# Hi, can somebody help to measure varicap diode

Hello to everobody,
i need help how works varicap
can somebody explain how it works, how to measure
i have bb205 VARICAP, i try to search on internet but i dont find something good, to learn

thanks

Merlin3189
Homework Helper
Gold Member
A capacitor is just two conductors (usually metal plates or metal films) separated by an insulator (air, mica, ceramic or various plastics.)
A reverse biased diode does not conduct because there is a region, the depletion region, which has few mobile charge carriers (electrons or holes) and behaves like an insulator. So you have a sandwich of an insulating region between two conductors, the anode and cathode, which makes it behave like a capacitor.*

Having got your varicap diode, I think you could measure its capacitance at various voltages by building an oscillator and measuring the frequency of oscillation. You would need to calibrate the circuit by using known values of capacitance in place of the varicap.
You might not need to measure the frequency, so long as you can detect and identify specific frequencies, say with a radio receiver, then adjust the varicap to match the frequency generated by a standard capacitor.
So make your oscillator circuit with a known capacitor about the value you expect for the varicap (err on the low side.) Design it to oscillate at a frequency near the middle of the range your radio can receive.
Tune the radio to the frequency you get, then leave it there.
Replace the fixed capacitor with the varicap circuit and adjust until you hear it on the radio the same as before.
Repeat with other values of known capacitor. (Or maybe use a known variable capacitor in different positions.)

*(Incidentally, this is one of the major limitations on switching transistors: the capacitances between electrodes have to be charged and discharged every time the transistor switches off and on.)

jim mcnamara
Mentor
Moved to Electrical Engineering....

A capacitor is just two conductors (usually metal plates or metal films) separated by an insulator (air, mica, ceramic or various plastics.)
A reverse biased diode does not conduct because there is a region, the depletion region, which has few mobile charge carriers (electrons or holes) and behaves like an insulator. So you have a sandwich of an insulating region between two conductors, the anode and cathode, which makes it behave like a capacitor.*

Having got your varicap diode, I think you could measure its capacitance at various voltages by building an oscillator and measuring the frequency of oscillation. You would need to calibrate the circuit by using known values of capacitance in place of the varicap.
You might not need to measure the frequency, so long as you can detect and identify specific frequencies, say with a radio receiver, then adjust the varicap to match the frequency generated by a standard capacitor.
So make your oscillator circuit with a known capacitor about the value you expect for the varicap (err on the low side.) Design it to oscillate at a frequency near the middle of the range your radio can receive.
Tune the radio to the frequency you get, then leave it there.
Replace the fixed capacitor with the varicap circuit and adjust until you hear it on the radio the same as before.
Repeat with other values of known capacitor. (Or maybe use a known variable capacitor in different positions.)

*(Incidentally, this is one of the major limitations on switching transistors: the capacitances between electrodes have to be charged and discharged every time the transistor switches off and on.)
yes but i dont have a oscillscope ;-(

Merlin3189
Homework Helper
Gold Member
I wasn't thinking of an oscilloscope. Just detecting the oscillations with a radio receiver.

I had in mind just constructing a simple oscillator similar to this idea. and c matching the output when you used ordinary capacitors.

But BB205 seems to be about 16pF down to 2pF, so really more for VHF than MF and that makes it a problem to find a suitable receiver.
Maybe you could make the oscillator, at a lower frequency that you can receive, using an ordinary capacitor, then seeing how much you can shift the frequency with the varicap. Again using an ordinary receiver to detect the frequency and to measure the shift.

davenn
Merlin3189
Homework Helper
Gold Member
Hi again Michael. Since my idea obviously didn't solve your problem, I decided to try it myself. It was not as simple as I thought!
Being accustomed to receiving CW signals, I'd forgotten that a MW radio is AM and does not respond simply to a single frequency signal. I'll look into producing a modulated signal.
What it has shown is that 10pF, say, doesn't produce a big shift and with the broad tuning of a domestic AM radio, the results are more qualitative than quantitative.
Also my breadboard setup is poor, because accidentally moving a wire or having my hand too close can produce a bigger shift than the varactor change. I might have to solder it up and give it the same attention as if I were really building a transmitter.

My mind at the moment is moving towards having two oscillators at higher frequencies, eg. 9 MHz and 10 MHz, with the 9 MHz fixed and the 10 MHz using the varicap. Then mixing these (just a diode ) should give a 1 MHz signal in the MW band. If the fixed 9 MHz is modulated, the heterodyne should also be modulated and audible on the AM receiver. Being at a higher frequency the shift should be greater for a given change in capacitance.

I'm still looking around for other ideas, but most use signal generators, oscilloscopes and frequency counters, which you don't want. (I must admit I've been cheating and using these things myself, but that's to help design my circuits and check what I'm finding.)
I see that there are very cheap (less than a multimeter) frequency meters available on the internet. If you really want to do this sort of experiment, maybe that would be the thing to do - and you'd probably find them useful or interesting for other things. Otherwise, just buy devices whose properties you can look up on data sheets, or with unknown devices, just try them in circuits and see what you get.