Calculator Inputs
Example Data Table
| Waveform | Impedance (Ω) | Voltage (Vrms) | Load Power (W) | Load Power (dBm) |
|---|---|---|---|---|
| Sine | 50 | 1.000 | 0.020000 | 13.010 |
| Sine | 50 | 2.236 | 0.100000 | 20.000 |
| Square | 75 | 1.500 | 0.030000 | 14.771 |
These are ideal calculations; real systems can differ due to mismatch and harmonics.
Formula Used
- P = Vrms2 / R (load power from RMS voltage)
- P = Irms2 · R (load power from RMS current)
- Vrms = √(P · R), Irms = √(P / R)
- dBm = 10·log10(P(mW)), dBW = 10·log10(P(W))
- Power ratio from loss: factor = 10(-Loss/10)
- Delivered scaling = Efficiency · Duty (entered as percentages)
Waveform conversions: sine uses √2, square uses 1, triangle uses √3 between RMS and peak.
How to Use This Calculator
- Select the input mode that matches your known quantity.
- Enter impedance, waveform, and any loss or scaling values.
- Provide power, voltage, or current with the correct unit type.
- Press Calculate to view results above the form.
- Use the export buttons to download CSV or PDF results.
Professional Notes on RF Power Calculations
1) Why RF Power Matters in the Lab
RF power sets the operating point for mixers, amplifiers, antennas, and filters. A few dB can shift gain compression, intermodulation, and noise performance. When you document power at the source and at the load, your measurements become repeatable across cables, attenuators, and test fixtures and compliance audits.
2) Typical Power Levels You See
Many signal generators deliver from about −120 dBm up to +20 dBm, while benchtop amplifiers can reach watts and beyond. For perspective, 0 dBm equals 1 mW, 10 dBm equals 10 mW, 20 dBm equals 100 mW, and 30 dBm equals 1 W.
3) Converting Watts to dBm and dBW
Because RF spans huge ranges, logarithmic units simplify comparisons. dBm references 1 mW, while dBW references 1 W. This calculator converts both ways so you can read datasheets, set instruments, and report results in the unit your team expects.
4) Using Impedance Correctly
Most RF systems assume 50 Ω, some video systems use 75 Ω, and certain audio or telecom interfaces use 600 Ω. Power from voltage depends on load impedance: doubling impedance halves power for the same voltage. Always use the impedance at the measurement point.
5) RMS, Peak, and Peak-to-Peak Inputs
Oscilloscopes often show peak or peak-to-peak, while power calculations need RMS. For a sine wave, Vrms = Vpk/√2 and Vrms = Vpp/(2√2). Square and triangle waves use different factors, so waveform selection is essential.
6) Accounting for Loss, Efficiency, and Duty Cycle
Real setups include cable loss and component insertion loss. A 3 dB loss cuts delivered power by roughly half. Efficiency matters for transmit chains, and duty cycle matters for pulsed RF. This tool applies a linear power ratio from dB loss and scales power by efficiency and duty.
7) Practical Measurement Workflow
Start by defining your reference plane: instrument output, amplifier output, or load input. Measure voltage or current where possible, or read power from a meter. Enter loss values for known cables and pads, then compare the calculated load power to device ratings and compliance limits.
8) Interpreting Results for Design Decisions
Use load voltage and current to check component stress and connector limits. Use source power to verify generator or amplifier headroom. If your target is a specific load power, back-calculate the required source power and confirm the chain can deliver it without overheating or distortion.
FAQs
1) What is the difference between dBm and dBW?
Both are logarithmic power units. dBm is referenced to 1 milliwatt, while dBW is referenced to 1 watt. Convert between them by adding or subtracting 30 dB because 1 W equals 1000 mW.
2) Why does impedance change the calculated power?
Power from voltage depends on the load: P = Vrms²/R. With the same Vrms, a higher impedance reduces current and lowers power. Always use the impedance at the point where the voltage or current is measured.
3) Should I enter source power or load power in Power mode?
Use “Input is source power” when you know the generator or amplifier output. Use “Input is load target” when you want a required source power for a desired delivered load power after loss, efficiency, and duty scaling.
4) How do I use Vpp readings from an oscilloscope?
Select Voltage mode, choose Vpp, and pick the correct waveform. The calculator converts Vpp to Vrms using waveform factors and then computes load power from Vrms and impedance.
5) How should I estimate cable or component loss?
Use datasheet insertion loss or measured loss at your frequency. Enter loss in dB as a positive number. For example, 3 dB approximates a 50% power reduction, and 10 dB is about a 90% reduction.
6) What do efficiency and duty cycle represent here?
Efficiency models how much source power becomes RF at the load, and duty cycle represents pulsed operation. For continuous-wave signals, set duty to 100%. If you do not need these factors, set both to 100%.
7) Does this calculator include mismatch, VSWR, or reflections?
No. It assumes matched conditions at the entered impedance. For significant mismatch, measured power and voltage can differ from ideal relationships. Use a directional coupler or VNA data to quantify reflected power separately.