Wrong measuring of diode I-V curve in pulsed mode

[simazPHYS]

Hi. In order to measure the I-V curve of a normal diode in pulsed mode, I set up the following experimental kit. The experimental results do not change with pulsewidth, which is quite long (us regime), and low frequency (200hz and less).
An HP pulse generator is connected via BNC cable to an oscilloscope (1MOhm input) to measure the input voltage, and to the series of the diode under investigation and a 50Ohm resistor. The voltage drop across this resistor is measured via the second channel of the scope. No terminators are used.
The problem is that the I-V I obtain is different from what it should be (using the curve tracer or the datasheet). However, when I replace the diode with a resistor (for instance a 550Ohm resistance), the I-V I obtain is reasonably good (straigh line, ~500Ohm slope obtained).
I cannot figure out what is wrong
I can post the graphs and the photo of the experiment if necessary.
Here is the schematics

Vin--------------------|>-----------/\/\/\/\/\--------GND
diode | 50 Ohm
|
|
Vout

Vdiode=Vin-Vout
Idiode=Vout/50


Thanks
Simone

 

[vk6kro]

When you do it like that, the current flowing is that of the diode plus the resistor across the input voltage.

This will be different to that of the diode alone. It will be more like a resistor when the diode is conducting, so you will get a sloping line instead of a vertical one.

To minimise this effect, you could try putting in just a 1 ohm resistor instead of the 500 ohm one.

You can then read the voltage across the resistor as milliamps. 10 mV = 10 mA