GB2097226A - A wide band transmitting/receiving switch - Google Patents

Abstract

The invention refers to a transmitting/receiving switch in PIN diode technology substantially intended for the frequency range of 1.5-30 megahertz and transmitter power up to 1200 watts. Included combinations of inductances and capacitances, which have the purpose of isolating high frequency signals from direct current supply circuits and which according to earlier known design methods would have been expensive and difficult to implement for the above-mentioned frequency and power range are chosen according to form high-pass filters. In this manner lower and more easily implementable values are achieved simultaneously with decreased time constants and thereby also faster switching times. The direct current supply circuits are provided with active circuits connected in such manner that no resistances which can increase the time constants in switching from transmitting to receiving and vice versa are disposed in series with the current supplies. The direct current biases are held constant and well-defined by current generators.

Description

SPECIFICATION A wide band transmitting/receiving switch This invention refers to a wide band transmitting/receiving switch which is implemented in PIN diode technology. The switch is mainly intended for the frequency range of 1.5-30 megahertz and transmitting power of up to 1200 watts.

In specific modern short wave radio communication systems, for example simplex ARQ systems, it has hitherto been necessary to utilise individual antennae for the transmitter and the receiver, in spite of the fact that in theory it should be possible to make use of a common antenna, as transmitting and receiving do not occur simultaneously.

The reason that individual antennae have been necessary is that the transmitting/receiving switches which have been available for the above-mentioned frequency and power range have been implemented in conventional electromechanical technology. This has resulted in the switching times between transmitting and receiving and vice versa becoming too lengthy, approximately 10 milliseconds, to be able to be utilised in simplex ARQ systems. The low length of life, calculated in the number of switchings, has also been a limiting factor for electromechanical transmitting/receiving switches.

In order for a transmitting/receiving switch to be utilised in practice in a short wave simplex ARO system i.e. the following requirements can be made: Fast switching from transmitting to receiving and vice versa, preferably '200 microseconds.

Low insertion loss, in transmitting state preferably c0.1 decibels and in receiving state preferably '0.5 decibels.

Good isolation towards the receiver in transmitting state 265 decibels.

A wide band so that a substantial frequency range may be utilised.

High linearity so that the transmitting/receiving switch does not distort transmitted or received signals. Disturbing harmonics more than 80 decibels below a desired signal.

A transmitting/receiving switch designed in accordance with the present invention will fulfil the requirements listed above among other ones.

According to the present invention, there is provided a transmitter/receiver switch implemented in PIN diode technology for frequencies in the range 1.5-30 MHz with a transmitted power of up to 1 200 watts, characterised in that included inductances and capacitances having the purpose of isolating high frequency signals from direct current supply circuits are chosen in accordance with filter characteristics, whereby decreased time constants and thus also faster switching times are achieved, and the direct current supply circuits are designed with active circuits coupled in such a manner that no resistances which can increase the time constants when switching from transmitting to receiving and vice versa are in series with the current supplies.

Transmitting/receiving switches in PIN diode technology with circuit solutions similar to the one in accordance with the invention have existed earlier, but they are then intended for higher frequencies and/or substantially lower transmitter power.

The invention will be described more specifically in the following with reference to the accompanying drawings, in which: Figure 1 shows the principle of a wide band transmitting/receiving switch implemented in PIN diode technology, Figure 2 shows a transmitting/receiving switch designed in accordance with the invention, Figure 3 is a high frequency equivalent diagram for transmission, Figure 4 shows a simplification of the diagram of Fig. 3, Figure 5 is a vector impedance diagram illustrating effects of the invention, and Figure 6 shows a Smith diagram which may be used in explaining the invention.

Fig. 1 shows a short wave radio communication system comprising a transmitter A, a receiver B and an antenna in common to the transmitter and receiver with its associated transmitting/receiving switch C. As transmission and receiving are not carried out simultaneously the mentioned antenna can be common to both the transmitter and the receiver.

Fig. 2 shows a circuit diagram of a transmitting/receiving switch in PIN diode technology in accordance with the invention.

Specific clarifying text referring to the connecting voltages has been introduced into said figure. The designation D refers to a so-called stripline waveguide.

The function of the transmitting/receiving switch is briefly the following, see also Fig. 2: In transmitting, PIN diode D1 will conduct in consequence of being supplied with direct current over L, and L2 from the current generator connected transistor T3. With the direct current bias (1.3 amperes) utilised in the present invention D1 exhibits an equivalent high frequency resistance RDHF of 0.8 ohms.

PIN diode D2 is reverse biased by T11 and T12 conducting and applying - 420 volts at L3 to the anode of D2. D2 will then exhibit a capacitance of approximately 1 picofarad. As all of the components are positioned on a pattern card in which the line pattern between the components forms microstrip waveguides, see also Fig. 2, a "transmission line" is formed between the coaxial connectors X1 and X2. Furthermore, T3 supplies direct current to PIN diode D3 which then becomes conductive and shunts the receiver input, whereby the attenuation towards the receiver is increased additionally.

In reception D2 is instead provided with direct current via L2 and L2 from T9 and T10. T4 is conductive so as to reverse bias D1 over L1 and D3 over L4. In the same manner as in transmission a "transmission line" is then created between the coaxial connectors X2 and X3 instead.

The choke coils L1, L2, L3 and L4 serve the purpose of high frequency isolating the produced "transmission lines" from the direct current supply circuits. The purpose of the capacitors C1, C2 and C3 is to block direct currents simultaneously with passing high frequency signals.

In earlier known solutions of transmitting /receiving switches of the type illustrated in Fig. 1 the inductances of L1 = L2 = L3 = L, 3 have been selected in such manner that their reactances have been very large as compared to 50 ohmns and the capacitances C1, C2 and C3 in such manner that their reactances have been small as compared to 50 ohms so that they would not affect the high frequency signal.

In order to provide an insertion loss '0.1 decibels in the frequency range of 1.5-30 megahertz in transmission the inductances would have to be of the magnitude of 100 millihenries and the capacitances of the magnitude of 1 microfarad in accordance with the design philosophy indicated above.

A selection of this type would i.e. include two problems: For one thin La, L2 and C, as above would be very expensive, as it is difficult to produce inductances for handling direct currents of 1.3 amperes and capacitances for handling passthrough power of the magnitude of 1 200 watts with good characteristics in the entire frequency range of 1.5-30 megahertz.

For another thing large values of LR 3 and C1, C2 and C3 will create difficulties in producing fast switching from transmitting to receiving and vice versa in consequence of increased time constants.

In the present invention these problems have been solved by enabling L, 3 and C1, C2 and C3 to have low values without the high frequency characteristics being affected. This has been made possible as the result of the combinations L1, L2 C1 and L3, L2, C2 C3 being made to form high-pass filters of the Butterworth type having a cut-off frequency considerably lower than 1.5 megahertz.

L4 has been chosen in such manner that its reactance is large as compared to 50 ohms, but in view of the direct current to D3 being comparatively small this choice does not affect the switching times.

As the reactance of D2, when it is reverse biased, is much greater than 50 ohms a high frequency equivalent diagram for transmission may be drawn up in accordance with Fig. 3, wherein E designates a transmitter, F designates an antenna, and G designates a transmitting/receiving switch.

As RDHF merely is 0.8 ohms with the direct current of 1.3 amperes utilised here the diagram may be simplified in accordance with Fig. 4, in which E, G and F have the same significances as in Fig. 3.

As the result of the high pass filter being calculated for an attenuation of 0.1 decibels at 1.5 megahertz the values C, = 7.9 microfarads and + Lea 3 = 1 7 microhenries were achieved.

Thus, although C, exhibits a reactance of 1 5 ohms in series with the high frequency signal simultaneously with + Lea 3 shunting the antenna with 1 60 ohms at 1.5 megahertz the insertion loss as the result of C13 and + L1 3 is less than 0.01 decibels. By means of a vector impedance diagram it is possible to see how the effects of C1 and + LR 3 on the antenna impedance ztot seen by the transmitter cancel each other when Zantenna = 50 ohms, see Fig. 5 in which the designation F refers to the antenna.

The Smith diagram of Fig. 6 shows Zantenna with a standing wave ratio of 2:1 and illustrates how ztot seen by the transmitter is affected by the filter combination of + L, 3 and C12 at 1.5 megahertz. In reception C2 = C3 = ZC 3 and L2 = L3 = LR 3 form a high pass filter in the same manner as in transmission so that the received signal may not be affected.

In order to achieve short switching times when switching from transmitting to receiving and vice versa, the direct current supply circuits have in the present invention been implemented with active components connected in such manner that no resistances which may increase the time constants in switching are included in series with the current and voltage supplies (see Fig. 2). R8 = 0.9 ohms, R7 = 2 ohms, and R20 = 2.7 ohms can be neglected in this regard. This has in combination with the low L1 3 and C1, C2 and C3 values made it possible to keep the switching times below 200 microseconds.

In order to prevent distortion caused by the change of RDHF in the transmitting/receiving switch the direct current supply circuits have in the present invention been implemented as current generators (T3 and T9) which provide the diodes with constant and well-defined direct currents.

In transmission D2 has to have a reverse bias which is greater than the peak value of the high frequency signal of the transmitter.

With 1 200 watts in 50 ohms this peak value is approximately 350 volts. In order to provide a good margin the reverse bias has in the present invention been selected at 420 volts.

If the direct current supply circuit of the receiving diode has been chosen on the same principle as that of the transmitting diode the included transistors would be subjected to a collector-emitter voltage of 420 volts. As transistors which can handle this are expensive a totem pole coupling has been utilised in the present invention in which the voltage over the included transistors never exceeds 300 volts, see also Fig. 2. The function is the following: In transmitting: T15 blocks, which cause T13 to conduct in consequence of it being provided with base current via R32. The voltage of the collector of T15 will then become approximately 280 volts. T13 in its turn provides base current to T11 which then also becomes conductive.

T12 is conductive in consequence of being provided with base current from T14. In this manner - 420 volts is connected to PIN diode D2 which then is reverse biased.

Transistors T9 and T10 block as the result of T6, T7 and T8 not being conductive.

Voltage divider R23, R24 in combination with conductive D4 as well as R22 causes the voltage of the collector of T9 to become - 210 volts. The function of diode D4 is to limit the negative base emitter voltage of T10 to - 0.7 volts. By the voltage of the collector of T9 being limited to - 210 volts, none of the transistors T6, T7, T8, T9 and T10 will be subjected to voltages higher than 210 volts.

In receiving: T6 conducts and delivers base current to the current generator connected transistors T7 and T9 which then become conductive. T10 is conductive in consequence of being provided with base current from T8 which also is conductive. In this manner PIN diode D2 is supplied with direct current.

Transistor T15 conducts, and voltage dividers R32, R33 and R30, R29, respectively, cause diode D6 to become conductive and transistors T13 to block, which in turn causes T11 to block. The voltage over T13 becomes approximately 230 volts. Transistors T14 and T12 block. Voltage divider R25, R26 causes the voltage of the collector of T11 to become approximately - 210 volts.

In transmitting the reverse bias PIN diode D2 alone provides attenuation towards the receiver of approximately 40 decibels, which would implicate a signal of approximately 2.5 volts root mean square over the input of 50 ohms of the receiver at a transmitter power of 1 200 watts. A signal of that magnitude would cause the automatic gain control of the receiver to decrease the sensitivity of the input stage. As this decrease of sensitivity most frequently does not cease immediately in shifting to receiving there is a danger that the beginning of the subsequent message is not perceived. In this invention the relevant problem has been reduced by PIN diode D3 shunting the input of the receiver in transmitting and increasing the attenuation towards the receiver to a minimum of 65 decibels.

Claims (5)

1. A transmitter/receiver switch implemented in PIN diode technology for frequencies in the range 1.5-30 MHz with a transmitted power of up to 1 200 watts, characterised in that included inductances and capacitances having the purpose of isolating high frequency signals from direct current supply circuits are chosen in accordance with filter characteristics, whereby decreased time constants and thus also faster switching times are achieved, and the direct current supply circuits are designed with active circuits coupled in such a manner that no resistances which can increase the time constants when switching from transmitting to receiving and vice versa are in series with the current supplies.

2. A switch according to claim 1 further characterised in that the direct current reverse biases are held constant and well-defined by current generators.

3. A switch according to claim 1 or 2 further characterised in that the maximum voltage over transistors included in the direct current supply circuits is limited to a maximum of 70 percent of the PIN diode reverse voltage.

4. A switch according to any preceding claim further characterised in that in transmission the receiver input is shunted by a conductive PIN diode in order to additionally increase the attenuation towards the receiver in this manner.

5. A transmitter/receiver switch substantially as described with reference to the accompanying drawings.