RF Switch Specifications Explained: Isolation, Insertion Loss, P1dB and Switching Speed

What an RF switch really does

An RF switch routes a signal along one path or another. Ideal switches would pass the "on" path perfectly and block the "off" path completely. Real switches do neither, and the gap between the ideal and the real is described by four numbers.

1. Insertion loss

Insertion loss is how much signal power is lost through the on path, in dB. Lower is better. It always rises with frequency, so a switch rated at 0.5 dB at 1 GHz may be 1.5 dB at 6 GHz. In a receive chain, every tenth of a dB of loss adds directly to your noise figure.

2. Isolation

Isolation is how much the signal is attenuated through the off path, in dB. Higher is better. It falls with frequency, so check isolation at the top of your band, not just at DC.

3. P1dB and power handling

P1dB is the input power at which gain drops by 1 dB from the ideal — the onset of compression. For transmit paths, choose a switch whose P1dB comfortably exceeds your peak output. PIN-diode switches generally handle far more power than small MMIC switches.

4. Switching speed and control

Switching speed is the time to change state, from nanoseconds (GaAs MMIC) to microseconds (PIN diode). PIN diodes are slower but more linear and higher-power; MMIC switches are faster and smaller. Choose based on whether you are doing TDD switching, antenna diversity or pulsed radar.

Choosing the technology

• GaAs / SOI MMIC switches — fast, compact, low control current; lower power handling.
• PIN diodes — high power, high linearity, low loss at RF; slower, and need bias current.
• Electromechanical relays — very high power and isolation, but slow and bulky.

Checklist

• Specify the band and check loss and isolation at both ends of it.
• Confirm P1dB and max input power for your worst-case transmit level.
• Match the control voltage and logic to your driver.
• Budget for the DC bias network (PIN diodes) or the driver (MMIC).

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