JP3360453B2 - Band switching circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a band switching circuit,
In particular, AG in CATV up-down converter
The present invention relates to a band switching circuit having a C function.

[0002]

2. Description of the Related Art Conventionally, this type of cable television (CAT)
V) The reception signal circuit configuration of the reception block in the broadcast system has the configuration shown in FIG. Figure 3 shows CATV
It is a receiving block diagram of a broadcasting system. In the receiving block A, a required frequency band (fa = 50 to 550 M in this case) supplied via a wired cable is used.
(Frequency band of Hz) is input to the input terminal IN. The reception block A includes a low-pass filter 1 that passes a low-frequency component from the signal fa input to the input terminal IN.
When the output is input and the radio wave is strong, lower the gain,
In the case of weak radio waves, an AGC circuit 2 that raises the gain and keeps the output constant, that is, automatically controls the amplification of the receiver and keeps the output constant regardless of the strength of the received radio waves.
And the AGC output is set to a high band and a low band.
w) 50 to 5 to perform frequency conversion on the band
An up-down converter 3 composed of a simple tuning low-pass filter for an input range of 50 MHz;
An amplifier 4 for amplifying the output, and a first local oscillator 5
And the first local frequency fb supplied from the first local oscillation circuit 5 via the buffer amplifier 6 to the first local frequency fb and the received signal fa to obtain a first intermediate frequency fc of a difference between the two frequencies as a beat. And a mixing circuit (mixer circuit) 7.

Further, an IF (intermediate frequency) band-pass filter 8 for passing a predetermined band of the output, an IF amplifier circuit 9, and a second local oscillator circuit 10 provide a buffer amplifier 1
The second mixing circuit 12 mixes the second local frequency fd and the first intermediate frequency fc supplied via the first unit 1 to obtain a second intermediate frequency fe having a difference between the two frequencies as a beat.
, The IF bandpass filter 13 and the output thereof from the output terminal OUT. An IF amplifier circuit (not shown) is provided downstream of the output terminal OUT,
A detection demodulation circuit for extracting an audio signal from the intermediate frequency fe, an audio low-pass filter, a secret circuit, an audio amplification circuit, a decompression circuit for restoring a waveform whose amplitude has been compressed during transmission, and an output amplification circuit are sequentially connected. It has a configuration. An audio signal is extracted from a carrier centered on fa received through a wired cable via these circuits. Note that Ca is a control input of the AGC circuit 2 and Vt is a power supply to the up-down converter 3.

In such a first mixing circuit 7, when a high-frequency signal (RF signal) in which two frequencies are superimposed and mixed is input, that is, a reception signal fa transmitted through an input terminal IN. And the first local frequency fb supplied via the first local oscillation circuit 25 are mixed and input, and are tuned by a tuning transformer, that is, a tuning circuit (parallel resonance circuit) including an IF transformer and a capacitor. First intermediate frequency fc of the difference between frequencies fa and fb
Is operated to obtain the following IF bandpass filter 8
Sent to The selectivity Q of the tuning circuit determines the quality of the characteristic. When the selectivity Q increases, only the signal component is amplified and the other components are attenuated. Therefore, it is usually necessary to design the Q value to be high. However, at this time, the tuning circuit is set so that the selectivity Q can be matched with the circuit including the IF band pass filter 8 at the subsequent stage. Thus, the input range is 50MHz to 550M
In the case of the input signal fa of 1 Hz, the receiving block A may be one band, and such a configuration is employed.

[Problems to be solved by the invention]

However, the circuit configuration of the conventional receiving block A has the following problems. That is, when the input range becomes about 50 to 860 MHz, the input range is separated into two reception blocks of a high band (High) and a low band (Low),
There is a need to switch and receive each. In such a case, if the conventional receiving block A is provided with two blocks in the same configuration, there is a drawback that the circuit configuration is double, which is wasteful and expensive, and that the size and weight cannot be reduced. SUMMARY OF THE INVENTION It is an object of the present invention to eliminate the above-mentioned conventional problems and to provide a high (High) band and a low (L) input range.
ow) To provide a band switching circuit having a simple and inexpensive circuit configuration and excellent switching characteristics for switching between two reception blocks of a band.

[0006]

SUMMARY OF THE INVENTION In order to achieve this object, the present invention has been developed in the sense that it has been provided by way of a cable.
In an up-down converter, a signal in a required frequency band is input to an input terminal, divided into reception blocks corresponding to two input ranges of a high band and a low band, and extracted as corresponding frequency components.
Switch so that the output is kept constant.
A first switch circuit, a high-band circuit, and a second switch that pass the predetermined high-band frequency band when the predetermined high-band frequency band is input from the input terminal. A high-band range loop including a switch circuit, and a third switch circuit, a low-band circuit, and a fourth switch circuit that pass the predetermined low-band frequency band when the predetermined low-band frequency band is input from the input terminal.
The high band comprising a low band range loop comprising a changeover switch circuit, a first switch circuit connected to a band switch control input, and a third switch circuit connected to the band switch control input via an inverter. A band switching function for selecting either a range loop or the low band range loop, a second switch circuit to which the first switch circuit is connected and an AGC control input is connected, and the third switch circuit A fourth switch circuit connected to the AGC control input and connected to automatically control the amplification of the input frequency band to keep the output constant regardless of the strength of the received signal.
It is characterized in that it comprises a control circuit having a GC circuit function.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to FIGS. FIG. 1 is a receiving block diagram of a CATV broadcasting system showing one embodiment of the present invention. In FIG. 1, FIG.
The same reference numerals indicate equivalents. B is the receiving block of the present invention, 14 is a band switching circuit, 14a is a switching circuit SW1, 14b is a high band circuit, 14c is a switching circuit SW2, 14d is a switching circuit SW3, 14
e is a low band circuit, 14f is a changeover switch circuit SW
Reference numerals 4 and 14g denote control circuits, and Bc denotes a control input for band switching. Note that the changeover switch circuit 14a,
14c, 14d, and 14f each have a high-resistance semiconductor (I layer)
One of them uses a San-Germanic structure PIN diode having a low-resistance P layer on one side and a low-resistance N layer on the other side, and controls a signal by utilizing a wide range of impedance between a reverse bias and a forward bias.

In the receiving block B, a signal f of a required frequency band (in this case, a frequency band of fa = 50 to 860 MHz) supplied via a wired cable is used.
a is input to the input terminal IN. The receiving block B receives the signal fa input to the input terminal IN, reaches the changeover switch circuit 14c from the high-band circuit 14b via the changeover switch circuit 14a, for example, fa = 551 to 860MHZ.
a high-band range loop that outputs a frequency band of z, and a signal fa input to the input terminal IN, from the low-band circuit 14e to the switch circuit 14f via the switch circuit 14d, for example, fa = 50 to 550 MHz.
And a low-band range loop for outputting the frequency band of each of the switching circuits 14c and 14f. Changeover switch circuit 14
a, 14c, 14d, and 14f are all controlled by the control circuit 14g. High band circuit 14b
Uses a tuned high-pass filter HPF and a tuned band-pass filter BPF. The low-band circuit 14e uses a tunable low-pass filter LPF and a tunable band-pass filter BPF. In the control circuit 14g, a circuit is formed using a small number of transistors, and the changeover switch circuits 14a, 14c, 14d, 14f
AGC function is provided by linearly controlling the amount of attenuation.

The signal f input to the input terminal IN
a is input to the band switching circuit 14 via the low-pass filter 1 that passes a frequency component corresponding to the input range,
In the case of the high band frequency band of fa = 551 to 860 MHz, the changeover switch circuit 14a and the high band circuit 14
b, switched to the high band range loop composed of the changeover switch circuit 14c and output, fa = 50 to 550M
In the case of the low band frequency band of Hz, the changeover switch circuit 14d, the high band circuit 14e, the changeover switch circuit 1
The output is switched to the low band range loop composed of 4f. This band switching control and the function of lowering the gain for strong radio waves and increasing the gain for weak radio waves to keep the output constant, automatically controlling the so-called receiver amplification to reduce the strength of the received radio waves The AGC circuit function for keeping the output constant regardless of the operation is performed by the control circuit 14g. That is, when there is a band switching control input Bc, a predetermined switching circuit 14a, 14c, 14d, 14f is operated to select either a high band range loop or a low band range loop, and to a subsequent circuit via that loop. Sent out. At the same time, since there is an AGC circuit function control input Ca, the AGC function operates.

The output signal fa is amplified by the amplifier 4,
By mixing the first local oscillation circuit 5 with the first local frequency fb supplied from the first local oscillation circuit 5 via the buffer amplifier 6 and the reception signal fa, a first intermediate difference between the two frequencies as a beat is obtained. The signal is sent to a first mixing circuit (mixer circuit) 7 for obtaining the frequency fc. Further, an IF (intermediate frequency) band-pass filter 8 that passes a predetermined band of the output, an IF amplifier circuit 9, a second local frequency fd supplied from a second local oscillator circuit 10 via a buffer amplifier 11, A second mixing circuit 12 that obtains a second intermediate frequency fe having a difference between the two frequencies as a beat by mixing the first intermediate frequency fc, and an IF bandpass filter 13;
The output is taken out from the output terminal OUT. At the subsequent stage of the output terminal OUT, an IF amplifier circuit (not shown), a detection demodulation circuit for extracting an audio signal from the second intermediate frequency fe, an audio low-pass filter, a confidential speech circuit, an audio amplifier circuit, and an amplitude compressed at transmission In this configuration, an expansion circuit for restoring the waveform and an output amplification circuit are sequentially connected. An audio signal is extracted from a carrier centered on fa received through a wired cable via these circuits.

Next, an embodiment of a control circuit 14g which is a band switching circuit of the present invention will be described with reference to FIG. FIG. 2 is a circuit configuration diagram of the control circuit 14g. In FIG. 2, the same reference numerals as those in FIGS. 1 and 3 denote equivalents. Q1 to Q4 are NPN transistors, R1 to R4 are resistors, and INV is an inverter. The band switching control input Bc is connected to the base of the transistor Q1 via the resistor R1, and the collector is connected to the power supply + V. AGC control input Ca is connected to the base of transistor Q2 via resistor R2, and its collector is connected to power supply +
Connected to V. The emitter of the transistor Q1 is connected to the collector of the transistor Q2.
The output to (SW2) is taken out. Further, the switching switch circuit 14a (SW) is output from the emitter of the transistor Q2.
The output to 1) is retrieved. Further, the base of the transistor Q3 is connected to the band switching control input Bc via a series circuit of the inverter INV and the resistor R3, and the collector thereof is connected to the power supply + V. AGC control input C
a is also connected to the base of the transistor Q4 via the resistor R4, and its collector is connected to the power supply + V.
And the emitter of the transistor Q3 and the transistor Q4
, And an output from the connection terminal to the changeover switch circuit 14f (SW4) of the control circuit 14 is taken out. Further, an output to the changeover switch circuit 14d (SW3) is taken out from the emitter of the transistor Q4.

With such a circuit configuration, 5
When there is a high-band signal input of 50 MHz or more, a high potential is applied to the band switching control input Bc, the transistor Q1 is turned on, and the switching circuit 14c (SW
2) is turned on. At this time, the AGC control input C
a, an AGC control signal is input to the transistor Q2
By controlling the PIN diode in the switch circuit 14a (SW1), the switch circuit 14a
(SW1) has an AGC function. Therefore, a stable output by the AGC function is sent to the amplifier 4 through a high band range loop that outputs a frequency band from the switch circuit 14a and the high band circuit 14b to the switch circuit 14c. On the other hand, transistor Q3
Is turned off because the low potential, which is an inverted potential, is applied to the base by the inverter INV, so that the transistor Q4 is also turned off, and the switch circuit 14f (SW4)
And 14d (SW3) are turned off. Therefore, the low band range loop that outputs the frequency band from the changeover switch circuit 14d and the lowband circuit 14e to the changeover switch circuit 14f is turned off.

When there is a low-band signal input of 550 MHz or less, a low potential is applied to the band switching control input Bc, the transistors Q1 and Q2 are turned off, and the switching circuits 14c (SW2) and 14a (SW) are turned off.
1) is turned off. Therefore, the changeover switch circuit 14
a, from the high band circuit 14b to the changeover switch circuit 14c
The high band range loop that outputs the frequency band up to is turned off. On the other hand, the transistor Q3 is connected to the inverter IN
Since a high potential, which is an inverted potential, is applied to the base by V, the base is turned on, so that the changeover switch circuit 14f (SW
4) is turned on. At this time, the AGC control input C
A is supplied with an AGC control signal, and the transistor Q4
By controlling the PIN diode in the switch circuit 14d (SW3), the switch circuit 14d
(SW3) has an AGC function. Therefore, a stable output by the AGC function is sent to the amplifier 4 through the low band range loop that outputs a frequency band from the changeover switch circuit 14d and the low band circuit 14e to the changeover switch circuit 14f. In this way, the high band range loop is switched to the low band range loop. The switching operation is performed when the switch circuit 14a (SW1) + 14c (SW2) is off or the switch circuit 14d (SW3) + 14f.
When (SW4) is off, the maximum amount of attenuation is obtained.
(SW2) or 14f (SW4) may be replaced with a PIN diode and configured with another ON / OFF switch circuit to further increase the attenuation.

[0014]

As is apparent from the above description, according to the present invention, when the bandwidth is widened so as to receive a received signal whose input range is about 50 to 860 MHz, the input range is set to a high band (High). There is an advantage that two low-band (Low) receiving blocks can be switched and received with a simple one receiving block configuration without having a separate side-by-side configuration, and there is no waste due to reduction of parts without double in the circuit configuration. There is a practical advantage that band switching with good switching characteristics can be performed at low cost, small size and light weight.

[Brief description of the drawings]

FIG. 1 is a block diagram of a band switching circuit according to an embodiment of the present invention.

FIG. 2 is a circuit configuration diagram of a control circuit which is a band switching circuit of the present invention.

FIG. 3 is a circuit block diagram showing a conventional band switching circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Low-pass filter 2 AGC circuit 3 Up-down converter 4 Amplifier 5 1st local oscillation circuit 6, 11 Buffer amplifier 7 1st mixing circuit (mixer circuit) 8 IF (intermediate frequency) bandpass filter 9 IF amplification circuit 10 2nd local oscillation Circuit 12 Second mixing circuit 13 IF band pass filter 14 Changeover switch circuit 14a Changeover switch circuit (SW1) 14b High band circuit 14c Changeover switch circuit (SW2) 14d Changeover switch circuit (SW3) 14e Low band circuit 14f Changeover switch circuit (SW4) 14g Control circuit A, B Receiving block Vt Power supply Q1 to Q4 NPN transistor R1 to R4 Resistance INV Inverter Bc Band switching control input Ca AGC control input

Claims (1)

(57) [Claims] 1. Supplied by way of a cable
In an up-down converter, a signal of a required frequency band is input to an input terminal, divided into reception blocks corresponding to two input ranges of a high band and a low band, and extracted as corresponding frequency components.
A to change the lock and keep the output constant
A band switching circuit for performing GC control, comprising: a first switch circuit, a high band circuit, and a second switch that pass a predetermined high band frequency band when the predetermined high band frequency band is input from the input terminal. A high-band range loop including a switch circuit; a third switch circuit, a low-band circuit, and a fourth switch circuit that pass the predetermined low-band frequency band when the predetermined low-band frequency band is input from the input terminal. The high band range loop comprising a first switch circuit connected to a band switching control input, and a third switch circuit connected to the band switching control input via an inverter. A band switching function for selecting one of low band range loops and the first switch circuit are connected. And a fourth switch circuit to which an AGC control input is connected and a second switch circuit to which an AGC control input is connected and a fourth switch circuit to which the third switch circuit is connected and to which an AGC control input is connected. AG that automatically controls the degree and keeps the output constant regardless of the strength of the received signal
A band switching circuit, comprising: a control circuit having a C circuit function;