Digital filter in ARB mode

Hello,
My question is about VSG60A Arb mode. (Great device, kudos to engineers who’ve built it).

I do not have deep knowledge about digital modulation, so I apologize in advance if my question will be very basic.

I’m using VSG60A to model power amplifier. In order to create a model of the amplifier I need to pass a known signal through it. I compare transmitted and received signals to evaluate amplifier’s effects on the signal.

I’m planning to do my comparison and model calculation in Matlab. I already know how to generate a file with an appropriate format and load it into the VSG.

My question is following. Let’s imagine I want to transmit a QPSK modulated integer sequence. I define some integer sequence ant apply QPSK modulation to it. I get a vector of complex number (it consists of four values: 0.707+0.707i, 0.707-0.707i, -0,707-0,707i and -0,707+0,707i).

Now if I upload this waveform into VSG60A Arb mode, I can stream it at my desired sample rate. But, as far as I understood, it gets filtered in the VSG, so I see a “smooth” signal at the receiver even though in Matlab it is not “smooth” at all.

Can I know the parameters of this filter that is being applied to my signal? Can I control it somehow? It is important for me, since I want to have the signal my amplifier “sees” at it’s input also in Matlab.

I saw that “digital mod” tab allows to control filter parameters and even define a custom filter, so it looked strange to me, that there is no same option in Arb mode.

I’m sorry for a long post, I was trying to communicate my question as clearly as possible.

Thanks.

ABorel,

Based on your comments, I think you want to be using the digital modulation mode rather than ARB. You are specifying symbols rather than I/Q samples. To clarify, it’s common to oversample the constellation symbols by upsampling and filtering with an ISI free filter like the Nyquist filters. If you had done all of the upsampling and filtering, then you would want to use ARB mode.

What I would try instead, is feeding your integer sequence into the sequence editor in digital mod mode. You can either adjust your sequence to match the constellation mapping, or create your own custom constellation mapping to match the one you have been using. Then the software will upsample and filter for you.

Without upsampling and filtering, you are creating sharp transitions, which will be filtered out by both the VSG software digital filters and hardware reconstruction filter which have ~40MHz of bandwidth. I suspect the smoothing effect is from these filters.

Let me know if you have follow up questions.

[ABorel]

Hello Andrew, thank you for your response.

I think I’ve got it, but still I will clarify.

Firstly, I would be glad to use VSG60A digital modulation tab if there would be an option to download Tx signal into Matlab. Is it possible?

Secondly, I’ve attached a screenshot of my Simulink code I use to generate Tx signal. “Square root” is a block which does upsampling and filtering for me. Now if I take “out.filtered” signal and upload it to VSG, then stream it using ARB mode, can I assume that my amplifier will “see” the same signal sequence that I’ve plotted real and imaginary parts of (of course, I mean it will see it as modulated carrier signal) in the second screenshot? I.e. can I assume that those filters you are talking about will not make any noticeable difference?

Thanks.

• This reply was modified 7 months ago by ABorel. Reason: Clarification

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[Andrew]

Unfortunately there is no way to save the waveform in the VSG60 software created in the various modes.

Everything else you said sounds correct. The signal in screenshot 2 looks like a valid modulated signal you can output with the ARB mode.

Let me know if you run into issues.

• This reply was modified 7 months ago by Andrew.

ABorel,
I’m not sure what you’re doing on the receive side, but something like the BB60D, using the digital modulation analysis tools, would be able to lock on and demodulate the (amplified then attenuated) QPSK waveform, and give you an EVM (error vector magnitude) related to the linearity of the amplifier. It can be fairly tricky to check your I and Q symbols without some sort of digital modulation analysis, as there is a delay and phase rotation between the transmitter and receiver, even on the same time base.

[Author]

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