Question on RF signal generator output power

[rf1008]

I have a question on RF Signal Generator.

When i setup the CW output at one frequency,the RF level defined manually is just the output power in dBm. BUT when i setup the modulated RF signal at the same frequency,for instance, 1GHz signal modulated by 40KHz squarewave signal with OOK modulation,40KHz squarewave has 50% duty cycle. My question is whether the actual output power is still equal to the RF level defined manually on the screen?

or the actual output power of the modulated RF signal needs to subtract 3dB result from 50% duty cycle of the OOK modulation? What about when the duty cycle is not defined?


Another, how to measure the modulated RF signal power?I think the result is not reliable from Zero-span screen by spectrum analyzer. Is RF power meter the best method? But it needs the correct trigger for modulated signal measurement.

I need your advice, please.. Thanks a lot!

 

[buark]

Spectrum analyzer is able to show you the power over the frequency range, so if you have a CW signal all of your power is at one frequency, if you use a square wave your total power will be distributed to different frequencies (see Fourier Series & Fourier Transforms of squarewave) but the sum of all frequency contributions will be equal to set output power of the SG.
While measuring the question is in which frequency are you interested in. If you want to measure all the power it is better you use a spectrum analyzer and sum contributions from different frequencies (be sure to sum correctly in dBm). It will not be possible to measure all power with power meter because it has got a limited band width and will miss higher frequency contributions
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[vfone]

Signal generator shows you the peak output power, whatever duty-cycle you have.
The same is for example in TDMA (GSM) signal generators situation, where the duty cycle is even lower (12.5%), but the generator shows the peak power (top of the burst).

 

[tony_lth]

Some power meter measures power using thermal coupler method, so it can measure some low frequency duty cycle power.
Such as, if your power meter sensor is up to 26.5GHz, you can surely measure the 2Ghz signal.

 

[pstuckey]

Another, how to measure the modulated RF signal power?I think the result is not reliable from Zero-span screen by spectrum analyzer. Is RF power meter the best method? But it needs the correct trigger for modulated signal measurement.

I would use the power meter instead of spectrum analyzer for this. Assuming it's an average power detector there is no triggering. You can modify that result by the duty factor to get the average transmit power during the waveform "on time".

 

[BigBoss]

Modulated signals are measured as RMS to my knowledge..

 

[rf1008]

Signal generator shows you the peak output power, whatever duty-cycle you have.
The same is for example in TDMA (GSM) signal generators situation, where the duty cycle is even lower (12.5%), but the generator shows the peak power (top of the burst).

I'll check and confirm it once my signal generator is on side. thanks.

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I really have measured GSM burst power by power sensor(R&S NRPZ11,10MHz to 8GHz),due to the 12.5% duty cycle, the direct power value will be added 9dB to get the final power. I think the measurement is correct, is right?

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Some power meter measures power using thermal coupler method, so it can measure some low frequency duty cycle power.
Such as, if your power meter sensor is up to 26.5GHz, you can surely measure the 2Ghz signal.

I don't understand it well.I really have measured GSM burst power by power sensor(R&S NRPZ11,10MHz to 8GHz,http://nrp-z11.com/),and the measurement result is reliable. NRPZ11 has only BW to 8GHz.. thanks.

 

[vfone]

For GSM you have to measure the peak power during specific portion of the burst, using a triggered RF power meter.

 

[rf1008]

for some applications， the specified instruments are available,such as R&S CMU200 and Agilent 8960 for GSM test including Tx power,WiFi/BT are also has dedicated measurement tools.

But for some "small" applications, need we ourselves try to measure it as reliable as possible.