@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.
From previous threads you have posted, it appears that you want to use the varactor diode to tune a radio. An earlier thread implied this was an AM radio tuning the broadcast band. If this is still your intention, the varactor diode you have selected is totally inappropriate. The BB205G varactor diode has a very small capacitance range, from 1.8 pF (minimum) to 17 pF (maximum). This is adequate for tuning an FM receiver from 88 MHz to 108 MHz, or TV tuners in the VHF band, but it is not enough range for AM broadcast band tuning from 540 kHz to 1600 kHz, which typically requires a tuning capacitance variable from about 25 pF at 1600 kHz to 250 pF at 540 kHz. The required tuning capacitance ratio varies inversely as the square of the frequency ratio and inversely with the inductance. While 1.8 to 17 is approximately 1:10 ratio, the same ratio at 25 to 250, it would require an inordinately large inductance to resonate from 540 kHz to 1600 kHz. If this is the range you are trying to tune, I would suggest using the NTE618 varactor diode which has at least a 1:15 capacitance ratio, a minimum capacitance of about 20 pF, and a maximum capacitance of about 420 pF.
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.
