Diode parametric language such as Vb, Vf, Ct, Rs, _L, θ, must convert into insertion loss, VSWR, isolation, output power at 1-dB compression (P1dB), input third-order intercept point (IP3), RF operating power, RF power dissipation, and DC power consumption specification terminology.

In addition to actual diode parameters, package parasitics play a significant role in determining switch circuit performance. Package capacitance, package inductance, package electrical resistance, and package thermal impedance are extremely important considerations to determine the effective frequency bandwidth and maximum incident power for reliable switch operation.

The manufacturing methodology dictates the type of diode selection. Surface mount assembly will mandate the usage of either plastic, HMIC SURMOUNT, or MELF and HiPAX ceramic devices. Chip and wire (hybrid) manufacturing will determine the usage of cermachips, flip chips, or beam lead devices. Schematics for some of the most common switch designs: series-exclusive (Figure 1), shunt-exclusive (Figure 2), and series-shunt (Figure 3) are detailed below.

Key Steps in PIN Diode Selection for Microwave Switch Designs The following procedure outlines an effective process for PIN diode selection for switch design:

1. Determine the preferred type of manufacturing for the PIN diode in the switch design: surface mount or chip and wire (hybrid) manufacturing.
2. Determine the frequency of operation and RF power handling of the switch design.
3. Use Table 1, "Relative Switch Performance and Design Evaluation Matrix" to determine the type of switch design that best satisfies the particular switch specifications and requirements.
4. Use Table 2, "Relative PIN Diode Performance Evaluation Matrix" to determine the type of PIN diode that best satisfies the switch design selected from Table 1.
5. Determine the PIN Diode P/N Series that best satisfies the type of PIN diode selected from Table 2.

Click for larger image

For example, the following assumptions could be true for an SP2T design: design is a reflective SP2T, two diodes are used per RF port, frequency bandwidth is 3.0 : 1 maximum. In this example, the series-shunt design is the best in terms of overall switch performance and value. Since each design has a specific advantage, the decision for a switch design selection is determined by the specific design priorities for the requirement.

Click for larger image

Based on the information in Table 2, the following conclusions can be drawn:

• Plastic devices are best suited where cost is a decision driver, the operating frequency is below 4 GHz, and the RF CW incident power is less than 1 W (+ 30 dBm).
• MELF or HIPAX ceramic devices are best utilized where highest average power (above 20 W CW) is the primary design goal and the operating frequency below 1 GHz.
• SURMOUNT devices are probably the best overall compromise in device selection. They can operate (in various bands) from 10 MHz to 20 GHz and perform well with RF incident power below 20 W CW (+ 43 dBm).
• Cermachip devices provide the best overall performance for operating frequency, (100 MHz to 20 GHz), and RF incident power below 200 W CW (+ 53 dBm ).
• Flip-chip devices are best suited for millimeterwave Frequencies below 60 GHz, where the RF incident power below 1W (+ 30 dB ) CW and conductive epoxy or soldering is required.
• Beam lead devices are best suited for millimeterwave frequencies below 60 GHz, where the RF incident power below 0.1W (+ 20 dBm) CW and thermo compression bonding is required.

The following schematics outline some common switch configurations, including series-exclusive (Figure 1), shunt-exclusive (Figure 2), and series-shunt (Figure 3).

Click here to access the related application note and more schematics and information.

Related Articles

SP2T 8 W switch targets WiMAX, base station applications

Basics of selecting the optimal frequency source for an application, (Part 1 of 2)