How to measure instantaneous RF power
Figure 1 shows the in-phase (I) and quadrature (Q) components of a 16-state quadrature amplitude modulated (16 QAM) baseband signal in the channel traces 1 and 2, respectively. The X-Y display shows the state transition diagram for the signal. There are distinct average power levels corresponding to the three possible combinations of the I and Q voltage levels of 0.58 and 0.18 V peak-to-peak levels.
The power levels associated with each combination of I and Q states can be determined by taking the square of the rms values of each component and adding them together. Note that since we will be taking power ratios, the power is expressed as a squared voltage,V2, to simply the measurements. If required, the square of the voltage can be divided by the impedance using the rescale function in order to read in Watts. The power levels are:
PA = 0.085 V2
PB = 0.047 V2
PC = 0.0083 V2
The expected average power in the waveform based on the 16 states included can be calculated as:
(4 PA + 8 PB + 4 PC) /16 = 0.046 V2
Figure 2 shows the steps used to measure the instantaneous power waveform and derive the peak and average power levels. The instantaneous power of the output signal is calculated by taking the sum of the squares of the I (F1) and Q (F2) waveforms. This is displayed in trace F3. The average and peak values were measured using the mean and maximum parameters reading over the sixteen states. The average power is read from the mean parameter as 46.4 mV2. This compares well with the expected value. The peak power is read by maximum parameter as 174 mV2.
Based on the measured values the ratio of peak to average power is:
PPEAK/PAVG = 174/46.4 = 3.75
or 10 log (3.75) = 5.7 dB
See EDN collection: Oscilloscope articles by Arthur PiniRelated Articles
Product How To: Use multipath diode sensors to make precision power measurements
Understand ripples in RF performance measurements: Theory and Experiments
If you liked this feature, and would like to see a collection of related features delivered directly to your inbox, sign up for the Test & Measurement newsletter here.