High Power Gain compression Test

So you've got your hands on an RF amplifier or similar 2-port device and want to measure its Gain or Output power as a function of Input power.

An ideal amplifier will have the same Gain at every given input power, as seen by the blue curves below. In practice, there is power lost (mostly in the form of heat dissipation) as you increase the input power. The red curves are more of a typical real-world response. This post will provide you with the techniques to measure these red curves so that you can compare them with their ideal counterparts.

Before you get started, you need to know the following information to gather the appropriate test equipment:

• Frequency(ies) of operation
• Maximum input power delivered to the (device-under-test)DUT
• This is usually dependent on the anticipated Gain-compression point of your device; Most tests deliver a maximum input power when the Gain is compressed by ~4 dB.
• Small-Signal Gain of the DUT
• Pulsed or continuous wave (CW) power deliver to DUT
• You may wish to do a pulse-measurement in order to reduce over-heating of components and to prolong the life of your DUT. This will be covered in a later post.

List of Test Equipment {all must operate at your frequency-of-interest (FOI)}:

• Signal Generator that can output a clean signal with good dynamic range (> 35 dB)
• High Power Amplifier (capable of delivering the required max input power to the DUT)
• Isolators, Attenuators, RF loads, couplers
• 2 Power Sensors {with meter(s)}

Here is a block diagram of your test bench:

Choose an amplifier that will boost your Signal Generator power sufficiently enough such that it will compress your DUT. Isolator #1 is to prevent any reflected power from the amplifier from going back into your signal generator; power reflected into the Sig Gen will damage it. Isolator #2 is to provide a good output-impedance match to your DUT.

Power Meter (& coupler) #1 is to measure Pin at the input of the DUT. The coupler and attenuator will have to be compensated for via the calibration sequence (next section). Power Meter #2 is to measure the output power.

The optional Power Meter (& coupler) #3 is used to measure the reflected power from the DUT. Return loss (in dB) can be found at any Pin by measuring Preflected (#3) - Pin (#1), in dBm.

{Choose attenuation values for each power meter so that they don't get damaged. For instance, if you expect your DUT to output 45 dBm max, and your power meter is rated for 20 dBm, you MUST provide >25 dB attenuation between the DUT-output and power meter #2.}

Calibration:
Zero and calibrate all power meters.
Calibrate Meter #1 for losses through the system:

• For now, Replace the DUT with output Power Meter #2.
• Turn on your signal generator and amplifier and adjust the power until you read 0 dBm on Meter #2.
• Adjust the Offset on input Power Meter #1 until it also reads 0 dBm. (If there is no "offset" function, just note the difference in power levels.)
• You can now accurately know the input power level to your DUT for any Sig Gen setting.

Calibrate Meter #3 for losses through the system:

• For now, Replace the DUT with an RF short (or leave it open, but a short is better).
• Turn on your signal generator and amplifier and adjust the power until you read 0 dBm on Meter #1.
• For an RF open, all power should be reflected back toward the source; Adjust the offset on Meter #3 until it reads 0 dBm too. Meter #3 is now calibrated.

Calibrate Meter #2 for losses through the system:

• For now, remove the DUT from the setup and connect coupler #2 directly to isolator #2.
• Turn on your signal generator and amplifier and adjust the power until you read 0 dBm on Meter #1.
• Adjust the offset on Meter #2 until it reads 0 dBm. Meter #2 is now calibrated.

Measurement:
For each Pin observed on Power Meter #1, you can record:

• Gain in dB (Meter #2 - Meter #1, in dBm)
• Pout in dBm (Meter #2)
• Preflected in dBm (Meter #3)
• Return Loss in dB (Meter #3 - Meter #1, in dBm)

If you want to get fancy, you can set up some software to take these measurements for you. The most commonly used is Labview. But you will need someone to write the code for you.

...Next time: Pulsed power measurements.