I am very new to the LabVIEW environment, and I am developing my Masers Degree project in LabVIEW. I am just reviewing and trying to learn things from the examples provided by the ni.
Whenever we want to transmit or recieve anything using USRP board, we need to use CDB cluster, and give the data to it. What I don't understand is that how is the modulation of the waveform done, i.e. for example, if I am transmitting a continuous IQ signal of 10kHz, and I have set the carrier as say 950MHz, then it is clear that the carrier of 950MHz will carry the information of 10kHz message signal. But I don't understand "how?".
So if please anybody can help me?
You told the answer in your question. "Modulation" is the part which is doing this. There are various types of modulation. The first is ASK (Amplitude Shift Keying).
Look into it, how the carrier wave's (950MHz) amplitude is modulated to get a modulated data on 10kHz.
Ooh!! missed one part, what type of modulation is also important! Make sure that you check OOK (On Off Keying), NRZ (Non Return Zero), Manchester...
It's real fun!
It can be a little confusing with the way that it works. The USRP is frequency translating the IQ signal that you write to it (The CDB Cluster) up to 950 MHz. So, say I have a 10 kHz CW tone in my baseband IQ data. All of the IQ data gets frequency translated up to 950 MHz through the USRP's single stage upconverter. If someone attached a Spectrum Analyzer to the USRP they would see a tone at 950.010 MHz.
All the modulation occurs at baseband whether it is AM, PM, FM, ASK, PSK, QAM, etc. At baseband, the carrier frequency is 0 Hz, the message signal modulates this based on your selected modulation scheme, and the USRP upconverts the resultant IQ waveform to the chosen center frequency.
I got a basic idea about the working of analog signal modulation, which as you said, is nothing but upconversion. It's imaginable. The next problem I'm facing is about modulation of data, that is complex thing that appears in thick pink color wire.
How is this types of data modulated?
I also wanted to know that what are the electrical parameters for the clock signal that is to be provided to the reference in for USRP if I am to connect a function generator as a clock signal source?
Hey You peeked (peaked)!!!
Nice to see this! Already you are in good shape. Again you said about up-conversion and modulation.
Did you work with SPI (Serial Peripheral Interphase)?
Just that! When you talk about modulation, there should be a clock with it indicating the baud at which it is running. All are bits, same as computer (more specific BINARY).
Example: ASK (Amplitude Shift Keying) when the carrier is ON, it is '1' and when carrier is OFF it is '0'. You need to keep the amplitude (carrier) on for a clock period to represent 1 in that baud (ex: 10kbaud case 100us as one bit).
Look out of the box! Manchester has clock embedded in the data! you'll enjoy that there is no need of clock when it comes to Manchester encoding.
Ghattu, The clock signal here is not a bit/symbol clock but the reference clock used by circuitry in the USRP (i.e. Reference clock for PLL's).
Himan, that wire is a called a cluster. Here is some background on basic data elements in LV: http://www.ni.com/getting-started/labview-basics/data-structures. The cluster you are seeing has three elements (start time t0, sample period dt, and array of complex data Y). The I/Q data in the Y array is what gets modulated.
I think it will help if you go through some of the RF and Communications tutorials. Specifically, the "What is I/Q Data?" and "Quadrature Amplitude Modulation (QAM)" sections will help you understand how the Complex Cluster Y array gets modulated before writing it out to the USRP for generation.
As for the reference clock signal you can find the specification for your specific USRP under "C:\Program Files (x86)\National Instruments\NI-USRP\documentation\specifications" or on ni.com/manuals for specifications and getting started guides. The following description for REF IN was found in one of the USRP getting started guides
"Input terminal for an external reference signal for the local oscillator
(LO) on the device. REF IN is an SMA (f) connector with an
impedance of 50 Ω and is a single-ended reference input. REF IN
accepts a 10 MHz signal with a minimum input power of 0 dBm
(.632 Vpk-pk) and a maximum input power of 15 dBm (3.56 Vpk-pk)
for a square wave or sine wave"
If you are indeed looking for a bit/symbol clock then that is specified in the modulation process of the I/Q data not the hardware level. The modulation process in general is bits --> Symbols --> IQ Samples. The IQ rate which the USRP plays data out at is derived from the Symbol Rate you want times the # of IQ samples per symbol that the modulation toolkit VI's ask for. For example, If I want my symbol rate to be 100 kSym/sec and I use the default # of IQ samples per symbol of 16 then my IQ rate will be 1.6 MSamples/sec and that is what the USRP will be set to.
I know it can be confusing but read through the documents I've linked and look through some more code and it will become clear over time.