How to use the AM Modulation Index Calculator
- 1. Choose your dataEnvelope Vmax and Vmin, or message and carrier amplitudes.
- 2. Add power and frequenciesCarrier power, carrier and message frequency are optional.
- 3. See the resultsm, efficiency, power split and spectrum.
AM formulas
The modulation index is m = Am / Ac, or from an oscilloscope envelope, m = (Vmax − Vmin)/(Vmax + Vmin). Total power is Pt = Pc (1 + m²/2); each sideband carries Pc m²/4. Efficiency (power in the sidebands) is η = m² / (2 + m²). The bandwidth is 2fm, with sidebands at fc ± fm. The antenna current rises as It = Ic √(1 + m²/2).
Worked example: Vmax = 10 V and Vmin = 2 V give m = 8/12 = 0.667. With Pc = 100 W, Pt = 100 (1 + 0.444/2) = 122.2 W, each sideband 11.1 W and η = 18.2%. At m = 1, Pt = 150 W and η reaches its maximum of 33.3%.
Exam questions
- A 400 W carrier is modulated to 75%. Find the total power (512.5 W).
- Find m when the antenna current rises from 8 A to 8.93 A.
- Find the bandwidth and sideband frequencies for fc = 1 MHz, fm = 5 kHz.
- What happens when m > 1?
When m is more than 1 the envelope folds over and an envelope detector distorts the message. This is over-modulation, and broadcasters keep m below 1.
Frequently asked questions
What is the maximum efficiency of AM?
33.3%, at m = 1. Even then two thirds of the power is in the carrier, which carries no information. That is why DSB-SC and SSB exist.
How do I find m on an oscilloscope?
Measure the peak-to-peak envelope at its widest (2Vmax) and narrowest (2Vmin), then use m = (Vmax − Vmin)/(Vmax + Vmin).
Does bandwidth depend on m?
No. For a single tone the bandwidth is 2fm whatever the modulation index.
Is this tool free?
Yes. You enter your name and WhatsApp number once to see your result, and all other Uncodemy tools then open without the form.