RMS Power
Understanding RMS Power
RMS power is the most important power metric for RF systems because it determines the actual heat dissipation, energy transfer, and long-term component stress. Peak power matters for voltage breakdown; RMS power matters for thermal effects.
Power Definitions
- RMS/Average power: Time-averaged power. Determines heating. What power meters typically measure.
- Peak power: Maximum instantaneous power. Determines voltage breakdown. P_peak = P_RMS + PAPR (dB).
- CW power: For a pure sine wave, P_peak = 2 x P_RMS (3 dB PAPR).
RMS Voltage and Power
V_RMS = V_peak / sqrt(2)
P_RMS = V_RMS^2 / R = V_peak^2 / (2R)
50-ohm system examples:
0 dBm = 1 mW = 224 mV RMS = 316 mV peak
10 dBm = 10 mW = 707 mV RMS
20 dBm = 100 mW = 2.24 V RMS
30 dBm = 1 W = 7.07 V RMS
40 dBm = 10 W = 22.4 V RMS
Frequently Asked Questions
What is RMS power?
RMS power is the time-averaged power in an RF signal. For a CW sinusoid, P_RMS = V_peak^2/(2R). RMS power determines component heating, power handling limits, and is what most power meters measure.
What is the difference between RMS and peak power?
RMS is the average power. Peak is the maximum instantaneous power. For CW: peak = 2x RMS (3 dB). For OFDM with 10 dB PAPR: peak = 10x RMS. Power meters measure RMS; voltage probes can measure peak.
Why is RMS power important for thermal design?
Component temperature rise is proportional to average (RMS) heat dissipation, not peak power. A 100W peak signal with 10% duty cycle dissipates only 10W average, requiring the same heatsink as a 10W CW signal.