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I write A wave signal and its anti-signal to FPGA (7851R), there is phase shift in out put


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The PC system is Windows 7.I use Labview-2012 to write this program.

I'm a beginner to study Labview. I'll be grateful if anyone could help me on this problem which nagged me for 3 days. Thanks!

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I suspect the problem might be you are essentially trying to do single point output from the RT side of things. The property node on the RT might look like it is doing everything at once, but I don't think it actually does update all the values as the same time.

Normally you would generate a waveform by either doing all the maths on the FPGA itself or using a DMA FIFO or something similar.

If you are determined to do the signal generation on the RT then try replace your 8 controls that you are using to send the points to the FPGA with a single cluster of 8 elements. This will guarantee the "atomic" transmission and might fix your phase shift.

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4 hours ago, Neil Pate said:

I suspect the problem might be you are essentially trying to do single point output from the RT side of things. The property node on the RT might look like it is doing everything at once, but I don't think it actually does update all the values as the same time.

Normally you would generate a waveform by either doing all the maths on the FPGA itself or using a DMA FIFO or something similar.

If you are determined to do the signal generation on the RT then try replace your 8 controls that you are using to send the points to the FPGA with a single cluster of 8 elements. This will guarantee the "atomic" transmission and might fix your phase shift.

  • Thank you very much! 

  • I did send single point from RT side to FPGA.

  • Before I try using DMA FIFO,  I tried two test methods.

  • First I bundled 8 point as cluster and sent it to FPGA. Then unbundled it in FPGA. But the phase shift still existed.

  • Then I transferred the case structure into FPGA,  phase shift still couldn't be fixed. In fact, I test these 4 signal and found that all the values of them were different! I'm going crazy!

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I think you need to start with a simpler example. (And sorry I mistakenly thought you were using RT, you are using an FPGA card in a PC, right?)

Try and make the most simple scenario you can think of. A simple VI generating a single point of the triangle wave at a time. Transfer this value to the FPGA but wire this to all the analogue outputs at the same time. If you still have a phase shift then something really weird is going on.

It has been a while since I used a PC based FPGA card, is is possible the FPGA analogue outputs have different configuration somehow in the .lvproj? Like perhaps different filters or something?

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46 minutes ago, Neil Pate said:

I think you need to start with a simpler example. (And sorry I mistakenly thought you were using RT, you are using an FPGA card in a PC, right?)

Try and make the most simple scenario you can think of. A simple VI generating a single point of the triangle wave at a time. Transfer this value to the FPGA but wire this to all the analogue outputs at the same time. If you still have a phase shift then something really weird is going on.

It has been a while since I used a PC based FPGA card, is is possible the FPGA analogue outputs have different configuration somehow in the .lvproj? Like perhaps different filters or something?

Thanks! I made a sine wave and tried your method, still failed. So maybe there is different configuration  in the .lvproj?  Is there any way to solve this kind of problem?

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What FPGA card are you using? Can you share your project and VIs?

One thing to point out, I would be a little surprised if you can get 1 MHz from your timed loop on the PC. However, this is not your problem though as it would manifest as a not quite perfect sine wave on the output and would not explain a phase shift.

 

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24 minutes ago, Neil Pate said:

What FPGA card are you using? Can you share your project and VIs?

One thing to point out, I would be a little surprised if you can get 1 MHz from your timed loop on the PC. However, this is not your problem though as it would manifest as a not quite perfect sine wave on the output and would not explain a phase shift.

 

I use 7851R card. Of course, I will share my project. You could check the "simple signal.vi" and "simple signal test.vi".

 I forgot to send you pictures which indicated the values all of these 8 line got are different when I stop the FPGA.vi.

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Is your amplitude correct? It looks like you are trying to command nearly +/-2000V output. Also you have a lot of coercion dots with your fixed points. Try to get those to be consistent. It's not as important with floats and integers, but you can get some strange stuff going on depending on how the bits are allocated going from FXP to FXP.

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2 hours ago, Neil Pate said:

I don't actually have the right toolkits installed on my PC so I cannot completely look

If you right click on one of these things, do you get any kind of dialogue box that allows you to configure anything?

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Yes, my dialogue box is showed as following.

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1 hour ago, Jordan Kuehn said:

Is your amplitude correct? It looks like you are trying to command nearly +/-2000V output. Also you have a lot of coercion dots with your fixed points. Try to get those to be consistent. It's not as important with floats and integers, but you can get some strange stuff going on depending on how the bits are allocated going from FXP to FXP.

In fact, if I input 10, it needs to be multiplied by a coefficient to get 10V. The only reason I use I64 point and FXP point is that the date type "DBL" is not allowed in FPGA.vi.

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49 minutes ago, 锋 said:

In fact, if I input 10, it needs to be multiplied by a coefficient to get 10V. The only reason I use I64 point and FXP point is that the date type "DBL" is not allowed in FPGA.vi.

Ah yes, you are reminding me of some specifics regarding the modules accepting integers when used in the FPGA and if the calibration mode is set to "Raw". Here are some notes I have from working with a 9263 (quite some time ago) which should be similar.

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Note explicitly casting types in the host sine wave properties VI (first picture) that produces the properties for signal generation that are passed to the FPGA code (second picture). The FXP constants wired in to the casting configures the bit word lengths.

https://www.ni.com/documentation/en/labview-comms/latest/data-types/intro-fixed-point-numbers/

Perhaps you have adequately addressed this, but I've been burned on this in the past. Anyway, some of these examples might help illustrate the signal generation being done in the FPGA itself if that helps. 

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