Lets say your are sending an RF pulse through a device. It is important to determine the maximum output power from the device before it settles to its steady-state value for the duration of the pulse.
Ideally, there would be no spike at the beginning of the output-pulse; this may damage other devices if they cannot handle the power spike.
How can we measure the power spike?
This measurement is nearly impossible with most oscilloscopes because:
Since the detector will submit a voltage to the oscilloscope- and we want to know the RF power of the spike, a calibration procedure will be needed to convert voltage to dBm. But first, lets go over the test setup.
Test Setup:
The test setup is similar to pulsed power measurements, except now we have added two important components: Variable attenuator and the Crystal Detector.
Why do we need a variable attenuator?
Typically, a detector cannot handle much RF power. During the test, we will constantly adjust the attenuator to known values so that we see enough signal on the oscilloscope, but not so much that we blow-up the detector. After, we can subtract this attenuation out to determine the spike power out of the DUT. The variable attenuator is surrounded by isolators so that when we adjust its value, we will not significantly change the impedance-match seen at the output of the DUT and the match seen at the input of the detector.
Calibration Procedure:
Ideally, there would be no spike at the beginning of the output-pulse; this may damage other devices if they cannot handle the power spike.
How can we measure the power spike?
This measurement is nearly impossible with most oscilloscopes because:
- the spike is typically extremely narrow (time).
- the sine wave that comprises the pulse is difficult to lock onto.
Since the detector will submit a voltage to the oscilloscope- and we want to know the RF power of the spike, a calibration procedure will be needed to convert voltage to dBm. But first, lets go over the test setup.
Test Setup:
The test setup is similar to pulsed power measurements, except now we have added two important components: Variable attenuator and the Crystal Detector.
Why do we need a variable attenuator?
Typically, a detector cannot handle much RF power. During the test, we will constantly adjust the attenuator to known values so that we see enough signal on the oscilloscope, but not so much that we blow-up the detector. After, we can subtract this attenuation out to determine the spike power out of the DUT. The variable attenuator is surrounded by isolators so that when we adjust its value, we will not significantly change the impedance-match seen at the output of the DUT and the match seen at the input of the detector.
Calibration Procedure:
- Calibrate the input arm to the DUT just like we did for pulsed power measurements.
- Characterize the variable attenuator at a number of settings:
- At your Frequency of Interest, use a Network Analyzer to measure S21 of the chain of RF components connected between the DUT and the detector.
- Make a lookup table (I): S21 vs Variable Attenuator dial reading.
- Now when we dial in a setting for the Variable Attenuator, we can know the exact attenuation that is being used.
- Calibrate the Crystal Detector:
- Use the signal generator to inject power directly into the detector and note the voltage displayed on the oscilloscope. (careful not to put more than rated input power of detector)
- Remove the detector and measure the power you injected with a power sensor.
- Create a lookup table (II) for the detector: Voltage vs. power (dBm).
- For a given input power to the DUT, you should be able to measure output voltage of the spike on the Oscilloscope.
- Convert the voltage to power using Lookup Table II. This is power incident on detector.
- Subtract the attenuation of the Variable Attenuation chain using Lookup table I to determine the actual value of the power spike.
ebadanza
ReplyDelete