Advanced Modulation Power Simulator
📡 How Is Signal Power Actually Measured?
Understanding the relationship between carrier frequency, signal amplitude, RMS voltage, and transmitted RF power.
Important Concept
The carrier frequency tells us where the RF signal is located in frequency , but frequency alone does not tell us how much power the transmitter is delivering.
Frequency → Where is the signal?
Voltage / Current → How much power is present?
🔍 What Can We Measure?
1. Frequency
Use a frequency counter, spectrum analyzer, oscilloscope, or SDR to determine the carrier frequency.
Frequency does not directly give transmitted power.
2. Voltage / Amplitude
Measure the RF voltage across a known load. From RMS voltage, power can be calculated.
3. RF Power
A calibrated RF power meter can directly indicate the RF power delivered to a properly terminated load.
From Signal Amplitude to Power
For a sinusoidal RF signal applied to a resistive load, the average power is calculated from its RMS voltage:
Therefore, if the RMS voltage increases, the power increases with the square of the voltage .
📊 What Does "Amplitude" Mean for Power?
Suppose the RF voltage amplitude across a 50 Ω load increases. The power does not increase linearly with voltage. It follows a square relationship.
RF Voltage
10 V
P = 2 W
RF Voltage
20 V
P = 8 W
RF Voltage
40 V
P = 32 W
📻 What About Carrier Frequency?
Consider two carrier signals having the same RMS voltage across the same 50 Ω load:
Signal A
fc = 100 kHz
Vrms = 70.7 V
P ≈ 100 W
Signal B
fc = 200 kHz
Vrms = 70.7 V
P ≈ 100 W
Same voltage + same load → approximately the same power
Changing carrier frequency alone does not determine the transmitted power.
📡 AM Signal: Where Does the Power Go?
In conventional AM (DSB-LC), the total transmitted power consists of the carrier power plus the powers in the upper and lower sidebands.
Thus, for AM, the modulation index μ affects the sideband power and therefore the total transmitted power.
🔬 Practical Measurement Procedure
AM Transmitter
Coupler / Attenuator
50 Ω Load
RF Power Meter
🎓 Remember
Carrier frequency: tells us the frequency position of the RF carrier.
Message frequency: determines the rate at which the AM envelope varies.
Signal amplitude: determines the voltage/current level of the RF signal.
RF power: can be determined from RMS voltage and load resistance, or directly using a calibrated RF power meter.
P = Vrms2 / R
Frequency identifies the signal; amplitude and load determine the delivered power.
RF Measurement Safety
For a real transmitter, do not connect a normal oscilloscope directly to a high-power RF output. Use an appropriately rated dummy load, RF attenuator or directional coupler and a suitable RF power meter/measurement instrument.