"External" Modulation

[cochenob]

After a study of a few of the communication toolkit examples...I've come to understand the following....

For example, if I want to generate an AM signal, in general...

- A waveform VI generates my message signal (sine wave, triangle wave, etc.) and writes it to memory in my AWG

- The AWG determines IQ samples....and generates my analog message signal (sine wave, triangle wave, etc.)

- This analog signal passes from AWG through a small cable, into the upconverter.

- The message signal is upconverted.

My question....what if I wanted to upconvert my own signal that came from an external source?  A simple example...say I want my message signal to be a real voice signal, or some other "external" signal.  I was hoping that I could just disconnect the AWG from the upconverter, and replace the input into the upconverter with my own signal, however, this didn't work. 

Some further questions....in using one of the Generate AM examples I disconnected the AWG from the upconverter and looked at the AWG on a scope to try and gain some insight as to what was going on.  Using a the sine wave option, I expected to see such on the scope...however it looks like the scope is unable to trigger off of whats coming out of the AWG.  That seemed to suggest that there isn't any phase continuity between "loops" as the AWG is generating it's signal over time.  It's at this point where things get nebulous, and I begin to think that there's more going on between the AWG and upconverter such that I might not be able to provide my own modulation signal.

Any thoughts?

(not to compound the problem...but the same argument can be made for digital schemes as well.  The examples have the option to use 1's and 0's from a user supplied file...but is there a way to provide 1's and 0's in some sort of "quasi-real time" externally?)

---
Brandon

[Andy_Hinde]

Hi Brandon,
When generating an AM signal:

- The message signal is generated in software but does not write this message signal directly to the arb.
- After generating the message signal, it is AM modulated using the MT Modulate AM vi from Modulation Toolkit (or the corresponding routine in C).
- The output of this modulation block is an IQ data waveform. This IQ waveform (alreeady AM modulted) is written to the AWG.
- The AWG generates this AM waveform on a IF carrier of 25 MHz.
- The PXI-5610 upconverter then takes the IF carrier at 25 MHz and frequency converts it to the RF carrier of interest.

If you had a voice signal you wanted to broadcast, you can do this. You would either have to digitize the voice signal and supply this signal in place of the message signal in the process described above, or provide the PXI-5610 with the modulated signal on a 25 MHz IF carrier frequency.

If you look at the arb output directly, it should be an AM modulated waveform with a carrier of 25 MHz.

Regards,
Andy Hinde
National Instruments

[ppx]

I have I/Q baseband complex data acquired on the third party hardware

and want to use modulation toolkit to do some analysis. I intend to use

some data format which can contains informtion such as

unit and sample rate so I only need to handle only one file.

I have a couple questions and would appreicate your help.

1) Does modulation toolkit accept all or some of the following

   

• Text (LVM)
• Binary (TDMS)
• Binary with XML Header (TDM)—

2) Does NI supply some C or C++ DLL to convert text files to

    LVM ? I know that such DLL is availabe for TDMS and TDM to write files.

 https://www.ni.com/en/support/documentation/supplemental/06/the-ni-tdms-file-format.html

[Matt_A]

Hello ppx,

Let me try to address your questions in order:

1) The NI Modulation Toolkit itself does not read from files. The VIs in the NI Modulation Toolkit will accept a number of data types depending on what operation you are trying to perform. For example, the MT Downconvert Passband.vi downconverts real passband signal data of a user-specified bandwidth. This VI accepts the data of the waveform data type, as described below:

---

Waveform inputs the signal for downconversion in passband form. This cluster contains the following elements:

• t0: trigger (start) time of the acquired signal.
• dt: time interval between data points in the acquired signal.
• Y: the complex array representing the signal for downconversion.

---

In your case, where you have stated that you have "I/Q baseband complex data", you would most likely pass that data directly to one of the demodulation VIs. For example, if you were using phase modulation, you would use the MT Demodulate PM.vi, which performs phase demodulation on the IQ signal. This VI accepts data in the form of a PM modulated waveform, which has the following description:

---

PM modulated waveform inputs the baseband (downconverted) time-domain data for demodulation. This cluster contains the following arrays:

	t0 specifies the trigger (start) time of the acquired signal.
	dt specifies the time interval between data points in the acquired signal.
	Y specifies the complex-valued time domain data array. The real and imaginary parts of this complex data array correspond to the in-phase (I) and quadrature-phase (Q) data, respectively.

---

You would need to use one of the File I/O VIs from LabVIEW to read the data from your data file. The implementation will depend on whether your data is stored as Text (LVM), Binary (TDMS) or Binary with XML Header (TDM). You can find more information about File I/O in the LabVIEW Help file here. So, to directly address your question, the NI Modulation Toolkit does not "accept" any of the file types that you specified, but uses data that can be stored in any of those formats.

2) I don't know if there is any DLL to convert text files to LabVIEW measurement file (.lvm), like there is for TDMS and TDM. However, .lvm files are already tab-delimited text-based measurement files, so I don't know what kind of conversion you would perform. Are you just talking about reformatting the text? You may find a better answer to your file conversion questions by posting to the LabVIEW Forums here.

[ppx]

Matt,

Thank you for your response. It definitely clears a few questions out of my mind.

Sounds like that the waveform format is the most straightforward way to go.

1) I wonder whether the waveform is the same for different modulation type, for example

FSK vs QPSK or analog demod. Assume that the input data is always baseband complex

I/Q data. In other words, I can write one conversion program which works for all modulation types.

2) Sounds like that the waveform format is a text based. Does it have bineary counterpart?

The text file could be big and slow for a large acquisition.

Thanks again.

PPx

[Matt_A]

Hello PPx,

Please see numbered responses below:

1) Yes, all demodulation functions in the NI Modulation Toolkit take the same complex waveform data type described above in the second case labeled PM modulated waveform. The demodulation functions include ASK, PAM, QAM, FSK, MSK and PSK.

 2) There is a difference between the data type and the format in which the data is stored. The data types described above defines how LabVIEW interprets the binary data in the computer. The format in which the data is stored does not matter, so long as LabVIEW can retrieve and interpret the data correctly. So, you could save a complex waveform data type in either a binary or text file. You could also retrieve this data from either a binary or text file, so long as you knew the format in which it had been saved. For more information about these options, I would recommend that you check out these resources: KnowledgeBase: How Can I Write and Read Binary Files Using LabVIEW? and Writing to Text Files LabVIEW Help

Again, you may find more information regarding file I/O on the LabVIEW forums, or you may want to consider taking a training course to learn some of the more advanced file I/O.

Message Edited by Matt A on 09-13-2007 08:20 PM