# Hello, varactor(varicap) formula help

Discussion in 'Radio and Wireless' started by michael1978, Feb 18, 2018.

1. ### michael1978

388
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Mar 17, 2012
hello can somebody help me how to measure maximum capacitance of varicap, or formula one example thanks

2. ### kellys_eye

4,277
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Jun 25, 2010
You can only get this from either the manufacturers datasheet or by actual measurement of a specific device (applying control voltage and measuring the change over the permitted range of reverse voltage).

3. ### michael1978

388
2
Mar 17, 2012
i do some experiment, connecting varicap to measure, but i had no succses, but is strange i read datachet, let say 6 to 16pF is, he dont tell the maximum capacitance, but i will like to know how much is maximum capacitance, i buy one bb205G they say 16pF and FM/TV tuning

4. ### davennModerator

13,500
1,830
Sep 5, 2009

HUH ?

6 - 16pF is the range what don't you understand about that ? 5. ### duke37

5,300
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Jan 9, 2011
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.

6. ### michael1978

388
2
Mar 17, 2012
yes is like that, but i take like example, but when you bias and apply voltage across, that change capacitance, i dont know how change

7. ### duke37

5,300
739
Jan 9, 2011
Look up Wikipedia which shows the internal structure and circuits to supply the bias voltage.
The graph of the capacitance against bias voltage is far from linear.
The AC voltage should be less than the bias voltage.
The diodes were often sold as three matched devices.
What do you want the diode to do?

8. ### hevans1944Hop - AC8NS

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Jun 21, 2012
Check this page for specifications. You generally have to isolate the varactor diode from the resonant circuit that it helps to tune. You do this by inserting a series capacitor of at least ten to hundred times the maximum varactor capacitance. Without this capacitor any inductance present in the (usually) parallel tuned circuit will "short out" the bias voltage applied to the varactor diode that is used to change its capacitance.

The bias voltage is applied in the reverse bias direction and serves to increase the width of the depletion region of the diode junction. hence decreasing the capacitance of the junction. I suppose you could increase the capacitance by applying a small forward bias, but I have never heard of anyone doing that. In any case, if you are trying to measure the capacitance of the varactor diode, use a 200 pF dipped mica capacitor in series with it and then apply your reverse bias voltage across the varactor diode. Make sure whatever AC signal is applied to measure the capacitance is small enough to not forward bias the varactor diode. Measure the series combination of the varactor and the 200 pF mica capacitor: the series-connected mica capacitor will contribute a negligible reduction to the varicap capacitance.

A better varicap or varactor diode for AM radio tuning purposes would be this NTE618.

Last edited: Feb 19, 2018
9. ### michael1978

388
2
Mar 17, 2012
i am reading the book abc of varactor, and i wil like to know more about tuning with varicap

10. ### michael1978

388
2
Mar 17, 2012
i will meausure

11. ### Audioguru

2,732
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Sep 24, 2016
Why do you want to try measuring such a tiny amount of capacitance? It is 0.00000000001 micro-farads.

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12. ### hevans1944Hop - AC8NS

4,378
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Jun 21, 2012
@michael1978: Because of the DC bias required to change the capacitance of varactor diode, the usual test equipment, such as a muiltimeter with a capacitance-measuring function, may not work very well. Also, because the capacitance is quite small and is easily affected by the connecting wiring and adjacent circuit construction, an alternative, non-contact, means of measuring the capacitance is desirable.

To measure small capacitance values accurately, you can use an instrument known as a grid-dip oscillator (GDO) meter. The GDO meter consists of an external inductance coil connected to an internal variable capacitance and to an internal circuit that causes oscillations to occur at tune-able radio frequencies from about 100 kHz to 200 MHz. A D'Arsonval meter measures power absorbed by an external resonant circuit when it is loosely coupled to the GDO inductor. When the external resonant circuit resonates with the GDO frequency, either a pronounced "dip" or "peak" in the meter reading occurs. Thus the resonant frequency of the external resonant circuit is identified. Some GDO meters have a built-in digital frequency meter to display an accurate representation of the resonant frequency, but you can usually purchase an inexpensive frequency meter on eBay for this purpose. Problem is, the resonant "peak" or "dip" is not particularly sharp or narrow, depending on the Q of the tuned circuit and the amount of coupling between the GDO inductance coil and the tuned circuit. You will be fortunate ot obtain two or three significant figures for the resonant frequency, but that is usually sufficient for experimental designs.

If the external resonant circuit is assembled from a known-valued capacitor and a suitable inductor, then the inductance can be calculated fairly accurately from the resonant frequency by using the formula:
L = 1 / [(4)(π)²(F)²(C)], where L is in henries, F is in hertz, and C is in farads.

Once the inductance value is known, you can substitute the varactor for the known-valued capacitor in the external resonant circuit. You can then measure the resonant frequency as a function of the reverse bias applied to the varactor diode, and from that information and the now known inductance, calculate the varactor capacitance as a function of the reverse bias.

So, to reiterate, the basic idea is to create an external resonant circuit using a capacitor of known value and then substitute your varactor diode for the known-valued capacitor, measuring the resonant frequency in both instances. A suitable inductor must be selected to resonate with both capacitors at some frequency within the range of the GDO meter, but it's inductance value is not critical or even very important as long as it remains constant.

You cannot directly connect the varactor diode across an inductor to form a parallel resonant circuit because the inductor resistance would "short out" the reverse bias voltage you must apply to vary the capacitance of the varactor. To get around this problem, simply select a capacitor to connect in series with the varactor diode to block the DC bias voltage. This series-connected capacitor must have a capacitance value that is much larger, at least ten times to as much as a hundred times larger, than the maximum varactor capacitance. If this condition is met, the effective capacitance of the series combination will be dominated by the smaller varactor capacitance. When you substitute a series-connected combination of the varactor diode and a much larger valued mica capacitor for the known-valued mica capacitor, the resulting effective capacitance will essentially be that of the varactor alone.

Apply a variable reverse-bias voltage across the varactor diode and again measure the resonant frequency, this time recording the frequency as a function of the bias voltage. Since you now know the value of the inductor, you can now calculate the varactor capacitance at each resonant frequency. The effective combination of two capacitors in series is Ceff = 1 / [1/Cv + 1/Cs], where Cv is the varactor capacitance and Cs is the capacitance placed in series with it to block the DC bias voltage. The limit of this equation as Cs becomes much larger than Cv is Ceff = Cv as 1/Cs approaches zero.

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13. ### michael1978

388
2
Mar 17, 2012
thank you very much................................i go buy the other varicap, because he told me is 500pF MAXIMUM
thanks a lot

14. ### michael1978

388
2
Mar 17, 2012
hi is ok i solved

15. ### michael1978

388
2
Mar 17, 2012
Mr i have one more question,

because 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 this say in the book bu after he write

the maximum capacitance 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 0V they dont write in datasheet, do you have and idea?
and also i dont see in datasheet from this book 40pf

so
first he write varies from 22pf to 4.4 pf, and after he says at 0volt is capacitance 40pf

he write those formula for one tuning circuits

how to find this 40pf at maximum, please explain me thank you

16. ### michael1978

388
2
Mar 17, 2012
exist a formula, how can i know what is the maximum, when they say 19.2pf min to max 24pf

17. ### hevans1944Hop - AC8NS

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Jun 21, 2012
It is now obvious to me that you still have zero understanding of how a varicap or varactor diode works. Also, it appears that you do not know how to read a datasheet. In the quote above that you made, you mention a book but did not bother to mention the title of the book, the author, or when and where it was published.

I have copied part of the datasheet for the Zetex 830 series of varactor diodes and it is shown below:  You will notice that there are minimum, nominal, and maximum values of capacitance listed for a reverse bias of 2 V, measured at 1 MHz. For the 832A, these three values are 19.8 pF, 22.0 pF, and 24.2 pF. They represent the range of capacitance values that you can except to find for any particular part when that part is reverse-biased with 2 V DC and the capacitance is measured at 1 MHz. In general, this range of values represents the maximum capacitance you should expect for 2 V reverse bias.

If the reverse bias is decreased toward zero volts, the varactor capacitance will increase, but how much it will increase is not specified. It may increase to 40 pF, but nothing on the datasheet indicates that. There is a set of curves in the datasheet that plot capacitance as a function of the reverse bias voltage, but the scales are logarithmic on both axes. These graphs only show capacitance in the range of 1 pF to 200 pF on the ordinate axis for reverse bias voltages from 1 V to 200 V on the abscissa axis. Since the graph does not show capacitance for reverse bias voltages less than 1 V, it is pure speculation what the 832A varactor diode capacitance would be at 0 V reverse bias. The capacitance as a function of reverse bias is not a linear relationship.

If the reverse bias is increased toward 25 V DC (the maximum allowable without causing damage to the varactor diode), the capacitance will decrease. How much it will decrease is specified only for a reverse bias of 20 V DC, and then only as a ratio to the capacitance at a reverse bias of 2 V DC. For the 832A varactor diode, the capacitance ratio at the two bias voltages of 2 V DC and 20 V DC will be some number between 5.0 and 6.5. Divide these ratios into the range of capacitance specified for 2 V reverse bias to find the minimum capacitance to expect when the reverse bias is 20 V DC.

18. ### Audioguru

2,732
616
Sep 24, 2016
ALL electronic parts are not made exactly the same. Some are typical (or nominal in some datasheets), some are minimum and some are maximum even if they have the same part number. The datasheet lists the range from minimum to maximum and the typical (nominal) value for some spec's. You need to design the circuit so that it works with any parts within the range of spec's, or adjust each product so it works with the part it has. Some product manufacturers by thousands of one part and test them all then group the parts into minimum, typical and maximum spec's.

19. ### michael1978

388
2
Mar 17, 2012
Mr thank you, you say that at 2v measured is 19,8 or 22, or 24pf the maximum, and if is 1V is unkown because they dont write in datasheet
sorry man, i dont know so much, i know diode when increase voltage decarse the capacitance if decrase voltage increase capacitance, i am a begginer, dont be nervous please, i want to learn the varicap because i have a book of radio, they explain very good, but they are with variable capacitor, is i decide to learn varicap
because they are difficult to find and expensive,

so to find the capactiance you mus divide 6.pf or 5pf with 2V and 20V, to find the capacitance ohh please dont be mad, i make you tired

Last edited: Feb 22, 2018
20. ### hevans1944Hop - AC8NS

4,378
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Jun 21, 2012
It is common practice for some end-users (or re-sellers) of large quantities of the same part number to test and "bin" all received parts according to the results of the test. With automated test equipment, it is not necessary to just sample a lot of parts to make an accept/reject decision of the whole lot, which was a common procedure in the previous century. While statistically any device from a given production run will have characteristics that fall between the published or "guaranteed" minimum and maximum values, with "most" of the parts having characteristics close to the typical value, the actual range of values depends on how well the production process is understood and controlled.

Unless there are specific quality control documents and specifications that all parts must adhere to, the only remedy an end-user has is to return to the manufacturer or the re-seller any parts that do not fall within the limits of the guaranteed values. Some purchase orders may specify that an entire lot will be returned if any of the parts fails to meet specifications. Obviously such practices will inflate the cost of the part. If the end-user has a very large volume commitment, they may bin the parts after testing and sell the outliers at a discount, reserving for themselves those bins closest to the "nominal" value. These practices clearly put the hobbyist buying in onesie-twosie quantities at a disadvantage, but there is little that can be done about it. The Golden Rule applies: He who has the Gold, Rules.

Somewhat disgusted, not mad, that you have failed to understand. Where did you find the value of 6.8 pF? What do you mean to "divide 6.8 pf with 2V and 20V? If you are going to divide anything, pick any number between the minimum capacitance ratio of 5.0 and the maximum capacitance ratio of 6.5 and divide that capacitance ratio number into any value between the minimum and maximum capacitance values that occur with 2 V reverse bias. This range of values represents the maximum varactor capacitance. The result after dividing by the capacitance ratio will be the minimum varactor capacitance.

The minimum varactor capacitance occurs with 20 V DC reverse bias. The maximum varactor capacitance occurs with 2 V DC reverse bias. The varactor will have more capacitance for reverse bias voltages less than 2 V, but the capacitance is not specified nor is it guaranteed. The varactor will have less capacitance for reverse bias voltage more than 20 V, but the capacitance in not specified nor is it guaranteed. Exceeding 25 V reverse bias may irrevocably damage the varactor. It is inadvisable to forward-bias the varactor diode without providing a means to limit the forward current, but it is your part, you own it: do what whatever you feel is necessary to advance your learning process.

Last edited: Feb 22, 2018
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