LeCroy oscilloscopes are capable of a wide variety of measurements. By combining a high resolution HRO-Zi oscilloscope with a LeCroy ArbStudio arbitrary function generator, it is possible to make a simple but accurate I-V curve tracer. In this application brief we characterize a simple diode, but the process is expandable to more complex devices.
The Basic Setup
Figure 1 shows the basic connections of the oscilloscope and arbitrary waveform generator to the device under test, in this case a small signal diode. The 12 bit Higher Resolution Oscilloscope was chosen to provide the largest possible dynamic range for these measurements. The ArbStudio was chosen because of its 16-bit resolution and its flexible triggering capability.
Channel 1 is set to 50 Ohm coupling, and the 50 Ohm termination is the load and current sense resistor. Channel 2 monitors the voltage driving the device and the load. Channel 3 is the trigger source.
Figure 2 summarizes the waveforms used to create the current vs. voltage curve for the diode:
The arbitrary waveform generator is programmed to output a series of pulses with stepped amplitudes. Each pulse is 500 ns wide, and the spacing between pulses is 10 s. Amplitudes vary from -5 V to +5V. This waveform is seen in the trace for Channel 2 (second from the top in the left column). Pulse waveforms are used to keep from heating the device under test. The duty cycle in this example is only 5%. The ArbStudio also generates a trigger pulse at the beginning of each pulse. This is applied to Channel 3 and is seen as the top waveform in the left column.
Because the waveform has such a small duty cycle the scope is set to sequence mode acquisition. In this mode the scope’s acquisition memory is broken into a user selectable number of segments. In this example we use 12 segments, one, for each pulse amplitude.
The voltage on Channel 1 is proportional to the current through the 50 Ohm scope input termination. We can calculate the current by dividing the measured voltage by 50 Ohms. This is done in the math trace F1 (bottom trace on the left column) where the rescale function is used to multiply Channel 1 by 0.02 (or 1/50). The displayed units are changed to Amperes.
The voltage across the diode is calculated by taking the difference between Channel 2 and Channel 1. This was done in math trace F5.
The parameter P1 and P5 read the amplitudes of each of the current and voltage pulses. Since it was desired to read the amplitudes after the device had time to settle, the parameter Level @ X was used. The level of each voltage and current pulse were read 400 ns into the 500 ns pulse using parameters P3 and P4 respectively. This is shown in Figure 3 which shows the 12 segments overlaid on a common grid and the help markers showing the point where the Level @ X parameter sampled the waveforms.
The result of this operation is that we will have 12 current /voltage pairs that we will plot on an X-Y display.
The readings from the Level @ X parameter are plotted as trend plots, one for voltage and the other for the current waveform. The trend plot is a waveform composed of a series of measured parameter values plotted in the order they are taken. In this example we have taken 12 values in each waveform corresponding to the current and voltage reads for each of the 12 input pulses. The trend plots are then cross plotted using the X-Y Dual display mode as shown in Figure 4:
The X-Y dual display shows both the X-Y plot, and the current and voltage source traces.
The X-Y plot in Figure 5 shows the 12 current/voltage sample pairs. The origin (center) of the X-Y display is the point 0,0. The voltage scale is 1 V/division horizontally, and the current scale is 12.4 mA/division vertically. The number of points on the X-Y display is a function of the number of pulse amplitudes in the test waveform. You may create as many as desired.
The trend functions, like all waveforms from the oscilloscope, can be exported to third-party programs such as MATLAB or Excel for plotting or further analysis.
The combination of the oscilloscope and arbitrary waveform generator can be employed for simple device characterization utilizing sequence mode acquisition and trends of sampled waveforms.