Ask again on the modulated RF signal power measurement
Ask again on the modulated RF signal power measurement
I did a simple measurement.The RF CW signal for example 1GHz is set to 0dBm at the Signal Generator(SG),connect SG to Spectrum Analyzer(SA) with one cable and did the compensation for cable loss,and then the signal peak power on SA is 0dBm.
After that, I added ASK modulation on SG itself which has internal AM option. 10KHz square wave is the modulation and the modulation index is 100%,RF carrier power is kept at 0dBm. Then read the power on SA.If the Span is larger than hundreds KHz,you'll find the wave is jittered up and down,and if the span is about 100KHz,you'll find several pulses on both sides of the carrier.Here the carrier is 1GHz and the sidebands are 1000.01MHz,1000.02MHz,1000.03MHz and so on, and also 999.99MHz,999.98MHz,999.97MHz and so on. All pulses are stable.
My question is that how to measure the modulated RF signal power. The question is asked before at https://www.edaboard.com/thread259589.html, Buark said that the sum of all frequency contributions will be equal to set output power of the SG. I did it and calculated the sum and found the sum is larger then the set power on SG. On SA, read the peak power of 1GHz and get -0.25dBm which is 0.94mW,1000.01MHz is -4.44dBm which is 0.36mW,999.99MHz is -4.36dBm which is 0.37mW. The sum of above three power is about 1.67mW which is 2.2dBm. It's larger than 0dBm set on SG.
This question is from my transmitter measurement in which i need measure my Tx power which is ASK modulated signal. thanks!
If you talk about AM only...
Average Power of AM Modulated signal is defined as
So, a AM modulated signal power will be higher than CW since a (modulation index is greater than 1)
This modulation is multiplication of carrier and modulating wave. Modulation index of 100% means that the magnitude of 10 kHz is the same as that of 1 GHz. So the power is equal too. It means that you are transmitting 2 mW. And if you add to your sum the other components (1000.02MHz,1000.03MHz and so on, and also 999.98MHz,999.97MHz) you should get 2 mW.
The sidebands power is even lower than that of carrier according to modulation index,when modulation index is 100%,the power difference is smallest which is still about -3~-5dB, not the same as you said.
Of course as you and Bigboss said that the power sum of all contributions is larger than the single carrier without modulation.
- - - Updated - - -
I still need a better method to measure the modulated RF power.
If I remember correctly from the prior thread you have access to a power meter. In my opinion this is a much better instrument than Spec An for measuring power of modulated signal. Assuming it is an averaging type sensor, you just adjust the reading by the duty cycle (ie 50% duty cycle, you would add 3 dB to get average power during transmit).
Yes ,but i used the power meter for such measurement before and in my now company, power meter is not available.
I dont understand why you dont like sum of powers of spectral components.
Try another way - there should be modulation signal outputs in your signal generator somekind baseband, LF or IQ. Just measure separately carrier and modulation signal and you should get the same 2 mW, I guess.
With the carrier modulation by meander ASK your spectrum is not concentrated in a narrow band near carrier. If you use a pseudo random sequence (with a period T) as a modulation signal and apply a filter (raised cosine), you then may measure the power in band +/-(1/T) beside a carrier and not count the single components.
The calculations in post #2 and #3 are presuming certain properties of the signal generator. But I don't think that it's obvious how the generator performs ASK.
From a linear AM scheme, we would assume a peak power of +6dBm, because the peak magnitude is doubled. If we assume OOK, the peak power is still 0 dBm. But before jumping into conclusions, I would check with the generator manual.