HW Help - Modulation depth vs mean output power.

[mower]

Hello everybody,
I am working on a design task for a communications system class. We are to design an amplitude modulator. The message is 0.5cos(2pi*f*t)+0.5cos(4pi*f*t) where f=1MHz. The system should have a mean output power of 15dBM and a modulation depth of +-5dB. My question is, how can i achieve both? the message signal has a maximum of 1, and a minimum of -0.56, and a mean of 0. Therefore, it is my understanding when the message is at 0, the output power should be 15dBM. How can it swing +5 and -5, if the max and min value of the message are not symmetrical about the mean? Also for more context the carrier is to be 10 GHz

[mikerj]

You wouldn't normally specify modulation index as a +-dB value around the carrier power as this will not give symmetrical modulation.  It's more usual to specify modulation index as a percentage of the absolute carrier power (W, mW, uW etc) e.g. 100% would be full modulation, doubling the carrier power on the peaks and giving zero power on the nulls.

[mag_therm]

I agree, in my day modulation depth was expressed in percent, so -5dB would be 56.2 percent which is OK.
The carrier level can then be determined  and the base power of the combination.
Better to ask your supervisor about the +-dB notation asap.

[Terry Bites]

[jonpaul]

read any classic theory of  communication text book.

Modulation theory (1953) Modulation theory by Harold S. Black

You will learn.

Jon