JPH077446A - Variable filter circuit - Google Patents

Abstract

PURPOSE:To provide a variable filter circuit small in circuitry, low in cost and capable of controlling pass band width in accordance with a receiving state. CONSTITUTION:In this variable filter circuit, the center frequecy f0 of a receiving signal is shifted upward only by f1 through the 1st shifting means to limit the upper limit pass band of the receiving signal by the 1st filter circuit 5, the center frequency of the hand-limited receiving signal is shifted upward only by f2 through the 2nd shifting means to limit the lower limit pass band of the receiving signal by a filter circuit 8 and to make frequencies f1 and f2 variable, thereby a receiving signal with required band width is obtained from the circuit 8. The circuit is provided with a multiplier 3 for multiplying the receiving signal by a signal of frequency f1 oscillated from a voltage control oscillator 4 as the 1st shifting means and a multiplier 6 for multiplying the receiving signal by a signal of frequency f2 oscillated from a voltage control oscillator 7 as the 2nd shifting means.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a satellite receiver, and more particularly to a bandpass filter (B) of a second intermediate frequency signal processing circuit.
PF) circuit. When the electric field strength of the received radio wave entering the receiver weakens and the influence of noise generated inside the receiver increases, a certain impulse level noise appears in the demodulated output of the receiver at a certain input electric field level. , Which impairs the reception quality and makes it impractical, but the level of this input electric field is called the threshold level. In recent years, in SHF broadcasting using satellites, the video signal is FM.
I am trying to modulate and transmit, but communication satellite (CS)
Since the transmission power is lower than that of a broadcasting satellite (BS), a large antenna is required.

Since the radio waves from satellites are attenuated by rainfall, even when the reception quality is good in fine weather, the radio waves are attenuated and fall below the threshold level in the rain, resulting in conspicuous noises on the receiving screen. There is. Therefore, in the FM receiver, if the threshold level can be improved, it is not necessary to use a large antenna in anticipation of radio wave attenuation when it rains, and reception can be performed with a small receiving antenna. Further, in order to properly receive the radio waves from the satellite, it is necessary to remove the interference caused by the microwave of the ground wave.

[0003]

2. Description of the Related Art FIG. 5 is a block diagram showing a conventional BPF circuit. As shown in FIG. 5, filter circuits 23 and 24 having a plurality of different pass bandwidths are used to generate a second intermediate frequency signal. A switch 22 for the converted satellite signal
, 25 are selected, the filter circuits 23 and 24 are selected according to the reception state, unnecessary signals other than the desired signal are removed, only the desired signal is extracted, and FM demodulation is performed to reproduce the satellite signal. Was there.

[0004]

However, if the band width of the output signal is narrowed by using the narrow band filter circuit, the threshold level is improved and the interference of the terrestrial microwave is also prevented. Although it can be removed, the reception quality is degraded. Therefore, if the filter circuit is switched according to the reception state, it is possible to receive favorably, but many filter circuits are required,
There is a problem that the circuit scale becomes large and the cost rises. It is an object of the present invention to provide a variable filter circuit that has a small circuit scale, low cost, and can control the pass bandwidth according to the reception state.

[0005]

A variable filter circuit according to the present invention comprises first shift means for shifting the center frequency of a received signal having a center frequency f0 upward by f1, and first shift means for the same.
A first filter circuit for limiting the upper limit pass band of the received signal shifted upward by the shift means, and a second shift means for upwardly shifting the center frequency of the received signal band-limited by the first filter circuit by f2. And a second filter circuit for limiting the lower limit pass band of the reception signal shifted upward by the second shift means, and by varying the frequencies of f1 and f2, A reception signal having a desired bandwidth is obtained, a SAW filter is used as the first filter circuit and the second filter circuit, and the reception signal and the first voltage control are used as the first shift means. A first multiplier for multiplying the signal of the frequency f1 oscillated by the oscillator is provided, and the received signal and the frequency oscillated by the second voltage controlled oscillator are provided as the second shift means. It constructed a second multiplier for multiplying the signal of several f2.

[0006]

According to the present invention, with the above-mentioned configuration, the band of the received signal is limited to extract the desired signal.
The pass bandwidth can be controlled by controlling the oscillation frequencies of the first and second voltage controlled oscillators.

[0007]

FIG. 1 is a block diagram showing an embodiment of a variable filter circuit of the present invention, and FIG. 2 is an explanatory diagram showing signal processing of each part of FIG. The embodiment of FIG. 1 will be described. At the input terminal 1, a satellite signal is converted into a second intermediate frequency signal, as shown in FIG.
The received signal of the center frequency f0 is input, and the input is input to the multiplier 3 so as to take out the signal through the wide band BPF2 over a band sufficiently wide with respect to the center frequency f0, as shown in FIG. . The voltage controlled oscillator 4 oscillates at the oscillating frequency f1 and inputs the oscillating signal to the multiplier 3, which multiplies the received signal by the multiplier 3 and, as shown in FIG. The SAW filter 5 by shifting only the center frequency f0 + f1
Are typing in.

The SAW filter 5 limits the upper pass band of the received signal and outputs it as a signal having a band as shown in FIG. The voltage controlled oscillator 7 oscillates at the oscillation frequency f2, inputs the oscillation signal to the multiplier 6, multiplies the received signal by the multiplier 6, and shifts the central frequency of the received signal upward by f2 to generate the central frequency f0 +.
It is set to f1 + f2 and is input to the SAW filter 8. S
The AW filter 8 limits the lower limit pass band of the received signal and outputs it as a signal having a band as shown in FIG.
Therefore, by changing the oscillation frequencies f1 and f2 of the voltage controlled oscillators 4 and 7, the received signal in the desired band is S
It can be output from the AW filter 8.

The output may be directly input to the FM demodulation circuit to reproduce the satellite signal, or, as shown in FIG. 1, the received signal from the SAW filter 8 is further input to the multiplier 9, Oscillation frequency f1 by voltage controlled oscillator 10
+ F2 oscillates and the oscillation signal is input to the multiplier 9, and the multiplier 9 multiplies the received signal for frequency conversion to pass through the wide band BPF 11, and as shown in FIG. It is also possible to output the received signal as a desired bandwidth through the output terminal 12 and input the output to the FM demodulation circuit to reproduce the satellite signal. Therefore, in order to improve the threshold level and remove the interference of the terrestrial microwave, the oscillation frequencies f1 and f2 of the voltage controlled oscillators 4 and 7 are changed to narrow the variable filter circuit, and When there is little noise and interference, if the variable filter circuit has a wide band, it is possible to receive satellite signals with good sensitivity.

FIG. 3 is an explanatory view showing the signal processing of each part showing another embodiment of the variable filter circuit of the present invention. In the above embodiment, the case where the center frequency of the received signal is shifted upward by f1 and f2, respectively, is described. However, as shown in FIG. 3, the multiplier 3 shifts the center frequency of the received signal downward by f1. 3, the multiplier 6 shifts the center frequency of the received signal downward by f2, and the SAW filter 5 shifts the upper limit pass frequency downward by 2 × f1.
Alternatively, the SAW filter 8 may be a SAW filter 8 ′ whose lower limit pass frequency is shifted downward by 2 × f 2, and the signal may be processed and output in the same manner as in the above embodiment.

FIG. 4 is a block diagram showing an example of control of the voltage controlled oscillator of FIG. For the control of the voltage controlled oscillators 4 and 7, the signal output from the variable filter circuit 15 of the present invention is FM demodulated by the FM demodulator 16, and the control signal is input to the variable filter circuit 15 according to the FM demodulated signal level. Then, the voltage-controlled oscillators 4 and 7 are controlled, the center frequency of the variable filter circuit is changed corresponding to the instantaneous frequency of the FM signal, and the received signal is output and input to the FM demodulator 16. Or using a microcomputer etc.,
The CN ratio of the received signal may be detected and input to the microcomputer, and the microcomputer may output a control signal according to the input so that the voltage controlled oscillators 4 and 7 are controlled by the control signal. An operation signal may be input to the microcomputer so that the voltage controlled oscillators 4 and 7 can be controlled based on the operation signal.

[0012]

As described above, according to the present invention,
By controlling the oscillation frequency of the voltage controlled oscillator, it becomes possible to change the bandwidth of the received signal to be output.By changing the bandwidth according to the receiving condition, the threshold level can be improved and the terrestrial wave It is possible to provide a variable filter circuit capable of removing the microwave interference of, and having a small circuit scale and a low cost.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a variable filter circuit of the present invention.

FIG. 2 is an explanatory diagram showing signal processing of each unit in FIG.

FIG. 3 is an explanatory diagram showing signal processing of each unit, showing another embodiment of the variable filter circuit of the present invention.

FIG. 4 is a block diagram showing an example of control of the voltage controlled oscillator of FIG.

FIG. 5 is a block diagram showing a conventional BPF circuit.

[Explanation of symbols]

 1 Input Terminal 2 Wideband BPF 3 Multiplier 4 Voltage Controlled Oscillator 5 SAW Filter 6 Multiplier 7 Voltage Controlled Oscillator 8 SAW Filter 9 Multiplier 10 Voltage Controlled Oscillator 11 Wideband BPF 12 Output Terminal 15 Variable Filter Circuit 16 FM Demodulator 17 Output Terminal 21 Input Terminal 22 Switching Device 23 Filter 24 Filter 25 Switching Device 26 Output Terminal

Claims (4)

[Claims] 1. A first shift means for shifting the center frequency of a received signal having a center frequency f0 upward by f1 and a first shift means for limiting the upper limit pass band of the received signal shifted upward by the first shift means. A filter circuit, a second shift means for shifting the center frequency of the received signal band-limited by the first filter circuit upward by f2, and a lower limit pass band of the received signal shifted upward by the second shift means. And a second filter circuit for limiting
A variable filter circuit, wherein a reception signal having a desired bandwidth is obtained from the second filter circuit by making the frequency of the variable. 2. A first shift means for shifting a center frequency of a received signal having a center frequency f0 downward by f1 and a first shift means for limiting an upper limit pass band of the received signal shifted downward by the first shift means. A filter circuit, a second shift unit that shifts the center frequency of the reception signal band-limited by the first filter circuit downward by f2, and a lower limit pass band of the reception signal that is shifted downward by the second shift unit. And a second filter circuit for limiting
A variable filter circuit, wherein a reception signal having a desired bandwidth is obtained from the second filter circuit by making the frequency of the variable. 3. The variable filter circuit according to claim 1, wherein the first filter circuit and the second filter circuit are SAW filters. 4. The first shift means comprises a first multiplier for multiplying a received signal by a signal of a frequency f1 oscillated by a first voltage controlled oscillator, and the second shift means comprises a received signal and a second signal. A second multiplier for multiplying the signal of the frequency f2 oscillated by the voltage controlled oscillator.
The variable filter circuit described.