JPH079271Y2 - RF switch circuit - Google Patents

Description

[Detailed Description of the Invention] "Industrial Application Field" This invention relates to the economicalization of an RF switch circuit used in a video tape recorder or the like.

"Prior Art" In a home-use video tape recorder, as shown in FIG. 6, a television signal given from a television antenna to an antenna input terminal 1 is divided into two by a two-division circuit 2, and each of them is an RF switch circuit. 3 is supplied to the input terminal IN and the tuner of the video recording / playback circuit 4. When watching this TV signal on a monitor TV, the TV signal is the RF switch circuit 3
Is transmitted as it is with almost no attenuation, and is supplied to the monitor television from the antenna output terminal 6 via the coupling circuit 5. When playing a video soft tape, the polarity of the control signal 7 given to the control terminal CTL of the RF switch circuit 3 is inverted by the video recording / playback circuit 4, and the RF switch circuit 3 outputs from the input terminal IN. Terminal OUT
The transmission of a television signal to the TV, especially a signal in the same frequency band as the video signal is blocked, and the video recording / playback circuit 4
A video signal 8 obtained by reproducing the video soft tape is supplied to the antenna output terminal 6 via the coupling circuit 5.

A conventional RF switch circuit will be described with reference to FIG. Capacitors C _{1 to} C ₅ are sequentially connected in series between the input terminal IN and the output terminal OUT. The input terminal IN is grounded through the coil L ₁ , the connection point P _{1 of} the capacitors C ₁ and C ₂ and the capacitor C ₂
And the connection point of the C ₃ P _2, capacitors C ₃ and C ₄ of the connection point P _3, the capacitor C ₄ and C respectively connecting point P ₄ coil L ₂ of _5, L _3,
It is grounded through L ₄ and L ₅ , and the output terminal OUT is grounded through a coil L ₆ . Further, the cathodes of the diodes D are connected to the connection points P ₁ , P ₂ , P ₃ , respectively, and the anodes thereof are grounded through the bypass capacitor C _p and connected to the control terminal CTL through the resistor R.

When watching a normal television, the control signal 6 applied to the control terminal CTL is at a low level (for example, zero volt),
Each diode D is controlled to be off, and the transmission loss frequency characteristic of the RF switch circuit becomes that of a high-pass filter having a cutoff frequency of about 30 MHz, as shown by the solid line in FIG. 8A. The input television signal is output only by suppressing low frequency noise of 30 MHz or less and being attenuated by about 1 dB.

When watching a video, the control signal 7 applied to the control terminal CTL is set to a high level and each diode D is controlled to be turned on. The connection points P ₁ , P ₂ , P ₃ are grounded through the small ON resistance of the diode D and the bypass capacitor C _{p, respectively,} and the transmission loss of the RF switch circuit is large over the entire band as shown by the solid line in FIG. 8B. Become. In particular, in the frequency band of the video signal (54 to 60 MHz and 60 to 66 MHz for North America in this example), if a TV signal leaks into this band, beat disturbance will appear on the TV screen, so an attenuation of 70 dB or more will be obtained. I am trying.

"Problems to be solved by the invention" While the performance of video tape recorders, which have been rapidly becoming popular in recent years, has become more sophisticated, their prices have been declining. Reduction of product cost is one of the important issues that lead to ups and downs of circumstances for manufacturers. This invention was made under such a background, and its purpose is to make the RF switch circuit economical.

[Means for Solving the Problem] A first capacitor c ₁ is connected between an input terminal and an output terminal, and the input end and the output end of the first capacitor c ₁ are respectively the first and second coils l ₁ , It is grounded through l _{2 to} form a π-type high frequency filter section, and the input terminal and the output terminal of the first capacitor c ₁ are turned on and off according to the magnitude of the control voltage applied to the control terminal. In an RF switch circuit including a π-type partial circuit grounded through a switch circuit, in the present invention, a series circuit of a second capacitor c ₂ and a diode d is connected in parallel with the first capacitor c ₁ and the second capacitor connection point between c ₂ and the diode d is connected through a resistor r to the control terminal of the diode d, which is controlled to be on parasitic inductance l ₀ and the second series circuit of a capacitor c ₂ The anti-resonance frequency of the terminal reactance circuit formed by connecting in parallel with said first capacitor c ₁ is set in the vicinity of the frequency band to be blocked transmission to the output terminal from the input terminal.

"Embodiment" In order to make it economical, the high-pass filter consisting of π type 5 sections in FIG. 7 is reduced to 4 sections, and the diode D connected to the connection point P ₃ and the corresponding bypass capacitor C _p and resistor R are connected. Consider the deleted conventional RF switch circuit (Fig. 9).
The transmission loss frequency characteristic of this circuit is shown by the dotted line in FIG. Regarding the characteristics in the transmission state, the loss is reduced as the number of sections is reduced, but there is no particular problem. In the transmission blocking state, the loss in the video signal band 60 ± 6MHz becomes 60 dB or less, and the standard value of 70 dB is not satisfied. Therefore, in this device, as shown in FIG. ₂ , a series circuit of a capacitor C _l and a diode D _l is connected in parallel with the capacitor C ₂ , and the connection point between the capacitor C _l and the diode D _l is connected to a resistor R _l. Through to the control terminal CTL. When the diode D _l is turned on by the control voltage, the reactance two-terminals are formed by connecting the parasitic inductance L ₀ of the diode D _l and the series resonance circuit of the capacitor C _l (whose resonance frequency is f _r ) in parallel to the capacitor C _2. The reactance frequency characteristics of the circuit are as shown in Fig. 3.
With a large anti-resonance frequency f _a from the resonance frequency f _r.
Therefore, in this invention, the anti-resonance frequency f _a is set in the vicinity of the video signal band 60 ± 6 MHz for which transmission is to be blocked (for example, in the vicinity of the center frequency 60 MHz of the band). Therefore, the transmission from the point P _{1 to the} point P ₂ in the vicinity of that frequency is effectively suppressed, and as shown by the solid line in FIG. 4B, the standard of 70 dB can be sufficiently satisfied in the video signal band of 60 ± 6 MHz. The characteristic shown by the dotted line in the figure shows the characteristic (shown by the dotted line in FIG. 8B) of the circuit of FIG. 9 without adding C _l , D _l , and R _l of this invention for comparison. . The characteristic of FIG. 4A is the characteristic in the transmission state of the circuit of FIG. 2, that is, when the diodes D and D ₁ are controlled to be off, and is the characteristic of FIG. 9 shown by the dotted line in FIG. 8A. It is almost equal to the characteristic of the circuit. The circuit of FIG. 2 has a series capacitor C ₅ and a parallel coil L _{6 in} the filter section reduced compared to the conventional circuit of FIG.

Another embodiment of this invention is shown in FIG. In this case, the transistor Q is used in place of the diode D and the bypass capacitor C _p shown in FIG. It is utilized that the signal applied between the collector and the emitter is almost short-circuited when the base current sufficient to saturate the transistor is passed. The transmission loss frequency characteristic of the circuit of FIG. 5 is almost equal to the characteristic of the circuit of FIG.

The switch circuit connected to the connection points P ₁ and P ₂ may be the series circuit of the diode D and the bypass capacitor C _p of FIG. 2, the transistor Q of FIG. 5, or any other suitable circuit. You can use a simple switch circuit. Therefore, generalizing these, as shown in Fig. 1, the switch circuit SW
Can be expressed as 2 and 5, the filter section between the input terminal IN and the connection point P ₁ and the connection point P ₂ and the output terminal
As for the filter section between OUTs, the number of sections or the circuit configuration can be appropriately changed. The feature of this device is the π-type partial circuit shown in FIG.
Note that the circuit symbols in FIG.
It is in accordance with the description of.

"Effect of device" According to this device, it is possible to obtain almost the same transmission loss frequency characteristic as the conventional one by using a smaller number of filter elements than the conventional one, which leads to economical and compact RF switch circuit. .

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a generalized essential part of an embodiment of the present invention, FIG. 2 is a circuit diagram showing a first embodiment of the present invention, and FIG. 3 is a diode D at the time of turning on of FIG. _l parasitic inductance L ₀ and illustrates the reactance frequency characteristic of the reactance two-terminal circuit formed by connecting in parallel a series circuit to the capacitor C ₂ of the capacitor C _l, and Figure 4 is a transmission loss frequency embodiment of Figure 2 FIG. 5 shows characteristics, FIG. 5 is a circuit diagram showing a second embodiment of the present invention, FIG. 6 is a block diagram of a video tape recorder, FIG. 7 is a circuit diagram showing an example of a conventional RF switch circuit, and FIG. FIG. 9 is a diagram showing the transmission loss frequency characteristics of the RF switch circuits of FIGS. 7 and 9, and FIG. 9 is a circuit diagram showing another example of the conventional RF switch circuit.

Claims (1)

[Scope of utility model registration request] 1. A first capacitor c ₁ is connected between an input terminal and an output terminal, and an input end and an output end of the first capacitor c ₁ are grounded through _first and second coils l ₁ and l ₂ , respectively. A high-pass filter section of π type is formed, and the input terminal and the output terminal of the first capacitor c ₁ are grounded through a switch circuit that is on / off controlled according to the magnitude of the control voltage applied to the control terminal. In a RF switch circuit including a π-type partial circuit, a series circuit of a second capacitor c ₂ and a diode d is connected in parallel with the first capacitor c _1, and the second capacitor c ₂ and the diode d are connected. point is connected via a resistor r to the control terminal, the parasitic inductance l ₀ and the second series circuit of a capacitor c ₂ of the diode d, which is controlled to be on is in parallel with the first capacitor c ₁ RF switch circuit and setting the anti-resonant frequency of the terminal reactance circuit formed by connection in the vicinity of the frequency band to be blocked transmission to the output terminal from the input terminal.