is for FM/TV tun ing i test the diode i read old book, abc of varactor,and i make a circuit like in book to measure capacitance, but hi change the value i dont understang nothing, some times is good tot 300 450pf, soms to 1000 to 1500p, i dont know maybe my multimter is not working

How did you measure varicap capacitance?

Maximum capacitance is with zero reverse voltage, but most capacitance meters apply a test signal that could skew the reading.

Many high end FM receivers use inverse series varicaps so a rising signal swing in one cancels the falling swing in the other - but I've never seen it in a TV tuner where capacitances and signal swings are vastly smaller.

 

How did you measure varicap capacitance?

Maximum capacitance is with zero reverse voltage, but most capacitance meters apply a test signal that could skew the reading.

Many high end FM receivers use inverse series varicaps so a rising signal swing in one cancels the falling swing in the other - but I've never seen it in a TV tuner where capacitances and signal swings are vastly smaller.

I read abc of varactor, so you connect one potentiometer, and 1mega, between positive and negative of varactor you measure capacitance

 

I read abc of varactor, so you connect one potentiometer, and 1mega, between positive and negative of varactor you measure capacitance

My capacitance meter would either be damaged by the bias - or shunt it.

Max capacitance at zero bias - but the test signal would skew result.

 

Why do you want to measure the capacitance of a low capacitance varactor diode? Simply use it in a VHF circuit.

The leads of my expensive Fluke multimeter are a little longer than 1m. They measure 200pF between them so I cannot measure low value capacitors. I unplugged the leads and the meter measures 150pF for its wiring inside it. A solderless breadboard would also cause over 100pF of capacitance between its rows of contacts and wires.

 

Why do you want to measure the capacitance of a low capacitance varactor diode? Simply use it in a VHF circuit.

The leads of my expensive Fluke multimeter are a little longer than 1m. They measure 200pF between them so I cannot measure low value capacitors. I unplugged the leads and the meter measures 150pF for its wiring inside it. A solderless breadboard would also cause over 100pF of capacitance between its rows of contacts and wires.

rely to use VHF and measure....i will try but today i have to buy thiing for home ;-) huis is empty zonder material;-) thank you man

 

i try but does not work

 

can somebody tell me, is there a formula for maximum capacitance of varicap thanks

 

can somebody tell me, is there a formula for maximum capacitance of varicap thanks

Mostly a product of doping level and junction area.

 

Mostly a product of doping level and junction area.

sorry i ask to much, is possible for you to show me one example?

 

sorry i ask to much, is possible for you to show me one example?

Google broken again?!!!

 

Hello,

Perhaps the info in the following links might help:
http://mysite.du.edu/~etuttle/electron/elect40.htm
https://en.wikipedia.org/wiki/Varicap
http://www.radio-electronics.com/info/data/semicond/varactor-varicap-diodes/basics-tutorial.php

Also the attached PDF's might be of interest.

Bertus

 

Hello,

Perhaps the info in the following links might help:
http://mysite.du.edu/~etuttle/electron/elect40.htm
https://en.wikipedia.org/wiki/Varicap
http://www.radio-electronics.com/info/data/semicond/varactor-varicap-diodes/basics-tutorial.php

Also the attached PDF's might be of interest.

Bertus

thank you very much for losing time for me....

 

Google broken again?!!!

no no i search a lot and still i am searching, but in datacheet, they dont explain the maximum pf, but they dont explain;-(, is difficult also the the abc of varactor book dont explain i still dont finish the book,

 

Michael, please post the schematic and parts list of your circuit that did not work.
The datasheet of the BB205 varactor diode says its minimum capacitance is 2pF when its reverse bias is 30V and is 16pF with 2V reverse bias. The actual pF vary a little for different diodes and stray wiring capacitance will add to their actual capacitance.

 

Michael, please post the schematic and parts list of your circuit that did not work.
The datasheet of the BB205 varactor diode says its minimum capacitance is 2pF when its reverse bias is 30V and is 16pF with 2V reverse bias. The actual pF vary a little for different diodes and stray wiring capacitance will add to their actual capacitance.

i have this problem i told you already, i just want to know the maximum varicap capacitance, you see, in datacheet is 2pf min and max 16pf
but when you apply the voltage they changes, this i dont know;-(

 

i just want to know the maximum varicap capacitance, you see, in datacheet is 2pf min and max 16pf
but when you apply the voltage they changes, this i dont know;-(

As you were told by Duke on the other website, "I looked up the Telefunken bb205b diode. The data says 1.8 to 17pF.
The 1.8pf will be at maximum voltage and the 17pF at minimum voltage."
Don't you believe the datasheet? The maximum capacitance is 16pF or 17pF when it has 1.2V or 2V of reverse bias. Stray wiring capacitance will add 5pF to 100pF to it.

Show us your schematic that did not work.

 

As you were told by Duke on the other website, "I looked up the Telefunken bb205b diode. The data says 1.8 to 17pF.
The 1.8pf will be at maximum voltage and the 17pF at minimum voltage."
Don't you believe the datasheet? The maximum capacitance is 16pF or 17pF when it has 1.2V or 2V of reverse bias. Stray wiring capacitance will add 5pF to 100pF to it.

Show us your schematic that did not work.

i dont have circuit, you know my friend, u are right i belive datacsheet, but this man in electronics shop he say is almost 500pf, for that i ask so much, maybe he does not know, what do you think?

but i read to find minimum and maximum from one book
for example diode Zetex 832A if c2 minimum = 22pf
(c2/c20)= ratio = 5 so increase by factor 5
so 22pF/5 = 4.4pF

this varies from 22pF TO 4.4 pF
but at reverse is 40pF at 0V, do you have any idea, how they calculate, of where they take this 40pF
now how did they get this 40pf, how they get 40pF at reverse 0V they dont write in datasheet, do you have and idea?
and also i dont see in datasheet from this book 40pf

 

The minimum reverse voltage of a Varactor diode is usually 1.2V or 2V, not 0V.

 

The minimum reverse voltage of a Varactor diode is usualnly 1.2V or 2V, not 0V.

Audioguru, I ask something else i ask at 0v the diode capacitance is ~40pF, so this ~40pf i dont see in datasheet, ???? i hope you understand me now

 

A varactor diode is not used at 0V so it is not on the datasheet. Whoever told you the capacitance is 40pF at 0V then ask them.
Maybe at 0V then the signal strength changes the capacitance too much and it is useless for tuning.