How To: RF Power FETs

The figure above shows WiPry-Power measuring the pulsed RF output of a high power (250 Watt) transistor. Power transistors are almost always used exclusively in pulsed applications. There are transistors capable to 250 W, 500 W, and even 1400 W, but only with pulsed signals and would not survive if used CW.

Measuring RF With Power Meters

Power meters like WiPry-Power are very useful for measuring key parameters, like the following:

  • Transitions: Rise Time and Fall Time
  • Timing: Pulse width and pulse (repetition) frequency
  • Amplitude: (Gain) Flatness over carrier frequency
  • Droop: Droop over a single pulse and over a series of pulses
  • Power: Measure peak and average power for a pulse
  • Time: Adjust delay and offset to include transitions or just the center in the measured pulse

Programmable Offsets For Attenuation

WiPry-Power has two ways to accommodate for RF attenuation in the setup. The application that runs on iOS devices includes separate dials for attenuation external to the WiPry accessory, and a programmable attenuator internal to the WiPry accessory.

Example: Suppose the output of an amplifier is 150 Watts and is terminated in a 50 Ohm load. To measure the RF safely, a directional coupler is inserted between the amplifier and the load, with the coupled power used for the measurement. Couplers are described by how much of the incident power couples over to the coupled port. A 20 dB coupler, therefore would pass the incident RF to its output with minimal loss, typically less than 0.5 dB, and puts (Incident RF – 20 dB) to the coupled port. The WiPry external attenuator dial can be used to “add” the 20 dB lost through the coupler back into the result.

iMSO Useful In Verifying Setup

Transistor amplifiers have individual Gate and Drain voltages that must be set properly to obtain the desired operating point. Oscium’s iOS accessories are incredibly useful for the pictured setup. The hand held oscilloscope was used to precisely set and verify the Gate and Drain voltages. After the DC bias is set, the WiPry accessory replaces iMSO for measuring the RF signal already described. The iPhone or iPad is the instrument; choose the correct accessory for the measurement.