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Building stimulus-response system with AWG, digitiser

18 Mar 2016  | Arthur Pini, Greg Tate, Oliver Rovini

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Matched diodes
Some circuits require diodes with matched I-V curves. While you can make these measurements with a curve tracer, you can also make automated measurements using an AWG and digitiser, shown in figure 2. Note that the maximum current supplied to the diode must be less than the maximum AWG output current otherwise an amplifier may be required. This works well for the 5082-2811 Schottky Barrier diode used in this example.


Figure 2: Simple setup for quickly characterizing signal diodes using a ramp function from the AWG. The 50 Ω input termination of the digitiser serves as the load.


Diode measurements, modulation, part simulation
Figure 3 shows the result of the measurement. The AWG is setup to output a ±2 V ramp waveform. This waveform is applied, through a splitter, to the digitiser's channel Ch1. The other leg of the splitter connects to the diode under test and then on to the digitiser Ch0. Both channels are set to 50 Ω termination. The voltage on channel Ch0 is proportional to the current through the diode. Rescaling is applied to that channel so that the readings for Ch0 will display in milliamps. The voltage across the diode is calculated by Ch1—Ch0.


Figure 3: With some basic math, you can derive the current through the diode and display it.


The centre bottom grid is the ramp waveform from the AWG shown in Ch1 of the digitiser. The centre-top grid shows the Ch0 waveform, which is proportional to the diode's forward current. Subtracting Ch0 from Ch1 yields the voltage across the diode (upper-right grid). The lower-right grid is the shows the diode current directly in milliamps. The X-Y plot in the leftmost grid is the diode's Current-Voltage (I-V) plot of the diode. To find matching diodes, look for closely matching I-V plots.

Although this is a very simple example of device characterisation, many AWG's can perform more complex small-signal device testing. Testing devices at higher voltage and current levels may require amplifiers.


Modular modulation
Because an AWG can generate almost any waveform, you can perform communications tests by generating modulated signals. Amplitude, phase, and frequency modulation can be achieved as was demonstrated in the first example on frequency response testing where a linear frequency swept sinewave was used. In figure 4, we used quadrature signal techniques to generate a "chirp" waveform at an intermediate frequency.


Figure 4: Using quadrature signal processing to generate a "chirp" waveform at an intermediate frequency.


Figure 4 shows the steps in creating the chirp. It starts in the upper left grid where linearly swept sine and cosine waveforms are shown in the time domain along with the equations used to create them. These signals are in quadrature representing the I (in-phase) and Q (quadrature) components of a quadrature modulation, but the phase difference doesn't show in the FFT's of these waveforms, which appear in the lower left grid. The FFT shows the sweep range from 22.7MHz to 27.7MHz. This is a base band signal.

We then up-convert the base band quadrature components by multiplying the I and Q base band signals by the cosine and sine of the intermediate frequency, respectively. The intermediate frequency is 100MHz. The upper central grid shows the time domain view of the up converted signals. The lower central grid shows the FFT's. Note that the multiplication by a sinusoid has converted the base band signals to double sideband suppressed carrier signals centred at 100MHz with upper and lower sidebands extending from 22.7MHz to 27.7MHz on either side of the centre frequency. The amplitude FFT doesn't, however, show the phase of the two quadrature components.

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