Turn-on Time of Transmitter Power Amplifier

In this picture Oscium’s WiPry-Combo measures and displays the rising output power of a radio transmitter’s power amplifiers (PAs). Walkie-talkies, cell phones, Wi-Fi transceivers and even satellites orbiting the earth use PAs for transmitting. The PAs are normally off (a low power dissipation state) and only come on when necessary for transmitting.

WiPry-Combo As A Receiver

High power transmitters use an interesting array of circuitry for pulsed and CW power applications while maintaining compliance with regulations (FCC in the US, EU in Europe). Remember that the WiPry accessory is an RF receiver, with maximum input power limits for linear input or no damage, and should be protected from high power signals. For the measurement in the picture above I used the setup described in the following section and diagram.

Conducted Peak-Power Measurement

For designing a measurement setup like this one, I usually start with the output power of the radio and work towards the input of the receiver.

  • The Transmitter - This radio is capable of transmitting 4 or 5 watts depending on the frequency band it’s operating in. There are many online tools for converting between “watts” and “dBm”. Be sure the connector and coaxial cable you are using is rated for the expected power level.
     
  • The Attenuator - The attenuator value is chosen by calculating the difference between the output power of the transmitter and the full scale input of the receiver (usually much less than the maximum). I use full scale input instead of maximum input because I want a linear, uncompressed or saturated measurement from the receiver. You can overdrive a receiver pretty hard before something “pops” but at the expense of degraded measurement accuracy.
     
  • The Receiver - WiPry-Combo is used in the peak power meter mode for measuring the instantaneous output power as the PAs come up. You can make fine adjustments to the front end attenuation (loss between the source and input to the receiver) through the WiPry interface. Adjustments made to the internal attenuator are self-correcting for loss in the measurement results displayed.
     
  • The Display - The integration between the iOS device touch screen interface and Oscium’s WiPry interface make setting up the display quick; quick to learn and quick to use. The image below is a screen shot Using the “pinch-zoom” technique on the display, you can adjust the time span for more resolution on the rising edge - to really see how the PA turns on, or zoom out to see the transmitter transition between on and off. The vertical axis has a fixed resolution of 7.2dB/Div, so you’ll need to adjust your setup to take full advantage of the provided resolution. Adjust the trigger position on the screen to align the waveform within the display.

More on Power Measurements

The next blog post will go into detail about high power RF testing including detailed discussion of a suggested setup with recommended test accessories.