Question, measuring reverse voltage dependency of capacitance of a varicap diode

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yungbasedgod

Joined Apr 9, 2022
1
Hi! i have an assignment to use labview along with the circuit below to plot the capacitance of a varicap diode as a function of the reverse voltage. My plan so far is to gradually increase the DC voltage of the power supply, and at each step calculate the instantaneous value of the capacitance using the rise time measured by the oscilloscope. However, i'm having trouble deciding what the amplitude of the square wave from the function generator should be, mostly because i don't know what effect the square wave will have on the diode. Will the capacitance jump up and down in sync with the wave? or does it not affect it at all?
Any help is appreciated, and thank you very much in advance!



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C_KD=Varicap diode, C_R=equivalent unwanted capacitance from wires and components, GND=ground. Oscilloscope will connect to the circuit via 10x probe.
 

DickCappels

Joined Aug 21, 2008
8,711
Generally, I would think that the smallest pulse possible would be best.

The question that may reveal more is whether or not the test circuit is similar to the eventual application circuit.

One other thing - I would consider using a choke in series with the 10k resistor feeding the bias voltage to be sure that the characteristics of the power supply do not affect the scope display. Alternatively, it would be a good idea to use a couple of capacitors across the input from the power supply, located in the test fixture, say a .001 uf multilayer ceramic capacitor in parallel with a 1 uf to 10 uf electrolytic capacitor.
 
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