GB2033181A - High-frequency electronic switch circuit - Google Patents

Abstract

An electronic switch circuit includes a diode to which a forward or reverse bias is applied to turn it on or off, thereby permitting or preventing transmission of a signal through the switch circuit. In order to prevent a leakage signal from passing through the junction capacitance of the diode when the switch circuit is turned off, an inductance element is parallel- connected with the diode to form, along with the junction capacitance of the diode which is reverse-biased, a parallel resonant circuit which is tuned to the frequency of a signal to be transmitted through the switch circuit. Thus, a switch circuit is realized which can handle a high-frequency signal of up to the UHF-band. <IMAGE>

Description

SPECIFICATION High-frequency electronic switch circuit The present invention relates to electronic switch circuits suited for handling high-frequency signals and more particularly to an electronic switch circuit suitable for switching high-frequency signals in a case where a television receiver is used in combination with a video signal reproducing apparatus such as a video tape recorder, a video disc player or the like.

In the drawings: Figures la to ic are circuit diagrams of the conventional switch circuits using diodes; Figures 2 and 3 show equivalent circuits of the switch circuit of Figure la in cases where the diode is conductive and nonconductive, respectively; Figure 4 is a circuit diagram showing a basic embodiment of the present invention; Figures 5a and 5b show equivalent circuits of the circuit of Figure 4 in cases where the diode is conductive and nonconductive, respectively; Figure 6 is a block diagram showing use of the switch circuit of the invention between a picture signal reproducing apparatus and a television receiver; Figure 7 is a circuit diagram showing a specific embodiment of the invention; and Figures 8a and 8b are graphs showing frequencyisolation characteristic curves to which reference is made in explaining the effect of the invention.

There have been widely used electronic switch circuits of which the switching operation is achieved by forward-and reverse-biasing of a diode. Some examples of such conventional switch circuits are shown in Figures la to 1c.

Figure 1a shows an example of using a single diode. Shown at 1 and 7 are high-frequency signal terminals, 2 and 6 DC blocking capacitors which present a sufficiently low impedance at working frequencies, 4 a diode, 3 and 5 resistors through which a DC bias is applied, and 8 and 9 control terminals. If a positive potential is applied to the control terminal 8 and the ground potential to the control terminal 9, the diode 4 is made conducting, providing a high-frequency conducting path between the terminals 1 and 7. On the contrary, if the ground potential is applied to the control terminal 8 and a positive potential to the control terminal 9, the diode 4 becomes nonconducting, thus providing a path through which high frequency signals cannot be passed between the terminals 1 and 7.

Figure 1 b shows an example of using two diodes.

When a positive potential is applied to the control terminal 8 and the ground potential is applied to the control terminal 9, a diode 12 becomes conductive, whereas a diode 13 is made nonconducting by the reverse bias. Consequently, the path between the terminals 1 and 7 allows high-frequency signals to be passed therethrough. If the ground potential is applied to the control terminal 8 and a positive potential is applied to the control terminal 9, the diode 13 is turned on by the forward biasing, while the diode 12 is turned off by the reverse biasing. As a consequence, the path between the terminals 1 and 7 does not allow high-frequency signals to be passed therethrough.The diode 13 serves to improve the isolation characteristic between the terminals 1 and 7 when the path between the terminals 1 and 7 is nonconducting, as compared with the example of Figure 1a. Use of more diodes will improve the isolation characteristic more.

Figure 1 c shows an example of using three diodes between the terminals. When the diode 12 is turned off and the diode 13 on in order that the path between the terminals 1 and 7 can be made nonconducting, a diode 17 is nonconductive to much improve the isolation between the terminals 1 and 7 as compared with the example of Figure 1 b.

In general, a diode, when forward-biased, exhibits an equivalent resistance of which the value changes depending on a biasing current and usually about several ohms. On the contrary, a reverse-biased diode is equivalently a capacitor the value of which is dependent upon a reverse biasing voltage and typically about 1 pF. When a diode is incorporated in a printed circuit board or the like, the inductance of its lead wire is added in series to the abovementioned resistor or capacitor. This inductance value is around ten and several nanohenries (nH).

Thus, when the diode 4 in Figure 1 a is forwardbiased, the equivalent circuit is as shown in Figure 2.

The equivalent circuit of the diode 4, when reversebiased, is illustrated in Figure 3. In these equivalent circuits, there are shown an inductance 19 of the lead wire of the diode 4, a resistance 20 of the diode 4 which is forward-biased, and a capacitance 21 of the diode 4 which is reverse-biased. The inductance 19 of the lead wire and the capacitance 21 are negligibly small at relatively low working frequencies. At frequencies as high as those in the UHF band, the series circuit of the lead inductance 19 and the reverse-biased diode capacitance 21 falls into a resonant region, and thus the impedance between the terminals 1 and 7 is lowered resulting in poor isolation. In order to improve the isolation, it is necessary to reduce the lead inductance.However, when components are practically connected on a printed circuit board, a lead inductance of a length cannot be removed which corresponds to the board thickness. Although the examples of Figures 1 band 1c using plural number of diodes provide a higher isolation than the example of Figure la, the isolation at high frequencies becomes poorforthe same reason.

An object of the invention is to provide an electronic switch circuit which presents an excellent isolation even at frequencies in the UHF band.

The present invention is arranged so that an inductance element is connected in parallel with a diode for switching and the resonant frequency of the parallel circuit consisting of the inductance element and the reverse-biased diode capacitance is selected to be close to the frequency of a highfrequency signal to be dealt with. Therefore, the parallel resonant circuit formed by the capacitance of the reverse-biased (cut off) diode and the inductance element is tuned to the vicinity of the frequency of a high-frequency signal and thus presents a high impedance to improve the isolation.

Figure 4 is a circuit diagram showing a basic embodiment of the present invention, which is a switch circuit with a single diode similar to that of Figure la, and in which like elements corresponding to those of Figures 1 a and 1 b are denoted by the same reference numerals. Shown at 22 is a coil, and 23 a capacitor which presents a sufficiently low impedance at working frequencies and which serves to prevent the diode 4 from short-circuiting through the coil 22 for direct current. Figure 5a shows an equivalent circuit of the circuit of Figure 4 in which the diode 4 is forward-biased, and Figure 5b shows an equivalent circuit of the same circuit in which the diode 4 is reverse-biased.The capacitor 23 presents a sufficiently low impedance when high-frequency signals are applied thereto as described above, and thus is negligible in the equivalent circuits.

When the diode 4 is reverse-biased, the capacitance 21 of the reverse-biased diode 4 resonates with the coil 22 to form a resonant circuit as shown in the equivalent circuit of Figure 5b. Thus, if the resonant frequency thereof is preselected to be around the working frequencies, the path between the terminals 1 and 7 provide a high impedance at the working frequencies, resulting in considerable improvement in the isolation as compared with the example of Figure 1 a. If, for example, the reversebiase capacitance 21 is 1 pF, and the working frequency is 600 MHz, the inductance of the coil 22 is selected to be 70 nH. When the diode 4 is forwardbiased, the resistance 20 of the forward-biased diode 4 becomes sufficiently small in the equivalent circuit of Figure 5a and thus the coil 22 connected in parallel with the diode 4 has no effect.

Figure 6 is a block diagram of an example of a system employing such a high-frequency electronic switch. This example shows that the switch controls the connection between a picture signal reproducing apparatus such as a video tape recorder, a video disc player or the like and a television receiver for displaying a reproduced signal from the reproducing apparatus. The home picture signal reproducing apparatus is generally constructed so that the reproduced signal therefrom can be viewed by a common television receiver. Referring to Figure 6, there is shown a high-frequency modulating device 27 for combining a picture signal 29 reproduced at a picture reproducing circuit 28 and an audio signal 30 into a signal like a television broadcasting radio wave signal. Shown at 26 is a high-frequency electronic switch according to the present invention.

A common terminal c of the change-over switch 26 is connected to a high-frequency output terminal 25 which is further connected to an antenna terminal of a television relceiver (not shown). Shown at 24 is an antenna input terminal which is connected to an antenna (not shown) for receiving a television broadcast radio wave. To one contact a of the change-over switch 26 is applied a received television signal which is supplied from the input terminal 24. To the other contact b is applied a reproduced high-frequency signal 31 which the high-frequency modurating apparatus 27 produces. Therefore, the received broadcast signal and the reproduced signal from the picture reproducing circuit are selectively supplied to the television receiver by the changeover switch 26.

This change-over switch 26 is arranged as in Figure 7 which shows one specific embodiment of the switch circuit capable of handling not only VHF-band signals but also UHF-band signals. This circuit arrangement is a combination of the threediode circuit as shown by Figure 1 c and the two cascade-connected single-diode circuits one of which is as shown in Figure 1a, each diode being parallel-connected with an inductance element.

An input terminal 32 corresponds to the contact a of the high-frequency change-over switch 26 in Figure 6, an input terminal 51 to the contact b, and an output terminal 42 to the common terminal c. If a positive potential is applied to a control terminal 52, and the ground potential is applied to a control terminal 53, diodes 35 and 39 are turned on, and diodes 36,45 and 48 off. As a result, a broadcast radio wave signal applied to the input terminal 32 can be transmitted to the output terminal 42, and thus the radio wave signal is applied through the high-frequency output 25, as shown in Figure 6, to a television receiver. On the contrary, if the ground potential is applied to the control terminal 52, and a positive potential is applied to the control terminal 53, the diodes 36,45 and 48 becomes conducting and the diodes 35 and 39 nonconducting.Consequently, a reproduced high-frequency signal 31 applied to the input terminal 51 is transmitted to the output terminal 42, but a broadcast radio wave signal is not transmitted to the output. In this way, selective application of a broadcast radio wave signal and a reproduced high-frequency signal to a television receiver can be easily performed by the electronic switch circuit. In this case, if the reproduced high-frequency signal 31 is leaked to the antenna inputterminal 24, the leaked signal is radiated from an antenna to the external, interfering with another receiver. Thus, in West Germany, for example, such a leakage voltage is specified to be not more than 20 dBu by the VDE standard.Since the voltage of a reproduced high-frequency signal is usually as much as 70 dibs, the isolation of the input terminal 51 from the input terminal 32 should be at least 50 dB. Thus, as illustrated coils 54 and 57 in series with DC blocking capacitors 55 and 56 are connected across diodes 35 and 39, respectively. The capacitance of the reverse-biased diode 35 and the coil 54 constitute a resonant circuit and the capacitance of the reverse-biased diode 39 and the coil 57 constitute another resonant circuit. The resonant frequencies of both the resonant circuits are selected to be in the vicinity of the frequency of a reproduced high-frequency signal. If, for example, the frequency of a reproduced high-frequency signal is 600 MHz, the inductance values of the coils 54 and 57 are selected to be 70 nH as described previously. The values of the other circuit elements are chosen as illustrated, and a positive voltage of 12 V is applied to either of the control terminals. The measurements of the isolation in the switch circuit of Figure 7 under these conditions are plotted as shown in Figure 8a.

Moreover, for the comparison with the effect of the invention, the measurement of the switch circuit without coils 54 and 57 is given in Figure 8b. From the graphs it will be seen that in the switch circuit of the invention, the isolation is considerably improved at high frequencies as compared with that in Figure 8b, and that in particular the isolation of 60 dB or more is achieved at a working frequency of about 600 MHz. Since the frequencies of a reproduced high-frequency signal from a picture reproducing apparatus are usually selected to be within an unused channel between the television broadcasting channels, variation in the frequency of the reproduced high-frequency signal may be as large as several channel bands about the center frequency of the high-frequency signal.However, if the aforesaid resonant frequency is selected to be the center value, a sufficiently high isolation can be effected in the range of the working frequencies as shown in Figure 8a.

Without the coils 54 and 57, the isolation of 50 dB or more is achieved only at frequencies in the VHF band, and not at frequencies in the UHF band as shown in Figure 8b, which means that the switch circuit can not be used at the UHF band.

Use of multiple switching diodes may cause a considerable transmission loss in the conductive state of the diodes. In the embodiment of Figure 7, however, when the diodes 35 and 39 are turned on and the diodes 36,45 and 48 off, the measured transmission loss in the conducting path from the terminal 32 to the terminal 42 is 1 dB or lower at several MHz to 1GHz, and in addition there is not any noticeable effect of the coils 54 and 57 on the transmission.

In accordance with the invention, as described above, the switch circuit is arranged so that a coil is connected in parallel with a diode, and the parallel resonant circuit of the coil and the capacitance of the reverse-biased diode is designed to have a resonant frequency close to high frequency signals to be handled by the switch circuit. Therefore, the impedance of diodes in the nonconducting state becomes increased at the working frequencies, thereby providing a sufficiently high isolation characteristic even at the UHF band.

While in the embodiment of Figure 7 a coil is connected in parallel with only each of the diodes 35 and 39, a coil, if necessary, may be connected to each of the diodes 45 and 48 on the terminal 51 side.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of this invention.

Claims (4)

1. A switch circuit wherein diodes used are forward-or reverse-biased to turn on or off thereby selectively transmitting high-frequency signals applied thereto, said switch circuit comprising at least one diode connected in series between terminals between which a high-frequency signal is transmitted, a means for applying a switched bias to said diode to forward bias or reverse bias said diode, and an inductance element connected in parallel with said diode through a DC blocking capacitor so that said inductance element resonates with the junction capacitance of said diode upon reverse-biasing to form a parallel resonant circuit which is tuned to the vicinity of the frequency of said high-frequency signal.

2. A switch circuit according to claim 1, wherein a second diode is connected between a reference potential point and a transmission path between the terminals through which said high frequency signal is transmitted, said second diode is reverse-biased or forward-biased when the first-mentioned diode connected in series between said terminals are forward-biased or reverse-biased, respectively.

3. A switch circuit for selectively transmitting high-frequency signals between a common terminal and two terminals, comprising at least one diode connected between the common terminal and each of the two terminals, a biasing means for biasing said diodes in such a way that when a forward bias is applied to the diode connected at one terminal side of said two terminals, the other diode connected at the other terminal side thereof becomes reversebiased, while when a reverse bias is applied to the diode connected at the one terminal side thereof, the other diode connected at the other terminal side becomes forward-biased, and an inductance element connected through a DC blocking capacitor to be in parallel with the diode which is connected at least at the one terminal side of said two terminals, said inductance element resonating with the junction capacitance of said diode which is reversebiased, to form a parallel resonant circuit which is tuned to the vicinity of the frequency of said high-frequency signal.

4. A switch circuit substantially as hereinbefore described with reference to and as shown by Figures 4 to 8 of the accompanying drawings.