ON-OFF modulation

[tucoi98]

Hi guys,

I am currently working on implementing a ON-OFF transmitter. Since a ON-OFF signal is very similar to a ASK signal, as a first step I tried to take benefit of the ASK VIs that LabVIEW provides (MT Generate System Parameters, MT Generate Filter Coefficients and MT Modulate ASK).

However, I am not getting the result that I am expecting.  Please, find in attachment two screenshots that show how is the signal that I want to get and how is the actual signal that I get.

If you please can guide me a bit, I would be very grateful!

Thanks!

[LekeR]

Hi tucoi98,

If you could attach your source code where you are generating those two graphs (expected and actual) would be very helpful, so that I can troubleshoot your issue further and get back to you with alternatives or solutions.

Looking forward to hearing from you.

Best regards,

Leke Raifi

AE Netherlands

Certified LabVIEW Associate Developer

[tucoi98]

Hi,

Thanks for your reply. 

Unfortunately I cannot attach the code used to generate the "expected signal" (since I do not have access to the code).  However, I can provide you the code that I used to generate the "actual signal". Basically I built a ASK signal and used the "scale.VI" to have values in the range 1, -1 (as the range of the expected signal)

Any suggestion on how to improve my design?

Thanks!

[LekeR]

Hi Tucoi98,

I had a first look at your transmitter block diagram and I saw that the MT Generate System Parameters, MT Generate Filter Coefficients you have put inside the loop whereas the modulation itselft expects the raw baseband IQ signal at the input. Therefore my suggestion would be to put only the MT Modulate ASK inside the loop and try out if this will generate the expected signal. Unforturnately, I don't have a NI-USRP in the office right now to try out your source code but I would suggest you to try the above comment and let me know of the outcome. 

Also, a second option would be to demodulate the signal yourself (without using MT Demodulation ASK.vi), as OOK is quite simple to handle. 
You can take some inspiration from the following tutorial --> http://www.ni.com/white-paper/13192/en/ 

Looking forward to hearing from you.

Best regards,

Leke Raifi

AE Netherlands

Certified LabVIEW Associate Developer 

[tucoi98]

Hi LekeR,

I tested what you suggested (I only leave the MT Modulate ASK.vi inside the loop) and did not get anything, meaning that the shape of the "actual signal" is very different compared with the expected one.

I am quite convinced that there must be other ways to achieve a perfect ON-OFF signal without using the MT Modulate ASK but I did not find any example. Could you please suggest me how to continue? I have already done many tests...

Regards,

Tucoi

[ErikL]

Note they is a DC filter in the USRP at the center of the LO. You will want to transmit just of center by on-off modulating a sine wave or move the LO using the advanced property node. The DC filter is made to correct DC offsets in the ADC and is implemented in the FPGA.

[tucoi98]

ErikL,

Thanks for your message. Could you explain in more detail how to do it, please? it is not very clear now...

Tucoi

[ThiCop]

Hello Tucoi,

I have made a small word-doc tpo show/illustrate how you can get to the specific property node.

Are you familiar with Property Nodes?

[tucoi98]

Hello ThiCop,

I know what a property node is, but I do not know how I can play with this parameter that in order to achieve a OOK signal. Could you explain it in more detail, please?

Thanks!

[ThiCop]

Hello,

To be able to correctly understand what is going wrong more complete information will be needed.

It is not a 100% clear to me where it is going wrong and what you exactly want to achieve.

1) Can you tell me what you exactly mean with the "signal(actual)"?

For me personally this would always correspond to an actual measured signaL

However, this does not seem to be the case over here.

1a) If this is also the meant definition at your side, then please let me know with which device (USRP or not) you are measuring the signal. (including used settings)

1b) If this is not what you mean with "signal (actual)", then please explain as accurately as possible what this corresponds to.

As far as I have understood you are in case 1b. The "signal (actual)" is the IQ data you seem to get after using the Modulation Toolkit VIs.

Please confirm that this understanding is correct.

Please also tell me how you determine/retrieve the data concerning this "signal (actual)"

Which Graph indicator in the provided VI shows this?

2) Can you tell me what you exactly mean with the "signal (expected)"?

Is this what you would like to generate yourself?

Is this the baseband IQ data you want to generate?

Or is this rather the actual time domain data of the "upconverted" RF signal (around the 868.3 MHz transmitter frequency)?

Thanks in advance for your feedback!

Once I fully understand what you mean and want to accomplish, it should be possible to provide advise from my side.