Varactor diodes

A collection of articles about varactor diodes

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Varactors and Parametric Amplification

(a) Capacitance and (b) charge variation against bias on diode. Note the distorted output in (b)

The curves for a capacitor are all markedly non-linear and this property can be used as an element in a circuit to modulate, demodulate and multiply frequency. This is non-linear reactance, rather than a resistance, however, and because of this, very little loss occurs.

The charge in a capacitor is equal to the capacitance multiplied by the voltage across the capacitor: Q = CV, and the capacitance/voltage curves above approximate very roughly to the reciprocal of the square root of the applied voltage. Combining Q = CV and C approx l/V*0.5, the charge on the capacitor can be seen to be proportional to the square root of the voltage. If an RF voltage is applied to a capacitative reactance which behaves in this manner, a heavily distorted charge, consisting mainly of the second harmonic of the input, is produced and can be tapped off using a tuned circuit. A diode used this way is called a varactor diode. This action can easily be demonstrated by using a zener diode as the non-linear element and a signal generator tuned to about 500 kHz giving an output of about 0.25 to 0.5V rms.

Circuit for demonstrating non-linear reactance effects.

The circuit broken down into its basic components.

In diagrams (a) and (b) above the circuit is in two halves: the left-hand part is tuned to 500 kHz and the right-hand part to 1 MHz. RF current is passed through the diode at 500 kHz and, due to the non-linear reactance of the diode, a proportion of its second harmonic will (or should be) circulating in the right-hand part of the circuit. L1 and L2 are IF transformer coils: L1 could be two coils in series (aiding), tuned to resonate at 500kHz with the diode in circuit, and L2 one coil tuned to resonate at 1 MHz. Resonance can be measured across the complete coil with a low-capacitance, high-impedance RF probe. Having tuned each circuit, apply 500 kHz to L1 and measure the output across L2. Tune this coil up very carefully (the resonance peak is very sharp). To prove this frequency has been produced by the diode and is not just generator harmonics slipping through, apply 1 MHz from the generator to L1 and see how much of this gets through to the output. With any reasonable Q in the coils, it will not be very much. Another proof would be to replace the diode with an equivalent capacitance; the output will go down to a very low proportion of the previous value.

Other harmonics can be produced in a similar way. The second harmonic can mix with the fundamental and produce the third, and the second can produce its own distorted charge for the fourth harmonic, although the output of the fourth harmonic is improved if the third is produced as well. In all these circuits for multipliers, current must be allowed to circulate through the diode at all the respective frequencies by circuits that are tuned to those frequencies. These circuits, excepting the output circuit, are called idlers. All tuned circuits have to be of the maximum unloaded Q to minimize circuit loss and they can be considered either as parallel- or series-tuned, depending on which way one looks at them

Various varactor multipliers: doubling in (a), tripling in (b) and quadrupling or quintupling in (c). (d) shows a way of altering the bias to make the input RF lie along some other point of the curve and possibly to improve the effect.

the diode average capacitance (at a certain voltage) is in series with the tuning capacitance and both of these tune the coil. A properly-designed doubler of this kind, using a diode specifically meant for this function, can give an output of about 90 per cent of the input, the rest being dissipated in circuit resistances. When multipliers of this sort were first used, some surprise was expressed that they could be fed with an AM input. This cannot normally be done with a frequency multiplier, at least to get a coherent output, but it seems that a non-linear reactance can demodulate at the input frequency and remodulate at the output frequency (with a little added harmonic distortion), to give an output which is at least intelligible.

Another use for varactor or charge-storage diodes is in, parametric amplification. If a certain charge is fed into the diode capacitance at the crest of an RF half-cycle and then the capacitance is suddenly decreased, the voltage associated with this charge becomes larger: Q = CV. With C reduced to half its original value, the voltage would double for the same charge. The potential energy of this charge, which is equivalent to ˝CV*2, would double as well. Obviously this would need some form of outside energy to make up the difference. If the capacitance is suddenly returned to its original value when the RF voltage becomes zero between its positive and negative half-cycles, no additional energy is needed because the PD across the capacitor is zero. Some energy has to be supplied to the circuit twice every cycle: RF energy at double the frequency. This RF energy, which is called the pump frequency energy, must also be in phase with the original frequency wave crests. Part of this pump frequency energy is transferred to the original frequency and amplifies it. The processs is called parametric amplification. As it stands, it would be quite difficult to carry out in practice because of the necessary phase relationship, but this relationship can be obtained simply by letting the pump frequency beat with the original frequency and providing an idler circuit for the beat frequency to circulate as is shown below.

(a) Basic series-tunes parametric amplifier. (b) Parametric amplifier with idler.

Of course, the idler circuit can provide a useful output of the difference frequency as a mixer, but care needs to be taken not to load the circuit too heavily. This method of amplication or mixing can, if done carefully, provide a really worthwhile gain and considerably less noise than most other methods but, as with all amplifiers, the parametric system can be considered to show negative resistance and can readily oscillate at the input or idler frequencies if given a chance: eg when the tuned circuit and diode losses are cancelled by this negative resistance. It thus seems to be a matter of balancing Q values in the various circuits against each other to get the best results.

This negative resistance and amplification mentioned above has been obtained through energy supplied from an outside RF source.

FM modulators using varactors

There are two types of FM modulators - direct and indirect. Direct FM involves varying the frequency of the carrier directly by the modulating input. Indirect FM involves directly altering the phase of the carrier based on the input (this is actually a form of direct phase modulation.

Direct modulation is usually accomplished by varying a capacitance in an LC oscillator or by changing the charging current applied to a capacitor.

The first method can be accomplished by the use of a reverse biased diode, since the capacitance of such a diode varies with applied voltage. A varactor diode is specifically designed for this purpose. Figure 1 shows a direct frequency modulator which uses a varactor diode.

This circuit deviates the frequency of the crystal oscillator using the diode. R1 and R2 develop a DC voltage across the diode which reverse biases it. The voltage across the diode determines the frequency of the oscillations. Positive inputs increase the reverse bias, decrease the diode capacitance and thus increase the oscillation frequency. Similarly, negative inputs decrease the oscillation frequency.

The use of a crystal oscillator means that the output waveform is very stable, but this is only the case if the frequency deviations are kept very small. Thus, the varactor diode modulator can only be used in limited applications.

The second method of direct FM involves the use of a voltage controlled oscillator, which is depicted in figure 2.

The capacitor repeatedly charges and discharges under the control of the current source/sink. The amount of current supplied by this module is determined by vIN and by the resistor R. Since the amount of current determines the rate of capacitor charging, the resistor effectively controls the period of the output. The capacitance C also controls the rate of charging. The capacitor voltage is the input to the Schmitt trigger which changes the mode of the current source/sink when a certain threshold is reached. The capacitor voltage then heads in the opposite direction, generating a triangular wave. The output of the Schmitt trigger provides the square wave output. These signals can then be low-pass filtered to provide a sinusoidal FM signal.

The major limitation of the voltage controlled oscillator is that it can only work for a small range of frequencies. For instance, the 566 IC VCO only works a frequencies up to 1MHz.

A varactor diode circuit for indirect FM is shown in figure 3.

The modulating signal varies the capacitance of the diode, which then changes the phase shift incurred by the carrier input and thus changes the phase of the output signal. Because the phase of the carrier is shifted, the resulting signal has a frequency which is more stable than in the direct FM case.