JPH0648988Y2 - Band filter switching circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial field of application) The present invention relates to a band filter switching circuit, and more particularly to a satellite broadcast receiver for receiving a plurality of satellite broadcast signals having different bandwidths for one channel. The present invention relates to a band filter switching circuit that can be switched and received.

(Prior Art) The bandwidth of a communication satellite based on video transmission varies depending on the manufacturer and application of the satellite. For example, the satellite bandwidth of Japan Communication Satellite Co., Ltd., which is scheduled to be launched in 1989, is 27 MHz, that of Space Communication Co., Ltd. is 36 MHz, and the latter is 18 MHz by dividing one channel into two. Operation in 2 is also considered. Therefore, for a receiver capable of receiving the signals of these satellites, it is necessary to switch the bandwidth of the IF frequency in the IF band filter of the tuner section.

FIG. 2 shows a conventional band filter switching circuit for switching the bandwidth of the IF frequency.

In FIG. 2, reference numeral 11 is an IF signal input terminal, and 12 is an output terminal. Input terminal 11 is a coupling capacitor
Three switching diodes D1, D via C1, C2, C3
Connected to the anode of 2, D3. The cathodes of the diodes D1, D2, D3 are connected to the reference potential point via the bias resistors R1, R2, R3, respectively, and the coupling capacitors C4, C5,
It is connected to the three band-pass filters 13, 14, 15 via C6, and the output ends of the band-pass filters 13, 14, 15 are both connected to the output terminal 12. The anodes of the diodes D1, D2, D3 are connected to the switching terminals 16, 17, 18 via choke coils L1, L2, L3, respectively.

In the above circuit, when a switching voltage is applied to any of the switching terminals 16, 17, 18 and one of the diodes D1, D2, D3 corresponding thereto conducts, one of the bandpass filters 13, 14, 15 becomes It is supposed to be selected.
The band filters 13, 14, 15 to be switched are, for example, SAW (surface acoustic wave) filters that do not require adjustment.

FIG. 3 shows an example of a logic circuit for supplying a switching voltage to the switching terminals 16, 17, 18 of FIG.
It is composed of three logic ICs (integrated circuits) L1, L2, L3 and three buffer transistors Q1, Q2, Q3. When switching the frequency band, the control means such as a microcomputer puts high level on the input terminals 21, 22. Level (H) and low level (L) signals are supplied, and any one of the buffer transistors Q1, Q2, and Q3 becomes conductive depending on the combination of the H and L levels, and the DC voltage + B (for example, from the power supply terminal 23). 5V)
Is supplied to any one of the switching terminals 16, 17 and 18 as a switching voltage. The combination of logic levels (H, L), (L,
H), (L, L), one of the transistors Q1, Q2, Q3 becomes conductive.

By the way, the conventional circuit configuration described above has the following problems.

First, since the pass loss of the bandpass filter differs depending on the bandwidth, the pass loss is not uniform when the band is switched. Generally, a filter having a wide bandwidth has a larger pass loss.

Secondly, it is necessary to use a large number of ICs and transistors (a total of 6 in the figure) in the logic circuit for supplying the switching voltage. Regarding this point, if three logic ICs are deleted and the three baselines of the buffer transistors are directly controlled by the micro computer, only three transistors will suffice. However, in such a configuration, the number of ports of the micro computer is limited. If there is a problem.

(Problems to be Solved by the Invention) As described above, in the conventional circuit configuration, there is a problem that a difference occurs in the pass loss of the bandpass filter at the time of band switching and the number of elements used in the logic circuit increases.

Therefore, the present invention is to eliminate the above problems,
It is an object of the present invention to provide a bandpass filter switching circuit which can correct the difference in the pass loss of a filter generated at the time of band switching, and can simplify or omit a logic circuit which supplies a switching voltage.

[Structure of the Invention] (Means for Solving the Problem) The bandpass filter switching circuit of the present invention includes an input terminal to which a signal is supplied and a first and second second terminal in which the cathode is commonly connected to the input terminal. One switching diode, a common bias resistor connected between a common connection point of these switching diodes and a reference potential point, and an anode of the first switching diode, and a control signal from the outside. A switching control circuit for switching the first switching diode between conducting and non-conducting; and a voltage generated at the cathode when the first switching diode conducts with respect to the anode of the second switching diode. A unit for constantly applying the same voltage or a voltage lower than that, and a unit arranged after the first switching diode. And a first band-pass filter, wherein arranged downstream of the second switching diode, configured to and a small second band-pass filter of pass loss than the first band-pass filter.

(Operation) In the present invention, the second switching diode is non-conductive when the first switching diode is conductive, and the second switching diode is non-conductive when the first switching diode is non-conductive. Since the diode becomes conductive, it becomes possible to switch between the two bandpass filters simply by controlling the first switching diode, and the second bandpass filter with low pass loss is connected to the second switching diode. When the second switching diode is conducting, its anode voltage decreases and the conduction resistance of the diode increases, and the other path (the path of the first switching diode and the first bandpass filter) It is possible to cancel the difference in passage loss between and.

(Embodiment) Hereinafter, the present invention will be described based on an embodiment shown in the drawings.

FIG. 1 is a circuit diagram showing a bandpass filter switching circuit according to an embodiment of the present invention.

In this figure, reference numeral 31 is an IF signal input terminal, and 32 is an output terminal. Input terminal 31 is coupling capacitor C11
Three switching diodes D11, D12, D13 via
, And a bias resistor R11 is connected between the connection point 33 and the reference potential point. Diode D
The anodes of 11, D12, D13 are bandpass filters 34,35,36, respectively.
, And the output terminals of each filter are both connected to the output terminal 32. The anodes of the diodes D11, D12 are connected to the collectors of the switching transistors Q11, Q12 via the choke coils L11, L12, respectively, and the anode of the diode D13 is divided by the choke coil L13 and the resistors R16, R17. It is connected to the power supply terminal 39 via a pressure circuit. The base of the transistor Q11 is connected to the switching terminal 37 via the resistor R12, the emitter thereof is connected to the power supply terminal 39, and the resistor R13 is connected between the base and the emitter. Similarly, the transistor Q
The base of 12 is connected to the switching terminal 38 via the resistor R14, the emitter thereof is connected to the power supply terminal 39, and the resistor R15 is connected between the base and the emitter. In addition,
The power supply terminal 39 has a power supply voltage + B from a DC power supply (not shown).
(For example + 5V) is supplied, and switching terminals 37 and 38 are set to logic H and L from a microcomputer (not shown).
Level switching signals S1 and S2 are supplied.

In such a configuration, when an L level switching signal is applied to the switching terminal 37 or 38, the transistor Q11 or Q12 becomes conductive, a DC voltage is applied from the power supply terminal 39 to the anode of the diode D11 or D12, and the diode D1.
A voltage obtained by dividing the power supply voltage + B of the power supply terminal 39 by the dividing resistors R16 and R17 is applied to the anode of 3.

Further, a specific example will be described. In this embodiment, as the bandpass filters 34, 35, 36, for example, SAW filters having a center frequency of 134 MHz and bandwidths of 31 MHz, 27 MHz, and 18 MHz are used. Also, for example, the power supply voltage + B is 5V, the resistor R16,
When R17 is 1 kΩ, 2.5 V is applied to the diode D13. When the switching signal S1 is at the L level and the switching signal S2 is at the H level, the transistor Q11 is turned on, the diode D11 receives about 5 V, and the voltage at the connection point 33 is equal to the forward voltage drop VF of the diode D11. I went down. About 4.3V appears. At this time, the transistor Q12
Is off, no voltage is applied to the anode of the diode D12 and the diode D12 becomes non-conductive. Since 2.5V is applied to the anode of the diode D13, the cathode voltage is higher and the diode D13 becomes non-conductive. Further, when the switching signal S2 is at the L level and the switching signal S1 is at the H level, similarly, only the diode D12 becomes conductive.

Next, when the switching signals S1 and S2 are both at the H level,
Both the diodes D11 and D12 are turned off and no voltage appears at the connection point 33, so that the diode D13 becomes conductive. At this time, assuming that the resistance R11 is 1 kΩ, the diode D13
The current that flows into the device is about 1.2mA, and the other two diodes D
It is less than the current (about 4.2mA) when 11, D12 is turned on. Therefore, when the pin diode 1SV128 manufactured by Toshiba Corporation is used as the diodes D11 to D13, the resistance value when the diode D13 is on is 20Ω, and the diodes D11 and D12 are
When it is turned on, the resistance value becomes about 5Ω, which makes a difference.

However, as the band-pass filters 34, 35 and 36 in the latter stage, in the present embodiment, the center frequency is 134 MHz and the bandwidth is 31 MHz.
A z, 27MHz, 18MHz SAW filter is used, and the insertion loss is 26dB at 31MHz, 25dB at 27MHz, and 21dB at 18MHz.
As a result, the loss when the diode D13 conducts is 18MHz.
The loss due to the resistance value of the diode D13 when the diode D13 is on is added to the pass loss of the bandpass filter 36 at. Therefore, no matter which band filter is used, the difference in the pass loss of the band filter is canceled by the difference in the resistance value of the diode, and the loss difference disappears.

According to the above-described embodiment, it is possible to switch three filter circuits by using two switching transistors, and use the difference in the on-resistance value of the diode generated at that time to detect the difference in filter insertion loss. You can create multiple effects of losing.

Although the above embodiment has described the case where the switching diode is arranged on the input side of the bandpass filter, the same effect can be obtained even if the switching diode is arranged on the output side of the bandpass filter. Needless to say.

[Advantage of the Invention] As described above, according to the present invention, the ethical circuit for supplying the switching voltage can be simplified, and the difference in the pass loss of the filter generated at the time of band switching can be corrected. Therefore, a simple and high-performance bandpass filter switching circuit can be realized.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a bandpass filter switching circuit according to an embodiment of the present invention, FIG. 2 is a circuit diagram showing a conventional bandpass filter switching circuit, and FIG. 3 is a logic circuit for switching control of the circuit of FIG. It is a circuit diagram showing. 31 ... input terminal, 32 ... output terminal, 34,35,36 ... band filter, 37,38 ... switching terminal, 39 ... power supply terminal, Q1, Q2 ... switching transistor, D11, D12, D13 ... switching diode, R11 … Bias resistors, R16, R17… Division resistors.

Claims (1)

[Scope of utility model registration request] 1. An input terminal to which a signal is supplied, two first and second switching diodes whose cathodes are commonly connected to the input terminal, a common connection point of these switching diodes and a reference potential. A common bias resistor connected between the points, and a switching control circuit connected to the anode of the first switching diode and switching the first switching diode to conductive or non-conductive by a control signal from the outside. A unit for constantly applying to the anode of the second switching diode a voltage equal to or lower than the voltage generated at the cathode when the first switching diode conducts; A first band-pass filter disposed after the switching diode of, and a second switching diode of It is arranged on the step, band filter switching circuit which is characterized by comprising a first second band-pass filter low pass loss than the band filter.