Features of the use of varicaps. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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Voltage-controlled semiconductor capacitors of variable capacitance - varicaps - devices with a strongly pronounced non-linearity. For this reason, in circuits where an alternating voltage of relatively large amplitude is applied to the varicap, it is able to surprise.

Essentially, a varicap is a reverse-biased semiconductor diode. The direct branch of its current-voltage characteristic, which is fundamental for the main purpose of the diode (rectification, detection), is insignificant for a varicap. In the general case, a diode and even a collector or emitter junction of a bipolar transistor can be used as a varicap (and in practice this is often implemented).

Unlike semiconductor diodes, for varicaps they normalize (and, of course, provide during production) the capacitance of the p-n junction at a certain bias voltage on it and the quality factor. Note that it is not easy to achieve a quality factor of a varicap significantly exceeding the quality factor of a contour coil. This is explained by the fact that in a varicap, as in any diode, the resistance of the base region of the semiconductor is always connected in series with the p-n junction, and in parallel - the equivalent resistance due to the reverse current through the junction. The relatively low quality factor of the varicap implies, in particular, the need to take it into account when calculating the quality factor of the oscillatory circuit

The dependence of the capacitance of the p-n junction on the reverse voltage applied to it has a power-law character of the form C-Un, where the value of the parameter n can be in the range from 0,33 to 0,5 (determined by the junction manufacturing technology). On fig. 1 shows a typical capacitance-voltage characteristic of the D902 varicap built in linear coordinates. Similar characteristics can be found in the reference literature. They allow you to determine the capacitance of the varicap at different values ​​of the bias voltage.

However, it is preferable to deal with the capacitance-voltage characteristic of the varicap, built on a "double" (ie, on both axes) logarithmic scale. It is known that a power function looks like a straight line on such a scale, and the tangent of the angle of its inclination to the y-axis is numerically equal to the exponent of the function. On fig. 2 shows this graph for the D902 varicap. Having measured the sides of a right-angled triangle ABC with an ordinary ruler, we obtain a value of 0,5 (AB / BC) for the module of the exponent. The falling nature of the characteristic indicates that this indicator has a minus sign. Thus, the dependence of the capacitance of the D902 varicap on the applied voltage has the form С = U-0.5.

The above applies to "classic" varicaps. To increase the control efficiency of modern varicaps, special technological measures are taken during their manufacture, therefore, capacitance-voltage characteristics may no longer have such a simple form.

Since the capacitance-voltage characteristic of a varicap is non-linear, its use in equipment inevitably leads to distortion. German radio amateur Ulrich Graf (DK4SX) measured second and third order intermodulation distortion in various bandpass filters containing semiconductor diodes (Ulrich Graf. Intermodulation an passiven Schaltungsteilen. - CQ DL, 1996, No. 3, s. 200-205). He applied to the filter input (input resistance 50 ohms) two signals with a level of +3 dB (10 mV at 50 ohms) and analyzed the spectrum of the output signal. Graf chose the frequency values ​​of the input signals so that the intermodulation products would fall within the passband of the filter.

In one of the experiments in a two-circuit input band-pass filter, the constant capacitors included in the oscillatory circuits were replaced by varicaps. In this case, the second-order intermodulation components at the filter output increased in level by 10 dB, and the third - by almost 50 dB!

In other words, varicaps in the input circuits of receivers can worsen their real selectivity, although, most likely, they "work" this way only in relatively high-class equipment (communication technology). However, even in a middle class receiver, intermodulation at the input varicap can become significant if the receiver is operated near transmitters.

There are, however, nodes in which, in principle, a relatively large alternating voltage must be supplied to the varicap - we are talking about generators. On fig. 3 shows a widely used scheme for including a varicap in the oscillatory circuit of the generator, and in fig. 4 - capacitance-voltage (C) and current-voltage (I) characteristics of the varicap and instantaneous voltage across the varicap (Ur) at two values ​​of the control voltage (Uynp). Please note that for clarity on the graph, the scale along the "U" axis to the right of zero and along the "I; C" axis down from zero is enlarged. As long as the control voltage is high (Uynp1) compared to the amplitude of the AC voltage (Ur), the varicap operates normally. But with a decrease in the control voltage (Ucontrol2), there may come times when, at the peaks of the negative half-wave of voltage, the operating point of the varicap will enter the direct branch of the current-voltage characteristic and it will begin to rectify the alternating voltage applied to it.

How to determine the boundary of the zone of normal operation of the varicap in the generator? You can, for example, measure the AC voltage on the varicap and compare it with the control voltage. This requires an RF voltmeter with high input impedance and low input capacitance (so that its connection does not change the generator operating mode).

The minimum allowable control voltage on the varicap can be determined without violating the generator's operating mode, and using a frequency meter. It is connected to the generator output and the dependence of the generator control slope on the control voltage is removed.

Steepness of control - is the ratio of the change in the frequency of the generator to the given change in the control voltage that caused it - ΔF / ΔU. When the varicap is fully included in the circuit, the slope can, for example, be described by a power function (at least for D902), the exponent of which depends on the type of capacitance-voltage characteristic of the varicap. Recall (see above) that such a function, if plotted on a "double" logarithmic scale, is a straight line. If the varicap starts to leave the normal mode of operation, the nature of the dependence of the slope on the control voltage will change. This is also true in a more general case, when the varicap is not completely included in the circuit or its capacitance-voltage characteristic is not a power function.

Since the capacitance-voltage characteristic is non-linear, the measurements must be carried out in a certain sequence. By setting some control voltage Uynp, the oscillator frequency Fr is determined. Then, first, this voltage is reduced to Uypr - ΔUynp, and then it is increased to Uynp + ΔUynp and the corresponding frequency values ​​Fr1 and Fr2 are read from the frequency meter display.

The control slope at the control voltage Uypr is calculated by the formula ΔF/ΔU = (Fr2-Fr1)/2ΔUynp. The absolute value of the voltage change ΔUypr should be minimal, but such that it is possible to reliably record the change in the frequency of the generator. Then set another value of the control voltage Ucontrol and repeat the measurements. This technique reduces the influence of the nonlinearity of the capacitance-voltage characteristic of the varicap on the accuracy of measuring the control slope.

The results of measurements of the slope of the generator frequency control with the full inclusion of the varicap in the circuit (see Fig. 3) are shown in fig. 5. It can be seen that when the control voltage on the varicap is below 3,5 V, it exits the normal mode. In other words, for the specified generator this voltage will be critical.

With a further decrease in the control voltage, the slope of the curve may even change its sign! This happens due to the already mentioned rectification of the high-frequency voltage applied to the varicap. The rectified voltage is subtracted from the control voltage and begins to prevail over it.

If the described situation happens, for example, with the local oscillator of your receiver, there will be something to be surprised at. Imagine - when you rotate the knob of the variable resistor "Setting" in the same direction, the reception frequency first changes in one direction, then practically stops changing, and then it can go back.

Author: B.Stepanov, Moscow

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