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##### AWR VSS sinewave spectral power level

03-22-2013
03:38 AM

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Hi all,

Could a fellow AWR user kindly help me with answering a question regarding my usage of a power spectrum measurement - using VSS's SPECTRUM -> PWR_SPEC measurement feature?

It is a simple question, but I believe that I will need the help of experienced AWR users to help me out.

In VSS, I simply used a sine-source to generate a 1 kHz sinewave with an amplitude of 1V (peak)....in other words 2V peak to peak. Then I simply put this sinusoid signal through a Real to Complex (R2C) converter module. And then I set up two simple VSS measurements. One measurement was just to observe the time-waveform of the sinewave - to make sure that I was getting 1 kHz and 1V (peak amplitude). The other measurement simply involved using PWR_SPEC measurement to look at the spectrum of the sine-wave.

For a 1V peak sinewave, I calculate that the RMS amplitude is 1/sqrt(2). And if the reference resistance is 50 ohm, then I calculate that the power in the sinewave is (Vrms^2)/50, which is 10 mW (ie 10 dBm).

For the spectrum of the sinewave, I expect to see two spectral lines, one at 1 kHz with a component amplitude of 0.5, and one at -1kHz (negative frequency) also with a component amplitude of 0.5. In terms of power, and with a reference resistance of 50 ohm, then the power in the 1 kHz component is (V^2)/50 = (0.5^2)/50 = 5 mW. And the power in the -1 kHz component is also 5 mW. So the sum of the power in these two components gives 10 mW, which is what I am expecting because this total is equal to the RMS power calculated earlier (in the sinewave).

Now, in the VSS simulations, the simulation results are indicating that the power in the 1 kHz component is 2.5 mW (ie 3.979 dBm). And VSS simulation results also indicate that the power in the -1 kHz component is 2.5 mW (3.979 dBm). So from the results of the simulation, the total power is 5 mW, which is exactly one-half of the value that I'm expecting.

So at the moment, it appears that the spectral power levels of components from VSS simulations are always 3 dB LESS than what I calculate. So half the power is being lost somewhere.

May I ask fellow AWR users if I'm setting up the simulation correctly (or not), and steer me in the right direction? I believe that I'm just not using the simulator correctly, and just need some pointers to put me on the right track. Or if I'm setting things up correctly already, then am I missing an important step such as a calibration (for measuring at some other reference point)?

Thanks very much for your help in advance. Also, there is one other small question too....I searched the internet and AWR guides, and there appears to be no information about what a .VIN file contains. I notice that AWR files have .emp project files, and it generates a .vin file too but I haven't come across information that discusses .vin files.

(also, I have attached my simple sinewave AWR project file)

Thanks again.

Kenny

4 REPLIES

03-25-2013
03:07 PM

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What you describe in the first few paragraphs is correct. You can see this behavior in VSS if you use a SINE block with its output node set to Real. Place a test point at its output and create a PWR_SPEC measurement with Frequencies Displayed to set to All. In this case you'll see two components, at +1 and -1 GHz, each at 7 dBm. Set Frequencies Displayed to Spectrum Analyzer style and you'll see a single tone at +1 GHz at 10 dBm since in this case the negative frequencies are folded.

If you set the output node of SINE to Complex or Complex Envelope, you'll see only one tone at +1 GHz at 10 dBm, as expected; please see the help of SINE block for the definition of the complex envelope signal. Also, please note that Real and Complex Envelope are two different representations and you should be careful when transforming a signal from one to the other - there can be a factor of 3 dB if this transformation is done incorrectly.

When you use the R2C block, you are effectively creating a complex signal with its imaginary part set to 0. Hence, it will contain only half the power of the complex signal generated by a SINE block with a complex output node, which you'll see on your power measurement.

To avoid any confusion in the future, use a SINE block with a complex output and everything will be scaled properly. All modulated signal sources also have complex outputs.

The VIN file contains information about the window layout. Everytime you close a VSS project, the window layout is saved in the VIN file. If you delete the VIN file and then you open the project (EMP file) you'll see that all windows are closed and you'll need to open them manually.

03-25-2013
07:06 PM

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Thanks very much for kind reply Gent_Paristo! Very much appreciated.

You mentioned that the usage of a SINE block with complex output will allow everything to be scaled properly. So if I use a SINE block and set the output to COMPLEX output (instead of real output), then it would mean that the Real-to-Complex (R2C) module can no longer be used because the output of the SINE block is complex already, and the R2C module accepts REAL data. So if I remove the R2C module and just carry out a PWW_SPEC measurement at the output of the SINE block, then the PWR_SPEC measurement will give me a positive frequency peak only.

This is ok....although, I was trying to obtain a spectrum that has *positive* and *negative* frequencies for frequency components of the 1 kHz (1V peak) sine-wave signal, so I was just expecting to see 7 dBm peak at +1 kHz and +7 dBm peak at -1 kHz. And, with the R2C block removed, I get a single dBm peak at + 1 kHz (which is the correct!). But is there a VSS configuration that gives us the +ve and -ve frequency peaks instead?

Also, regarding the operation of the R2C block. I believe I'm lacking full understanding of how it works. I originally thought that VSS required a conversion of the real data (from the sine source) to complex data ONLY for purposes of frequency analysis and processing. But, as far as I understand at the moment, the sine source outputs a real-valued signal, such as cos(wt) ---- but don't understand why converting to a complex signal (by simply making the imaginary parts equal to zero) results in HALF-of-original-power at the output of the R2C converter. I was thinking that if a signal is REAL, then the complex output representation is also essentially 'real' (ie the complex parts are just all zero). So I don't understand why the conversion process causes loss in power. Is the reason for the power loss easy to explain or is there a web-link where I can learn more to understand the power loss? Thanks very much again Gent.

Also, thanks for helping me to understand the purpose of the VIN file Gent. Very greatly appreciated. And thanks for your time again for replying to my original post Gent!

03-25-2013
07:31 PM

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The SINE block with a complex output, by default, uses the complex envelope representation with a center frequency equal to that of the sine. Hence, if you place a Waveform measurement at its output it will show a constant (1+j0) output. Setting the CTRFRQ=0 will allow you to "center" the signal at DC and see a sinusoid at its output. The spectrum of a complex tone signal will contain only one tone at +FRQ value. The two spikes at +FRQ and -FRQ, each with 7 dBm power, will only appear for a real signal if you display all frequencies in the spectral measurement.

You can find detailed info on this topic, along with the corresponding math, by doing a search on "complex envelope representation" online. The bottom line is that the complex representation of a tone signal will be A*[cos(2*pi*f*t) + j*sin(2*pi*f*t)]; however, when you use R2C you get only the real part, A*cos(2*pi*f*t), therefore only half the power you'd get from a SINE block with a complex output node.

03-25-2013
08:54 PM

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Hi again Gent! I definitely see what you mean now regarding the complex representation of a tone signal. That really came clear after you mentioned that the complex representation of the tone signal will be A*[cos(2*pi*f*t) + j*sin(2*pi*f*t)]. Thanks for all your help again Gent. I will definitely pass on to others what you explained to me - to others. The information you kindly provided will go a long way to help others too. Very greatly appreciated. Thanks again Gent! (also - thanks for that reference information - I will refer to that one).

==

I will just attach two project files, one with complex output for the sine source, and the other with real output. Both projects are set up with the 'frequencies displayed' option (in the measurement's options) set to 'ALL' (instead of 'spectrum analyzer style'). The spectral peaks obtained from the measurements (for both of these projects) are at the correct 7 dBm level (thanks to the help of Gent).

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