ELEC 241 Lab

Writing a Labview Program

In Labs 7 and 8 we used Matlab to do our digital signal processing. Matlab has the advantage of being familiar, and it would be easy to program the core of our transmitter and receiver in Matlab. But Matlab has several serious disadvantages for this application.
  1. It's easy to access the sound card from matlab, but we need the higher sample rate of the Labview DAQ card to achived our required carrier frequency.
  2. Matlab works with fixed length vectors, corresponding to signals of fixed duration. Our radio system has to work on continuous signals of arbitrary length.
  3. Our previous Matlab DSP has been in batch mode. We recorded a signal to a disk file, processed it, and played it back. Our radio would be much more useful if it operated in real time, i.e. we could hear the transmitted signal as it was being spoken.
Fortunately, all of these problems can be solved by doing our DSP in Labview rather than Matlab. Unfortunately, that means we have to write a Labview program. Although we've used Labview many times this semester, these programs have all been written by someone else. Fortunately, our requirements (at least for the transmitter) are fairly simple, so let's try to write a Labview program to compute $\displaystyle x(t)=A_c(1+m(t))cos(2\pi f_ct)$ .

Part 1: A Very Simple VI

Although the process we want to perform is a fairly simple one, if you have never written a Labview program before, even simple processes are difficult. So we will approach our final goal in a sequence of smaller steps. The first step required for either our transmitter or receiver is to digitize the analog input signal, so let's write a VI that does that and displays the result.


Step 1:

 Since we are going to build a VI from scratch, we will start with a new, blank VI. Select "New VI" from the "File" menu. A pair of windows should appear. The one on top will be a blank Front Panel window:
and the one on the bottom is a blank Block Diagram window:


Step 2:

 Our first task is to read and display an analog input signal. Since the front panel window is on top, let's take care of the display first.

Right click over the panel window. You will get the Controls popup:

Place the cursor over the "Graph Inds" button to get the Graph Indicators popup:
Left click on the "Graph" button.

Step 3:

 A group of boxes with an open hand cursor will appear on the front panel.
Move this to an aesthetically pleasing location and left click. You will get a numeric display labeled "Waveform Graph."
The fact that the words "Waveform Graph" are highlighted means that they are selected and can be edited. Let's change the name so that we can remember what is being displayed.

Type "Output Signal" and left click the check-mark box at the upper left of the window ( ). The indicator is now ready for use.

Step 4:

 Now we need something for the indicator to display. Click on the Block Diagram window to bring it to the top. This may also bring up the Functions palette. If so, move it out of the way or click the close box. Note that placing the indicator on the front panel has also placed a block on the block diagram.
This block is the icon associated with the indicator. We can provide a signal for it to display by connecting it to a signal source. The source we will use will be one of the A/D converter inputs.

Step 5:

 If you closed the Functions palette in the previous step, right click to bring it up.
From the Functions popup, select "Input". This will bring up the Input palette.
From this select "DAQ Assist" This will bring up a block labeled "DAQ Assistant, attached to a hand cursor.
Position this somewhere to the left of the VT indicator block and left click.

Step 6:

 Wait patiently. You may briefly see a dialog labeled "Initializing". After a second or two the "Create New .." wizard will appear.
Click on "Acquire Signals" in the left hand column, then click on "Analog Input" then on "Voltage" in the list that appears.


Step 7:

 From the list of Supported Physical Channels that appears, select "ai5",
then click the "Finish" button at the bottom of the frame.

Step 8:

 After another brief wait, the "DAQ Assistant" dialog appears. In the "Input Range" block set "Max" to 10 Volts and "Min" to -10 Volts. Set the "Terminal Configuration" field to "RSE". The upper half of the panel should look like this when you are done.
When you're done, click the "OK" button.

Step 9:

 Things will click and whir for several seconds. When it's all over, the "DAQ Assistant" box will have expanded, and should have a white band with the word "data" in it.


Step 10:

 We're almost done. All that remains is to connect the source (A/D converter block) to the destination (numeric indicator block). This process is called wiring.

Place the cursor over the small black triangle in the "data" field of the DAQ Assistant block. It should change into an icon representing a small spool of wire. Left click once and move the cursor to the small white triangle in the center of the left edge of the numeric indicator icon and left click once more. That completes our first Labview program. It should look something like this:



Data Types:

 Notice that the color of the icon for the waveform graph changed from orange to blue. This is an indication that the type of data which it is displaying has changed. Like many programming languages (e.g. C) Labview maintains the notion of data types. Labview's data types include familiar ones such as integer, floating point, boolean, and string, as well as a number of unfamiliar ones (which we will try to avoid for the time being). Labview denotes data type by the color of the wire which carries it: integer wires are blue, floating point wires are orange, boolean wires are green, and strings are pink. Labview also supports collection types, such as arrays and structures. Scalars are denoted by thin solid lines, arrays by thick solid lines, and other collections by various patterned lines. The wide dark blue lines with internal dashes are a composite data type called dynamic data.

Dynamic data contains a lot of information in addition to the value of the sample, for example, the time at which the sample was taken, whether any errors were made in previous handling of the sample, etc. This means that when we connect our signal to the waveform display, it can automatically display the correct time scale on the x-axis, rather than just the sample number.

Step 11:

 Let's try out our VI. First we need an analog signal to display. Set the function generator to produce a 1 V pp 100 Hz sinewave. Connect the output of the function generator to analog input channel 5. This is labeled ai5 in the DAQ Assistant and ach5 on the interface socket strip (pin 47).

Step 12:

 Click on the Front Panel window to bring it to the top. Run the VI by clicking on the Run arrow or by typing Ctrl-R. The graph indicator should display about 10 cycles of a sine wave.

Step 13:

 It's always a good idea to save your work from time to time, and since we currently have a working VI, this would be a good time. Select Save As... from the File menu. Set the Save in: field to an appropriate directory (e.g. the desktop, your network home directory, or your local Group nn folder).

Caution

Always set the destination directory when saving a VI. Never try to save a VI in the default directory.

If "Untitled.vi" seems like an inadequate name for such a momentous work, think of a more descriptive one (e.g. "lab9.vi") and enter it into the File name field. When everything is in order, press the Save button.

Part 2: Continuous Display

Although we have displayed what our signal was at the time we pressed the Run button, we really want a continuous display, as with the oscilloscope. Fortunately, this is a simple enhancement to make. Like more conventional programming languages, Labview has looping and branching constructs. What we need here is the while loop which repeats its contents until an exit condition is met.


Step 1:

 Right click in the block diagram to bring up the Functions palette. Move the cursor to the Exec Ctrl button to bring up the Execution Control palette.
Click on the While Loop button.

This is a variable sized block and we want it to enclose everything we currently have in our VI. Bring the cursor into the block diagram window and click above and to the left of all blocks, then drag until everything is enclosed and release.

Note the green-bordered block labeled stop in the lower right corner of the while loop. This is a free accessory that allows us to stop the loop, which would otherwise run forever. This block is an example of a control. A control is the dual of an indicator, i.e. it provides input from the front panel to the block diagram. Go to the front panel and notice that there is a new object, a button labeled STOP. Pressing this button causes the associated icon to output a True value (when the button is not pressed, the output is False).

Step 2:

 From the front panel, press the Run button. The graph should now display continuously updates signals. To convince yourself that these aren't random, vary the frequency or amplitude of the function generator. When you get tired of twisting knobs and watching the resulting display, press the STOP button.

Step 3:

 continuous While we're at it, let's set the sample rate and blocksize to the value's we'll need for the transmitter.

Part 3: Signal Processing

While it's nice to be able to display our signal, what we really need to do is add one to it, multiply it by $\cos(2\pi f_ct)$ and convert it back to an analog signal. Let's do the arithmetic first.


Step 1:

 Go to the block diagram window and right click to get the Functions palette. move the cursor to the Arith/Compare block to bring up the Arithmetic and Comparison palette.
Move to the Numeric block to bring up the Express Numeric palette.
This has everything we need: addition, multiplication, and constants (for +1).

Step 2:

 Let's start by adding one. Move the cursor over the symbol labeled Add and left click. Position the icon below the existing components on the diagram and left click to put it down. Note that while the cursor is over the symbol there are three small circles near the vertices of the triangle.
These indicate the connection points and will disappear when the cursor moves away.

Step 3:

 Now let's do the one. Select Num Const from the Express Numeric palette and place the resulting block above and to the left of the subtract icon. The highlighted text indicates that you can enter a new value. Type 1 and press Enter or click the check box at the upper left of the window.


Step 4:

 At this point you may have noticed a change in the appearance of the toolbar: the run button has changed from a white arrow to a gray broken arrow ( ). This means that there is an error in the block diagram and the VI won't run. To find out what the error is, click on the broken arrow button.

If you do this you should find that the problem is an unwired or bad terminal. This is because the adder currently has no inputs. We will fix this problem shortly.

Step 5:

 We need to disconnect the A/D output from the input to the waveform graph so we can replace it with the output of the adder. Place the cursor over the wire and double click. This converts the wire to a highway of marching ants, which indicates that it is selected. Press the DELETE key to remove the wire.

Step 6:

 All that remains is to make the connections. We already know how to do this from the previous Part: place the cursor over a connection point, wait for it to turn into a spool of wire, click, move to the other terminal, and click again.

When you're done it should look something like this:



Step 7:

 Let's test what we have so far. Go to the front panel and click on the Run button. If all is well, the output should now be offset by one volt. All that's left is to multiply this new signal by $\cos(2\pi f_ct)$ to get $x(t)$ , the transmitter output.

Step 8:

 Stop the VI. Go to the block diagram and disconnect the adder output from the Waveform Graph input.

Step 9:

 From the Express Numeric palette, select a multiply block and place it between the adder and the waveform graph icon.

Step 10:

 From the Functions palette, select Input, then Simulate Sig. Place the Simulate Signal block above the DAQ Assistant block and left click to place it. In the Configure Simulate Signal accept the default values and click OK A white band labeled Sine will appear at the bottom of the block.

Step 11:

 Connect the Sine output of the Simulate Signal block to one input of the multiplier and the output of the adder to the other. connect the output of the multiplier to the Waveform Graph When you're done it should look like this:


Step 12:

 Click Run. The signal on the waveform graph probably doesn't look much like the AM waveforms you've seen in class. This is because the default frequency for the Simulate Signal block is 10.1 Hz which is much less than the 100 Hz frequency of the function generator signal. Our model for modulation assumes that the carrier frequency is much greater than the signal frequency. Let's fix that before we proceed.

Step 13:

 Double click on the Simulate Signal block. In the Signal sub-panel, set Frequency (Hz) to 175000 (this is the center of our 160-190 kHz band). In the Timing sub-panel, set Samples per second (Hz) to 1000000, set Number of samples to 16384, and uncheck the Automatic box.


Step 14:

 We also need to set the sample rate and block size of the A/D converter to the same values. Double click on the DAQ Assistant block. In the lower sub-panel, set Acquisition Mode to Continuous, set Samples to Read to 16384, and set Rate (Hz) to 1000000.


Step 15:

 Restart the VI. The display should look more like a proper AM signal. (If necessary, adjust the function generator frequency for a stable display.)

Step 16:

 Stop the VI and save it in a persistent location.

Part 4: Analog Output

We now have an AM signal displayed on the Labview control panel. Before we can proceed, we need to convert it to an analog signal.


Step 1:

 Recall that the DAQ card has two analog voltage outputs: dac0 and dac1. Unlike the analog inputs which have different sensitivities, there's no real difference between these, so we'll arbitrarily choose dac0 for our output.

Use a BNC patch cable to connect J1-1 to CH 1 of the oscilloscope. On the interface module connector strip, connect J1-1 (socket strip pin 1) to dac0 (pin 51).

Step 2:

 We create D/A output blocks in nearly the same way we did A/D input blocks, using the DAQ Assistant. From the Functions palette, select Output, then Place the resulting block inside the while loop, above the waveform graph icon.

Step 3:

 Once it is placed, you will get a Create New ... wizard, just as you did with the input block in Part 1. This time, select Analog Output, then Voltage. From the list of Supported Physical Channels, select ao0, then click the Finish button at the bottom of the frame.

Step 4:

 When the DAQ Assistant dialog appears, set the Output Range to have a Max of 10 Volts and a Min of -10 Volts. Set the Generation Mode to Continuous. When finished, click the OK button.

Step 5:

 There will be a brief flurry of activity, the DAQ Assistant block will expand, and a white band containing the word data will appear.

Step 6:

 Wire output of the multiplier to the data input of the D/A output block. To connect to an existing wire (in this case the one between the multiplier and the Output Signal display block), move the cursor near, but not directly on, the wire. If you place the cursor on the wire it will turn into an arrow, indicating that you can select the wire.

Step 7:

 When complete, your VI should look like this:


Step 8:

 Press Run and verify on the oscilloscope that the output is the same AM signal shown on the waveform graph.

Step 9:

 We now have a working transmitter VI. Save it in a safe place.

Part 5: Bells and Whistles

If you have had enough of Labview, you can move on to the next section, since our transmitter VI is now fully functional. However, it will be much easier to use if we add a couple of enhancements. In particular, since a number of groups may be transmitting at the same time, it would be convenient to change the carrier frequency without having to stop the VI, edit the Simulate Signal block, and restart. Fortunately that's easy to do.


Step 1:

 Go to the front panel and press the Stop button. VIs can't be edited while they are running.

Step 2:

 Right click to bring up the Controls palette. Move the cursor to the Num Ctrls button to bring up the Numeric Controls palette.
This looks a bit like the Numeric Indicators palette, except that these are tools for producing rather than displaying numeric values. Click on the Num Ctrl button
and place the control in a convenient location on the front panel. Change the label to Fc.

Step 3:

 Return to the block diagram and notice the orange icon labeled Vout.
(An alternate way to get to the icon associated with a control or indicator from the front panel is to right click over the object and select Find terminal. This will bring up the block diagram with the associated icon selected.) This icon looks similar to the numeric indicator icon we used in Part 1, but the small triangle denoting the connection terminal is on the right-hand side.

Step 4:

 Stop the VI. Move the cursor over the small double arrow in the middle of the bottom edge of the Simulate Signal block. It will turn into a small black square and the cursor will become a resize arrow.
Left click and drag down slightly until the dashed outline grows by one increment, then release. Another white band, labeled error out will appear (the pink color indicates the data type of the output). Left click in this new box to bring up the menu of choices. This is the list of all available inputs and outputs for this block. Select Frequency.

Step 5:

 Wire the Fc numeric control output to this new input.

Step 6:

 Go to the front panel and start the VI. Type different values into the Fc control and observe that the carrier frequency changes corresponding to the entered value.

Step 7:

 We should also provide a means for setting the carrier amplitude $A_c$ . Place another numeric control on the front panel and label it Ac. Add another multiplier to the block diagram, connect one input to the output of the existing multiplier and the other to the Ac numeric control. Connect its output to the D/A converter. When done this is what your block diagram should look like:

Congratulations! You have completed the transmitter portion of this Lab. Be sure to save it in a safe place.