Performance and application analysis of PIN parallel switch
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Figure 1
Figure 1 shows the performance of an ELL-type switch using the MA4P709 series diodes. These diodes are rated for 3.3 pF maximum capacitance and 0.25 Ω with an RS maximum of 100 mA. In comparison, a simple series connection using the same diode switch will have similar insertion loss to the 100 MHz profile, and isolation will be 15 dB maximum at 100 MHz, falling at a rate of 6 dB per octave.
A tuned switch can be constructed by spacing two series diodes or two shunt diodes by one wavelength, as shown in Figure 15. The isolation value in a tuned switch is twice that obtainable in a single diode switch. The insertion loss of a tuned series switch is higher than that of a simple series switch and can be calculated using the sum of the diode resistances as the RS value in Equation 1. In a tuned shunt switch, the insertion loss can even be lower than that of a simple shunt switch due to the resonant effect of the spacing diode capacitance.
Quarter wave spacing need not be limited to frequencies where the wavelength is short enough to fit discrete lengths of line. There is an equivalent lumped circuit that simulates a quarter wave band that can be used in the RF band. This is shown in Figure 16. These tuned circuit techniques are effective in applications where the bandwidth is approximately 10% of the center frequency.
Send and receive switch
A class of switches is used in transceiver applications whose function is to connect the antenna to the transmitter (exciter) in the transmit state and to the receiver in the receiver state. When PIN diodes are used as components in these switches, they offer high reliability, better mechanical robustness, and faster switching speeds compared to electromechanical designs.
The basic circuit for an electronic switch consists of a PIN diode in series with the transmitter, and a shunt diode connected across a quarter wavelength (λ/4) section (Figure 2), which is, of course, preferable for transceivers operating at longer wavelengths.
Figure 2
Quarter wave line equivalent
When switched to transmit, each diode becomes forward biased. The series diodes present a low impedance to the signal toward the antenna, and the parallel diodes effectively short the antenna terminals of the receiver to prevent overload. Transmitter insertion loss and receiver isolation depend on the diode resistance. If RS is 1 Ω, greater than 30 dB isolation and less than 0.2 dB insertion can be expected. This performance can be achieved over a bandwidth of 10%.
Under receive conditions, the diode is at zero bias or reverse bias and presents essentially a low capacitance CT, which creates a direct, low insertion loss path between the antenna and the receiver. The off transmitter is isolated from this path by a high impedance series diode.
The power PA that the switch can handle depends on the power rating PD of the PIN diode and the diode resistance. For a maximum SWR of the antenna of σ, the equation showing this relationship is as follows:
In a 50 ohm system, the case where the antenna is completely mismatched must be considered, and this equation simplifies to:
Using these equations, it can be shown that using a MA4P709 (or equivalent) insulated stud and a MA4P709-150 stud mounted diode biased at 1 amp with an RS value < .2 Ω and mounted in a heat sink at 50°C, where the MA4P709-985 is rated for 20 watts, a power level of 2.5 kW can be safely controlled even for a completely mismatched antenna. For a perfectly matched antenna, 10 kW can be controlled.
The MA47266 is an axial lead PIN diode rated for 1.5 W at 50°C with a total contact length of 1/2" (12.7 mm). The diode has a resistance of 0.5 Ω (max) at 50 mA. It can then be calculated that a quarter wave switch using 2 MA47266s can handle 40 Watts with a completely mismatched antenna.
It should be noted that the shunt diode of the quarter-wave antenna switch dissipates about the same power as the series diode. This may not be apparent in Figure 3; however, it can be seen that the RF current in the series and shunt diodes is virtually the same. A wideband antenna switch using PIN diodes can be designed using the series diode circuit shown in Figure 4. The frequency limitation of this switch comes primarily from the capacitance of D2.
In this case, a forward bias is applied to D1 during transmission, or to D2 during reception. In high power applications (<50 W), it is often necessary to apply a reverse voltage across D2 during transmission. This can be accomplished by biasing D2 with a negative polarity supply or by passing a forward bias current through D1 through resistor R to apply the required negative voltage.
Figure 3: Quarter wave antenna switch
Figure 4: Broadband antenna switch
The performance and application of parallel switches are mainly discussed. By using the MA4P709 series diodes, an ELL type switch with good performance can be constructed. Compared with a simple series switch, the use of parallel switches can achieve insertion losses similar to the 100 MHz profile, and the isolation degradation rate is 6 dB per octave. Tuned switches are achieved by building two series or parallel diodes separated by one wavelength, and their isolation is twice that of a single diode switch.
Quarter wave spacing can be used to simulate the tuning circuit of the RF band, which is very effective for applications with a bandwidth of 10% of the center frequency. The transmit receive switch is a special switch used in the transceiver, which is implemented by a PIN diode. Compared with mechanical switches, PIN diode switches have higher reliability, better mechanical stability and faster switching speed.
In the transmit state, the PIN diode switch connects the antenna to the transmitter and in the receive state, it connects the antenna to the receiver. Low insertion loss and high isolation can be achieved with proper forward and reverse biasing. The power handling capability of the switch depends on the power rating and resistance of the PIN diode. The authors also provide corresponding equations to calculate the insertion loss and power handling capability.
In summary, this article has discussed the performance and application of parallel switches in detail, and provided some reference equations for design and calculation. This information is of practical significance to engineers and researchers in the RF field.
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