NMR microwave radio system design and application switch

RF systems typically have many input and output ports, with multi-port network analyzer scattering more expensive price. So the general use of the multiple input multiple output switching signal to switch, then use a relatively simple two-port network analyzer and analyzed. In NMR systems, the channel receiving system generally smaller than the number of the number of antenna coil, it should be applied on route ring switch to switch options.

At present the general design of the switch chip using existing switching function. But most of the switch chip reliability is not good, easily damaged, and power lines are more complicated. Such as SW-437 chip, although you can complete a simple switch function, but it has very high anti-static requirements, general conditions of laboratory and production workshop difficult to achieve the requirements of manufacturers, so the practical application of which are very convenient and easy to damage. In this design, the design of a novel application of pin diodes for RF switching converter circuit, the function of 4-way RF input signal choose any 2-way RF signal output.

Design the overall structure

Switch will be applied to the resonance of the test system is based on LabView software platform provided by the computer to the voltage control signal. The control signal is a digital signal can only provide the high and low and high voltage of 5V, the low voltage is 0V, so the need for voltage conversion can be made available to the switch circuit. The circuit consists of two parts: the voltage conversion circuit and RF switches. In the end, so when the LabView to provide 5V voltage input to the switch voltage 10V and 0V; when LabView to provide 0V voltage, the input to switch the voltage 0V and 10V.

Voltage conversion circuit design:

LabView-based platform provided by the computer to RF switch control signal is a digital signal voltage, extremely high as 5V, low to 0V, while the RF switch control signal is the voltage required 10V, 5V, therefore need to be converted to 10V, Figure 1 for the conversion circuit. When the input signal is 5V, input1, Q3 turns on, Q5 end, Q1 turns on, so output1 to 0V. Then end Q4, Q6 turns on, Q2 ended, output2 output VCC is 10V. In the end, so when the LabView to provide 5V voltage input to the switch voltage 10V and 0V; and when the input signal is 0V, input1, Q3 end, Q5 turns on, Q1 ended, so output1 to 10V. At this point turn Q4, Q6 closing, Q2 turns on, output2 output is 0V, the voltage input to the switch to 0V and 10V. Meet the operating voltage of microwave RF switch.
Switch Circuit:

Design: DC signal control using diode pin diodes off, input RF signal through conduction of the diode output; change control logic, thereby changing the control input RF signal output.

Design steps are as follows:

1) DC Control Circuit Design

Diodes in this circuit is INFINEON technologies used in the company's BA592, the best performance of conduction current is 5mA. So to meet the requirements of diodes added in the design of the control voltage is 10V, circuit resistance R7, R8, R11, R12 size are 10K.

2) According to the exchange scattering signal circuit design requirements

As working in the center frequency of 63.6MHz, a high frequency, so to ensure the matching input and output ports. Specifically, the way the RF signal output, another way the signal should be connected 50R resistor matching. As the DC Circuit both signals another AC signal, so to separate the two, so independently of each other is very important. Frequency of application according to the requirements of 10nF coupling capacitor, the AC signal short-circuit, and circuit for DC signals; application 18μH coupled inductor circuit for AC signal, while the DC signal short-circuit.

3) The basic connection between the module and the module

Figure 2 and Figure 3 is the basic module. Figure 2 is a two-input two-output modules (2x2): In CTRL3, CTRL4 inserted between the 10V DC voltage, that is, increase in CTRL3 10V voltage, CTRL4 0V voltage increases, making diode D6, D9 conduction. At this point the input signal input1 output through the diode D9, the input signal input2 output through the diode D6. When the control signal the reverse, that CTRL4 plus 10V voltage, and CTRL3 plus 0V voltage, the diode D5, D10 turns on, the input signal input1 output through the diode D5, the input signal input2 output through the diode D10. So as to achieve simultaneous output of two input signals, and can be changed by the logic control signal input output direction of purpose.
Two input-output module (2x1): 7,8 control diode control signal off, realize diodes D13, D16 also conduction 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.

Figure 4 is a block diagram of the circuit modules, and clearly that the logical relationship between the module and the signal transmission. For example, when control logic 1111, the input signal input1 and input3 access through the diode from the above input 2x1 output module, due to control logic high, and only from output1 input1 output; the input signal input2 and input4 access through the diode from the following input the bottom of the 2x1 output module, the same control logic as high, only input2 output from output1, thus achieving the four input signals only from the output1 input1 and input2 and output2 output, respectively. When the change of control logic, you can select the desired input signal output. For example, control logic, if it is 1110, the output signal input1 and input4. 4 way control signals to control 12 species of state, corresponding to set up the database, the corresponding program written by LabView applied to the test.

NMR microwave radio system design and application switch

4) printed circuit board design

1. 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 or receiving magnetic field radiation ring antenna. Each module in the design of the shortest path, the RF signal ground, reducing the differential mode radiation; common mode radiation is the ground because there is access to potential caused by the potential of this land 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 of the network and the ground plane with ground cloth as a double edition, all in the upper traces, lower all the shops, the reasonable choice of a ground point; the circuit are high-frequency high-speed circuits to meet the criteria 2W (W is printed plate wire width, wire spacing is not less than twice the wire width), to reduce crosstalk. In addition, the RF wire short, wide, uniform, straight, bend by 45 ° angle, wire width of no mutation, no abrupt corner.
2. Ground design: ground design is the most important design, often the most difficult part. "Ground" can be defined as the signal flow back into the source of the low-impedance 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, completely paved, the sites have no potential difference then. PCB version in production, set the jumpers between modules, so modules independent of each other, that the purpose is: the module can be individually tested, and when the circuit problems, to facilitate detection, quick to identify the problem.

3. PCB version in production, set the jumpers between modules, so modules independent of each other, that the purpose is: the module can be individually tested, and when the circuit problems, to facilitate detection, quick to identify the problem.

Performance and design advantages

1) Because the design is reasonable and symmetry to ensure a certain bandwidth (120MHz) in very low transmission loss, Figure 5 (S21) as shown. S21 which said that: For a microwave network, while other ports are matched, that is connected 50R resistor match, the measured two-port transmission, the physical formula: S21 = Uout / Uin. Curve at the center frequency of 63.6MHz, 120MHz bandwidth, the conditions, to maintain the low transmission loss, is about-0.29dB, and the entire bandwidth performance is stable.

2) The exchange of roles across the inductor and capacitor DC blocking function, input and output ports to ensure a good match, get a good reflection coefficient, shown in Figure 5 (S11, S22)) shown in the center frequency of 63.6MHz Department, Reflection coefficient can reach-30dB or so. Center frequency of the size of the B0 field by the NMR determined the size, the 1.5T system resonance frequency 63.6MHz.

3) to ensure a good isolation, as shown in Figure 6, the center frequency to -30 dB isolation is the following.

NMR microwave radio system design and application switch

4) In practice, the use of high frequency electronic components and one of the most important performance indicators is required for the application environment can not be too harsh, better reliability, difficult to damage. The use of this design 中 了 pin-diodes, circuit reliability, and obviously Tigao, Kefu the Yi Wang of the switch chip easily damaged, poor reliability of the shortcomings.

Modular design and its application in

1. Made of the finished components, the functional circuit and the pin functions shown in Figure 7.

2. Application: As the NMR system in the receive path 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 (4x2) application shown in Figure 8, the logical combination, can achieve 16-way signal output of any 2-channel signal, and then received a received signal imaging system.

NMR microwave radio system design and application switch

NMR microwave radio system design and application switch

Conclusion

As the design is reasonable, the microwave radio frequency switch parameters (reflection coefficient, transmission coefficient, isolation) is very good. The highly modular design makes it easy to fault detection circuit. In addition, the design application of flexible, 4 input 2 output can be like a certain combination of logic input and output combinations. In the NMR system, 16 input 2 output has been widely used