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Modulation and upconverter application in Labview

Dear all,
I am working on a wireless system. A block diagram was created to simulate detection of the Psuedo Random Bit Sequence in noise environment. It involves the use of an ASK modulator and Upconverter. I set the parameter as stated below.
1. Carrier frequency: 100MHz
2. Samples per symbol: 300
4. Symbol rate: 1M Symbol per sec.
I got the error of "Invalid dt: To satisfy the Nyquist criterion, the input sampling rate (d/dt) must be strictly greater than 2* carrier frquency:
Actually, my setting should fulfill the Nyquist criterion. I do not know what happens.
I would be grateful if any experts can help in this program.
Attached is a Block diagram of the system.
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Message 1 of 6
Hi mr Tam,

You state that each symbol lasts 300 samples. At 100 megasymbols per second you get 300 Msamples per second. At 100 MHz you can only transmit 50 MSamples/s.

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Message 2 of 6

Hi Ton,

Thanks for your answer.

For 100MHz carrier frequency, sample rate should be 200MHz or more. 50MSamples/s is just 1/2 carrier frequency.


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Message 3 of 6

To be precise you need to include the modulation bandwidth. The Nyquist criterion applies to the highest frequency of interest which would be the carrier frequency plus the largest modulation sideband.

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Message 4 of 6


Thanks for your response. I have eventually fix the problem by reconstructing the block diagram.

Unfortunately, I am facing another problem in display of simulation data. I want to display both the original pseudo random sequence and the resulted sequence after processing. I used the waveform graph but not working.

Attached is a new block diagram. Hope you or anyone can give me a hand.


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Message 5 of 6
Hi WK,

You can use the Merge Waveforms VI to display two different waveforms/signals on the same graph.  This VI can be found on the Express >> Signal Manipulation palette. 
Try to wire the original pseudo random sequence and the resulted sequence after processing to the Merge Waveforms VI and wire the output to a Waveform Graph.

Best Regards,
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Message 6 of 6