SSG IQ power conversion methodology

Siglent's RF vector signal generators including the SSG5000X-V and SSG6082A-V have the ability to load and produce arbitrary or designed IQ waveforms on carrier. These waveforms may be created in SigIQPro or from a variety of customized data sources. They can be loaded into the instrument in a number of ways. See the related articles for more information on this.

These generators load the IQ waveform and then the user sets an output level in power for the signal. The internal system of the generator calculates this by finding the average Vrms of the I and Q data and setting up the instrument so that the average RF output is at the requested level in dBm.

This is a standard method for setting up the instrument, but for some types of custom signals engineers may have different needs. One good example is arbitrary IQ data that contains a RF pulse as well as off time. In this case, the engineer may want to set the peak power during on time, but depending on the duty cycle the peak power and average power can vary significantly.

In this case, there are some formulations that can be used to estimate the setting level needed to attain accurate peak output levels. 

To start, we note that the combined power of an I and Q signal in dBm, when given the I and Q values in Vrms is 

=10*LOG(10*(I^2+Q^2))

Specifically, for pulsed RF signals we also note that the relationship between average Vrms and peak Vrms for an On/Off pulse with fixed duty cycle (DC) is: Average_Vrms = Peak_Vrms * SQRT(DC)

For truly arbitrary I and Q data, you would need to estimate the ratio of Peak Vrms to Average Vrms yourself.

These formulas allow us to back calculate the difference. Now, given the known power level the user wants to output at peak dBm (or Vrms) which is a simple conversion in a 50 Ohm system, and a better understanding of the ratio of average to peak power in the IQ signal as well as how I and Q power are combined, we can create a worksheet that calculates the correct average dBm power setting to input into the instrument in order to attain the correct peak power level.

You can download our Excel worksheet for this conversion from here.

This may be a useful tool for estimating the average power setting for IQ waveforms that require a known peak power level.

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