terminal, and an unmodulated second IF signal that is input from a second input terminal. Of the IF signal and the first IF signal, and an IF switch that selects and outputs one of them, and defines the first or second IF signal input from the control unit. An amplifier having a variable gain function for amplifying to a specified IF output level, an AGC circuit for sending a control signal to the amplifier so that the output level of the amplifier is constant, and I of the amplifier.
In a transmitter including a high-frequency unit having a local oscillator that mixes an F output signal and a local oscillation signal and converts the signal into a high-frequency signal, and an IF cable that connects the control unit and the high-frequency unit, the AGC circuit In the case where there is no IF signal that does not detect any of the first and second IF signals, the oscillation frequency control circuit added to the local oscillator changes the local oscillation signal to the same frequency signal as the transmission frequency. Convert.

〔Example〕

 Next, the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. The embodiment shown in FIG. 1 is the same as the configuration of the conventional example except that the configurations of the oscillation frequency control circuit 1 and the AGC circuit 2 are different from those of the conventional example.

Next, the operation of this embodiment will be described. Now, the signal input from the baseband input terminal 5 is FM-modulated by the IF modulator 6 and is input to the IF amplifier 8 through the IF switch 7 for operation. In the middle of this operation, IF switch 7 to the IF input terminal 4 side
Switch and the IF input signal is suddenly cut off.

In this case, since the IF amplifier 8 becomes full gain under the control of the AGC circuit 2, a control signal when the AGC level is in abnormal operation is sent to the oscillation frequency control circuit 1. The oscillating frequency control circuit 1 controls the oscillating frequency of the variable local oscillator 3 so that the oscillating frequency of the variable local oscillator 3 becomes the desired frequency output from the output terminal 13, that is, the transmission frequency of approximately the center frequency of the filter 11. The output of the unmodulated variable local oscillator 3 is output from the output terminal 13. The noise generated by the IF amplifier 8 having the full gain is mixed with the changed oscillation frequency of the variable local oscillator 3 and is out of the band so that it is removed by the filter 11 and is not output from the output terminal 13. In addition, IF amplifier 8
Is an IF amplifier 8 that amplifies only the IF band, but
The mixer 10 may be connected to the output of the IF amplifier by using an IF band filter.

[Effect of device]

As described above, in the present invention, the IF input becomes no signal.
Even if the IF amplifier outputs the noise component due to the AGC amplifier becoming full gain, the frequency of the local oscillator is changed and the output frequency, that is, the transmission frequency is automatically changed, so that the noise signal is output as the transmission power. Since it does not occur, there is an effect that the receiving side detects noise and the receiving side does not affect the received electric field.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a block diagram of a conventional transmitter. 1 ... Oscillation frequency control circuit, 2 ... AGC circuit, 3 ... Variable local oscillator, 4 ... IF signal input terminal, 5 ... Signal input terminal, 6 ... IF modulator, 7 ... IF switch, 8 ... IF amplifier, 10 ... Mixer, 11 ... Filter, 12 ... Power amplifier, 13 ... Output terminal.

Claims (1)

(57) [Scope of utility model registration request] 1. An IF modulator for outputting a first IF signal modulated by a baseband signal input from a first input terminal, and an unmodulated second IF input from a second input terminal.
A control unit having an IF switch that inputs a signal and the first IF signal and selects and outputs one of the signals; and a first or second IF signal input from the control unit. An amplifier having a variable gain function that amplifies to an IF output level, an AGC circuit that sends a control signal to the amplifier so that the output level of the amplifier is constant, an IF output signal of the amplifier, and a local oscillation signal are mixed. In a transmitter including a high-frequency unit having a local oscillator for converting to a high-frequency signal, and an IF cable connecting the control unit and the high-frequency unit, the control signal of the AGC circuit is the first and second IFs.
A transmitter, wherein the local oscillation signal is converted into the same frequency signal as the transmission frequency by an oscillation frequency control circuit added to the local oscillator when there is no IF signal that does not detect any of the signals.