TABLE OF CONTENTS SECTION 6.0

Transcription

1 TABLE OF CONTENTS SECTION 6.0 SECTION 6.0 FUNCTION GENERATOR (VFG)... 1 MEASUREMENT OBJECTIVES... 1 BASIC OPERATION... 1 Launching vfg... 1 vfg Quick Tour... 1 CHANNEL CONTROL... 2 FUNCTION TYPES... 2 Periodic Functions... 2 Chirp... 3 Two-Tone... 3 Random and Translated Random... 3 Arbitrary & Translated Arbitrary... 3 Open Arb (vcap)... 5 DSP Technology Inc. SigLab User Guide i

2 SECTION 6.0 FUNCTION GENERATOR (VFG) Measurement Objectives The Virtual Function Generator (vfg) is a general purpose signal source to be used with the dynamic signal analyzer, oscilloscope and spectrum analyzers. It features a selection of predefined functions as well as the ability to generate user-defined outputs. Basic Operation Launching vfg The vfg is the simplest of all the VIs to operate since no measurement data or plotting is involved. Type vfg at the MATLAB prompt and the vfg VI will appear. In the lower righthand corner, click the All Enabled/All Disabled button to the All Enabled state. Assuming the DUT is connected, it will be producing a faintly audible tone. If a small active audio speaker is available, this could also be used to get a feel for vfg operation. vfg Quick Tour Output Channel, 1 or 2 On/Off Toggle Button Frequency Control LEDs Burst/Continuous Function Type Level Offset Burst Parameters (On/Off Time in Seconds) All Enabled/ All Disabled Toggle Button Figure 6-1 vfg User Interface DSP Technology Inc. SigLab User Guide 6-1

3 Channel Control Section 6.0 Function Generator (vfg) Try the following sequence of operations to take a quick tour of the vfg: 1. Open testd1.vfg using the File menu. 2. Vary the chirp start-end frequencies with the sliders. 3. Vary the on/off time and output level. 4. Toggle burst off. 5. Try all the function types. Note that files for Arb and TrArb have not been loaded yet. 6. Quit with the Quit button when you are finished. DO NOT SAVE THE STATE. Channel Figure 6-2 Function Types Function Types Each output channel can have an independent setup of its function and function parameters unless two or more channels are set to arbitrary. The channel being set up is selected by the popup when the Channel Select button in the upper left corner of the vfg VI is selected. The on/off toggle button turns the selected channel's output level from 0.0 volts to the specified value. Periodic Functions The periodic functions include: sine square triangle sawtooth impulse 6-2 DSP Technology Inc. SigLab User Guide

4 Section 6.0 Function Generator (vfg) Function Types The periodic functions, other than the sine, have a non-uniform frequency resolution that becomes coarser as the target frequency of the function becomes higher. The edit box on the frequency slider always displays the actual frequency being generated. All the functions pass through the sigma-delta D/A converter, which contains an integrated interpolating digital filter as well as a switched capacitor-smoothing filter. The pre-shoot and overshoot in the time domain are a result of the filtering operations. Chirp The chirp function with independent start and end frequencies is capable of burst-mode operation. The length in samples or duration in seconds can be specified by the Samples edit box or the On Time slider. Moving the slider automatically changes the edit box and viceversa. Two-Tone The two-tone function features independent frequency settings for the tones (sine functions) as well as the ability to adjust relative amplitudes. Two-tone signals are useful in studying the linearity of systems by measuring intermodulation products with the spectrum analyzer (vsa). Random and Translated Random Random noise can be generated with a specified bandwidth or translated random noise (TrRand) can be generated with a specified bandwidth and center frequency. If continuous mode (Burst/Cont toggle button) is selected, a very long pseudo random number sequence is generated (see SigLab detailed specifications). If burst mode is selected, the sequence length is proportional to the on time selected and exactly repeats every burst interval to allow time domain averaging to be performed. Figure 6-3 shows a typical burst random signal. To simplify trigger setup, the output hardware generates a digital trigger signal coincident with the beginning of the burst sequence, or in the case of periodic functions, coincident with the periods of the function. This greatly simplifies measurement trigger setup. Arbitrary & Translated Arbitrary The SigLab output subsystem is capable of reproducing user-defined signals with an adjustable bandwidth and center frequency (for Translated Arbitrary). When burst mode is selected, the on time is determined by the record length and the selected sampling rate. The off time is set by the off time slider control. The effective sample rate of the output is 2.56* the selected bandwidth. To use the arbitrary output mode, select Arb from the function selection popup. Go to the File menu, and select Open Arb. A file select dialog box appears to allow the selection of a file with the proper arb format (filename.arb). If both output DSP Technology Inc. SigLab User Guide /17/98INSL2

5 Function Types Section 6.0 Function Generator (vfg) channels are set to arbitrary at the same time, the hardware/software make the following restrictions: 1. Both output channels must be set to the same bandwidth. 2. Both channels must be set to continuous (CONT), or both channels must be set to burst mode with identical on and off times. Figure 6-3 A Typical Burst Random Signal Figure 6-4 User Defined Output File Produced by arbgen.m in the vfg Directory 6-4 DSP Technology Inc. SigLab User Guide

6 Section 6.0 Function Generator (vfg) Function Types Open Arb (vcap) Standard arbitrary output records are limited to 8192 samples. However, it is also possible to output much longer records. This data may have been captured by the vcap program or generated in MATLAB. In any case, the data must be stored in vcap files (filename.vca). To use this selection do the following in vfg: 1. Select Arb from the list of function types. This list also contains sine, square, chirp, etc. 2. Select File/Open Arb (vcap) in main menu. 3. Select the desired.vca file from the open file dialog box. If there is not enough memory available in SigLab to accommodate the long record, an error message will occur. The File menu also includes the selection Clear Arbout Buffer to dispose of the buffer when it is no longer needed. See Appendix B for further details on long record arbitrary output. DSP Technology Inc. SigLab User Guide /17/98INSL2

7 Function Types Section 6.0 Function Generator (vfg) 6-6 DSP Technology Inc. SigLab User Guide