A new microwave RF switch (4 × 2) design and application
RF systems typically have many input and output ports, with multi-port network analysis analyzed scattering Jiage expensive, Yi Ban by switching pairs of Multiple Input Multiple Output of Xinhao Jinxingqiehuan, and then use a relatively simple two-port network analyzer and analyzed.
MR in the magnetic resonance system, generally the number of channels receiving system antenna coil is less than the number of cycle routes have so many application options switch to switch.
At present, most existing switching devices used to achieve switching. But most of the switching devices low reliability, easy to damage, power lines complicated. Although devices such as SW-437 can complete a simple switch function, but it demands a very high anti-static, laboratory and production workshop in general difficult to meet the requirements of the device, it is not convenient in practical applications, easy to damage. This paper presents a novel application of pin diodes for RF switching converter circuit, 4-way RF input signal choose any 2-way RF signal output.
2 Circuit Design
DC signal control using diode pin diodes off, input RF signal through the conduction of the diode output; change the control logic to control the output of input RF signals. Design steps are as follows:
1) According to the design requirements for DC control circuits
This circuit uses diode Infineon's BA592, the best performance of conduction current is 5 mA. So to meet the requirements of diode plus people in the design of the control voltage is 10 V, loop resistance R7, R8, R11, R12 size are 10 kΩ.
2) According to the exchange scattering signal circuit design requirements
The center frequency circuit 63.6 MHz, a high frequency, so to ensure the matching input and output ports. That is the way RF signal output, another way the signal should be connected 50 Ω resistor matching. As the DC Circuit both signals another AC signal, so to separate the two, so independently of each other is very important. Selected according to the requirements of the frequency coupling capacitance 10 nF, AC signal short-circuit, while the DC signal circuit; and choose 18μH coupled inductor circuit for AC signal, while the DC signal short-circuit.
3) The circuit design of the basic module and the connection between the module
2 output module (2 × 2): In CTRL3, CTRL4 inserted between the 10 V DC voltage, that CTRL3 plus 10 V voltage, CTRL4 plus 0 V voltage, the diode D6, D9 conduction. This time the input signal input1 output through diode D9, the input signal input2 output through the diode D6. When the control signal the reverse, that CTRL4 plus 10 V voltage, and CTRL3 plus 0 V voltage, the diode D5, D10 turns on, the input signal input1 output through the diode D5, the input input2 output through the diode D10. Two inputs to achieve the same output signal, and can be converted by the logic control signal to change the direction of the input signal output purposes.
A new microwave RF switch (4 × 2) design and application
Figure 2 is a 2 input 1 output module (2 × 1): 7,8 control diode control signal off, realize diodes D13, D16 or Diode D14, D15 also turn with the same module 1. But the only way the two input signals the output, while an output connected 50R resistor matching to achieve the two-input all the way out, but can be achieved through the control signal to select which output functions.
A new microwave RF switch (4 × 2) design and application
Figure 3 is a block diagram of the circuit modules, said that the logical relationship between the module, the signal transmission process is as follows: When the control logic 1111, the input signal input1 and input3 through the diode from the above 2 × 1 channel input output module, As the control logic for the high, only input1 output from output1; the input signal input2 and input4 through the diode from the following channels enter the following 2 × 1 output module. Also thanks to the control logic for the high, only input2 output from output1, thus achieving the four input signals only from the output1 input1 and input2 and output2 output, respectively. Similarly, when changing the control logic, you can select the desired input signal output. When the control logic 1110, the output signal input1 and input4. 4 way control signals to control 12 species of state, corresponding to a database, you can write Labview procedures applied to the test.
4) printed circuit board design
Electromagnetic compatibility design: printed circuit board to control the differential mode radiation, the signal and return lines should be close together, reducing the loop area formed by the signal path. Because the role of the signal loop is equivalent to radiation or magnetic loop antenna receiver. Each module in the design of the shortest path, the RF signal ground, reducing the differential mode radiation; common mode radiation is due to ground potential exists to create, and the ground potential is the common mode voltage. When connecting an external cable, cable been encouraging the formation of common-mode voltage common-mode radiation. Control of common-mode radiation, we must first reduce common mode voltage. The design used in ground network and ground plane, cloth into a double version, all in the upper Zouxian, lower all the floors, Gelixuanze Liaojiedidian; the Dianlushuyu high frequency circuit and meet 2W Zhunze Gao Su, W is printed plate wire width, wire spacing is not less than 2 times the wire width to reduce crosstalk. In addition, the RF wire short, wide, uniform, straight, bend by 45 °, wire width of no mutation, no abrupt corner.
Ground design: design is the most important ground. "Ground" can be defined as homologous to the low impedance signal flow path, it can be a dedicated return line can also be a ground plane, and sometimes you can use the product metal. Ideal "to" should be zero resistance entities, then there is no potential difference between the sites. This design, the lower plate into the access floor cloth, fully grounded, then place the potential difference between the no.
In the PCB board production, set the jumpers between modules, so modules independent of each other, so the module can be individually tested performance, when the circuit there Wenti O'clock, and convenient testing, Ke quickly brings-out the problem. 3, the performance and advantages of the design
As the design is reasonable and symmetry to ensure the bandwidth (120 MHz) in the low transmission loss. In the center frequency 63.6 MHz, 120 MHz bandwidth of the conditions, to maintain low transmission loss of about -0.29 dB, but in the entire bandwidth performance is stable.
Since the exchange effect across the inductor, capacitor DC-blocking effect of the input and output ports to ensure a good match, get a good reflection coefficient, the center frequency of 63.6 MHz Department, the reflection coefficient of -30 dB or so. Frequency is determined by nuclear magnetic resonance of the B0 field, for the 1.5T system resonance frequency 63.6 MHz. Ensure good isolation, the center frequency to -30 dB isolation is the following.
In practice, the use of high frequency electronic component performance and the most important indicators is the application of environmental requirements can not be too harsh, high reliability, and easily damaged. The design of manipulation to cut off the feet of the pin diodes, switching devices easily overcome in the past damage, poor reliability shortcomings.
4 modular design and its application in
4.1 Modular Design
According to the practical application of the RFSW (4 × 2) test case made by the 4-way RF input, 2 RF output port and 4-way digital control signals, whose function circuit and the pin functions shown in Figure 4.
A new microwave RF switch (4 × 2) design and application
4.2 Application
MR system in the MRI often need to test the coupling of two loop routes, which tests the transmission parameters of the two lap route S21, but in general there are many road coil, such as knee coil with 8, 2-way connection need to be tested. Application RFSW (4 × 2) test box to connect the two signals need to be tested, changing the logic control signal to turn can be measured.
MR system due to magnetic resonance receive channels is much smaller than the number of the number of its antenna coil, so it is necessary to switch the selection application switches. One application example is the use of seven RFSW (4 × 2) to achieve 16-way signal any two signals in the output logic of the combination of the received signal and image receiving system. RFSW (16 × 2) combinations of logic diagram shown in Figure 5.
A new microwave RF switch (4 × 2) design and application
5 Conclusion
In nuclear magnetic resonance (MR) systems, require the application of microwave RF switch to switch channels to receive the coil selection. Application of pin diodes circuit design, reliability has been improved to address the poor reliability of the general switching device, easy to damage problems. As the design is reasonable, the microwave reflection coefficient of RF switch, transmission coefficient, the isolation is very good. The highly modular design makes it easy to fault detection circuit. In addition, the application of this design is very flexible, 4 input 2 output can be some combination of the logical combination of input and output we want, in the MRI system, the 16-input 2 output has been widely used.