JPH0422574Y2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Field of Industrial Application The present invention relates to a high frequency switch circuit suitable for use in wireless equipment connected to a television receiver, such as a character multiplex decoder.

(b) Prior Art FIG. 5 shows a schematic configuration of a conventional RF modulator with a high frequency switch. In Figure 5, 1 is connected to an external antenna (not shown).
An antenna input terminal connected to a 75Ω coaxial cable, 2 a tuner output terminal connected to a tuner (not shown) of a VTR (video tape recorder) in which this RF modulator with high frequency switch is built in, and 3 a tuner output terminal connected to a television receiver. This is a TV output terminal for coupling with the antenna input terminal.
(T ₁ ) has one end (A ₁ ) connected to the high frequency switch circuit 4, the other end (B ₁ ) to the tuner output terminal 2, and the intermediate tap (P ₁ ) to the antenna input terminal 1. It is also a transmission line transformer with a matching resistor (R ₁ ) connected between both ends, and functions as a two-way divider that supplies the signal input to the intermediate tap (P ₁ ) to the switch circuit 4 and tuner terminal 2, respectively. do. In addition, such a transmission line transformer is described in "Toroidal Core Utilization Encyclopedia" published by CQ Publishing Co., Ltd., page 360. Reference numeral 5 denotes an RF modulator, which RF modulates a paceband signal generated in a VTR in which this RF modulator 5 is incorporated and supplies it to the switch circuit 4. +B
are power supply terminals for supplying power supply voltage to the switch circuit 4 and the RF modulator 5 through power supply voltage supply resistors (R ₂ and R ₃ ), respectively.

Now, assuming that the power supply voltage is off, the switch circuit 4 leads the distributed output obtained from one end _A1 of the transmission line transformer _T1 to the TV output terminal 3, and from the other end _B1 of the transmission line transformer _T1 . The resulting distributed output is led out to the tuner output terminal 2. At this time, the RF modulator 5 is not supplied with power supply voltage and is in an inactive state.

On the other hand, when the power supply voltage is turned on, the RF modulator 5 operates, RF modulates the paceband signal reproduced in the VTR, and supplies it to the switch circuit 4. The output is guided to the television output terminal 3, and acts to block the distributed output from one end _A1 of the transmission line transformer 4.

Therefore, if a wireless device with a recording function such as a VTR is equipped with such an RF modulator with a high frequency switch, it is possible to record a program while watching the TV screen by turning on the power supply voltage. Furthermore, by turning off the power supply voltage, the reproduced image can be displayed on a television receiver.

In the case of a wireless device having a recording function such as a VTR, a transmission line transformer is required as a distributor in order to record a counter program while watching a TV screen. However, for example, with a text multiplex decoder, there is a choice between watching the TV program as it is, or extracting the text signal superimposed on the television signal and viewing an image of the text projected on the screen. When an RF modulator with a high frequency switch is built into a wireless device such as this, if the transmission line transformer _T1 is used as a two-way divider as described above, a signal transmission loss will occur between terminals 1 and 2, and this transmission loss will be This becomes more noticeable as the frequency increases from the VHF band to the UHF band. In addition, the noise figure of the VTR's internal tuner connected to tuner terminal 2 is worse in the UHF band than in the VHF band, which, together with the above-mentioned transmission loss, causes
The overall noise figure of a VTR will deteriorate further in the UHF band compared to that in the VHF band. American
According to the FCC standards, compared to that of a television receiver, the UHF band overall noise figure of a tuner built into a VTR takes into account only the transmission loss in the distributor due to the necessity of recording the opposite program mentioned above.
Although a deterioration of 4 dB has been recognized, the character multiplex decoder does not need to divide the signal into two parts at the same time and can be easily switched using a mechanical changeover switch, so the transmission loss is low even under the FCC standards. minutes are not recognized.

(c) Problems to be solved by the invention This invention uses an existing transmission line transformer and simply adds a simple configuration to electronically switch signals. The present invention provides a high frequency switch circuit that eliminates the deterioration of the noise figure.

(d) Means for solving the problem One end of the transmission line transformer with an intermediate tap is connected to the antenna input terminal, and the other end is connected to the first terminal through a switching element such as a switching diode.
connected to a first output terminal coupled to a wireless device;
Either one of the output of an RF modulator that modulates the paceband signal obtained by the first wireless device and the output from the intermediate tap of the transmission line transformer is selected and coupled to the second wireless device. A switch means is provided which leads to the second output terminal and grounds the intermediate tap when the output of the RF modulator is selected, so that the switching element is turned on and off and the RF modulator and the switch means are driven by a single power source.

(E) Effect When power supply voltage is supplied to the switching element, RF modulator and switching means, the switching element becomes conductive and the intermediate tap is grounded, so the transmission line transformer acts equivalently as a 1:1 transformer coupling, The signal input to the antenna input terminal is supplied to the first output terminal, and
The output of the RF modulator is supplied to the second output terminal via switch means.

Further, when the power supply voltage is not supplied to the switching element, the RF modulator, and the switching means, the switching element becomes non-conductive, and the signal input to the antenna input terminal is supplied to the second output terminal via the switching means.

(F) Embodiment FIG. 1, in which the same parts as in FIG. 5 are given the same reference numerals, is a diagram showing an embodiment of the high frequency switch circuit of the present invention, in which one end A ₂ of the transmission line transformer T ₂ is connected to the antenna input. The other end is connected to the tuner output terminal 2 through a series circuit of a switching diode D and a DC blocking capacitor _C1 .
Further, a small capacitor C ₂ and a resistor R ₁ are connected in parallel between both ends of the transmission line transformer T ₂ , and the capacitor C ₂ is coupled with the transmission line transformer T ₂ to achieve coupling in the UHF band. The purpose is to increase the amount of energy. 6 is capacitor C ₃ ~
_C7 , various filter circuits consisting of coils _L1 to _L4 , etc., and an off-through switch circuit consisting of first, second, and third switching transistors _TR1 , _TR2 , _TR3 , and each switching A power supply voltage is supplied to the bases of the transistors TR ₁ , TR ₂ , and TR ₃ via base bias supply resistors R ₂₁ , R ₂₂ , and R ₂₃ . Also, power terminal +B
is connected to the connection point of switching diode D and DC blocking capacitor _C1 via resistor _R4 .

Now, when power supply voltage is supplied from power supply terminal +B, switching diode D becomes conductive and each switching transistor TR ₁ , TR ₂ , TR ₃
conducts. Therefore, due to conduction of the third switching transistor _TR3 , the intermediate tap _P2 of the transmission line transformer _T2 is grounded, and thereby both ends _A2 and _B2 of the transmission line transformer _T2 are connected to the ground as shown in FIG. They are transformer-coupled as shown below, and the input signal input from one end _A2 is output as is to the other end _B2 . However, it is not a completely ideal transformer, so as mentioned above, a small capacitor C ₂ is used to transform the UHF
The circuit configuration has been designed to increase coupling to UHF band signals and to reduce loss, especially in the UHF band. The signal outputted from the other end _B2 is outputted to the tuner terminal 2 through the switching diode D and the DC blocking capacitor _C1 which are in a conductive state. The tuner output terminal 2 is coupled to a built-in tuner in the character multiplex decoder unit in which these circuits are incorporated. The baseband signal generated within this decoder unit is RF
RF modulation is performed by the module radar 5, and the RF modulation output is the first switching transistor in a conductive state.
It is supplied via TR ₁ and capacitor C ₃ to a television output terminal 3 which is coupled to an antenna terminal of an external television receiver (not shown). Furthermore, when the second switching transistor _TR2 is turned on, a filter circuit is formed by the capacitor _C5 and the coil _L3 , and this filter circuit prevents the modulated output of the RF modulator 5 from flowing to the transmission line transformer _T2 side. do.

Next, if the power supply voltage is not supplied from the power supply terminal +B, the RF modulator 5 becomes inactive, and each switching transistor TR ₁ , TR ₂ , TR ₃ and switching diode D become non-conductive, so that the transmission line transformer _T2 has the other end _B2 open due to the non-conduction of the switching diode D, and the third
The configuration is as shown in the figure. Then, the circuit of FIG. 3 is equivalently as shown in FIG. 4, assuming that the turns ratio of the transmission line transformer T ₂ is 1:1, R ₁ = R ₁ '/4, C ₂ =
The circuit is converted to 4C ₂ ', and a passing state is established between one end A ₂ of the transmission line transformer T ₂ and the intermediate tap P ₂ . Therefore, the high frequency signal output from the intermediate tap P ₂ passes through the capacitors C ₅ , C ₄ and C ₃ and is output to the television output terminal 3. In this manner, the present invention does not use a transmission line transformer as a distributor, so there is no loss due to signal distribution, and deterioration in tuner noise figure caused by transmission loss, especially in the UHF band, can be reduced.

(g) Effect of the invention According to the invention, the transmission loss between the antenna input terminal and the tuner output terminal is 1 to 2 dB in the VHF band.
3 to 5 dB better than before in the UHF band, especially
A high frequency switch circuit with low transmission loss in the UHF band can be realized using existing transmission line transformers.

Furthermore, since the present invention is constructed using an electronic switching system using a single power supply, the circuit construction is not only simple, but also extremely advantageous in terms of operability and cost.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing an embodiment of the high frequency switch circuit of the present invention, Figure 2 is an equivalent circuit diagram of a transmission line transformer when power supply voltage is applied, and Figures 3 and 4 are respectively when power supply voltage is released. FIG. 5, an equivalent circuit diagram of a transmission line transformer, is a diagram showing a conventional high frequency switch circuit. 1... Antenna input terminal, 2... First output terminal, 3... Second output terminal, 5... RF modulator, 6... Off-through switch circuit, T ₂ ... Transmission line transformer, P ₂ ... Intermediate Tap, D...Switching diode.

Claims (1)

[Scope of utility model registration request] One end of the transmission line transformer with an intermediate tap is connected to an antenna input terminal, and the other end is connected to a first output terminal coupled to a high frequency input terminal of a first wireless device via a switching element, the output of an RF modulator that modulates the output of the device;
Switch means is provided for selectively selecting one of the outputs of the intermediate tap and guiding it to a second output terminal coupled to a second wireless device, and for grounding the intermediate tap when selecting the output of the RF modulator. High frequency switch circuit.