What Is a Frequency Multiplier?

Frequency multiplier A circuit that makes the output signal frequency equal to an integer multiple of the input signal frequency. The input frequency is f1, then the output frequency is f0 = nf1, and the coefficient n is any positive integer, which is called the frequency doubling times. The frequency multiplier is widely used. For example, after the frequency multiplier is used in the transmitter, the main oscillator can be oscillated at a lower frequency to improve the frequency stability; the frequency modulation device uses a frequency multiplier to increase the frequency offset; The frequency multiplier is an important component of the carrier recovery circuit.

Chinese name: Frequency multiplier
Foreign name: frequency multiplier

Sexual performance: An important component of carrier recovery circuit
Form of formation: Non-linear circuits generate higher harmonics, etc.

Frequency Doubler Overview

Frequency multipliers can be constructed using non-linear circuits to generate higher harmonics or using frequency control loops. The frequency doubler can also be composed of a voltage controlled oscillator and a control loop. Its control circuit generates a control voltage, so that the oscillation frequency of the voltage-controlled oscillator is strictly locked at the multiplication value f0 = nf1 of the input frequency f1.

Frequency multipliers include transistor frequency multipliers, variable capacitance diode multipliers, and step recovery diode frequency multipliers. Frequency multipliers can also be constructed with other non-linear resistors, inductors and capacitors, such as ferrite frequency multipliers. The frequency doubler composed of non-linear resistors has a large frequency doubler noise. This is caused by the large number of harmonics generated during the non-linear transformation process, which makes the output signal phase unstable. The higher the number of frequency doublings, the greater the frequency doubling noise, which limits the application of the frequency doubler. In equipment that requires low frequency doubling noise, phase-locked loop frequency multipliers and synchronous frequency multipliers constructed according to the principle of phase-locked loops can be used. However, this type of multiplier circuit is relatively complicated, and the number of frequency multiplications is generally not too high, and problems such as phase loss lock may also occur.

The frequency stability of the microwave oscillator is not very high. A high-order frequency multiplier is added after the crystal oscillator of tens of megahertz to 100 megahertz to obtain microwave oscillation with crystal frequency stability. In addition, the cascade of multi-stage frequency multipliers can greatly increase the number of frequency doublers. For example, the cascade of the doubler and the tripler can generate six times of octave, the m-level N-folder is cascaded, and the total number of octave is Nm. However, as the number of octave stages increases and the octave noise also increases, the upper limit of the octave is still limited.

Frequency Doubler Transistor

The circuit of this kind of frequency doubler is similar to the tuning amplifier, but the operating point of the transistor is usually placed in the cut-off region of the volt-ampere characteristic, and the output circuit is tuned to the nth harmonic of the input frequency. Since the transistor is turned on only during a part of the positive half cycle of the input voltage, its collector current is a pulsating current containing the fundamental frequency of the input signal and each harmonic. Utilizing the frequency selection effect of the loop tuned to f0 = nf1, the frequency doubler can output the required frequency. In order to make the amplitude of the output signal sufficiently large, the frequency multiplication times of such a frequency multiplier are low, and generally n = 3 to 5. Increasing the output amplitude will decrease significantly. The advantage of this type of frequency doubler is a certain power gain.

Variable frequency multiplier

A negatively biased varactor D is connected between the input and output loops. The high-Q filter formed by L1C1 only allows the input signal with frequency f1 to generate current i in the left loop. Due to the non-linear characteristics of the varactor diode, the terminal voltage of the diode contains harmonic frequencies such as the fundamental frequencies f1 and 2f1, ..., nf1. At the output end, due to the role of the high-Q band-pass filter, only the component with frequency nf1 can pass through the right loop and output useful harmonic power to the load. The varactor diode doubler is sometimes called a parametric doubler. Its frequency doubling efficiency is inversely proportional to the number of frequency doublings n. In order to make the output large enough, generally n <10 prevails.

Frequency doubler step recovery

The step-recovery diode with steeply changing capacitance characteristics works in two states of conduction and step under the action of the excitation voltage, and forms a short duration pulse with a large amplitude at the instant of the step. The energy spectrum of this pulse is uniformly distributed in a comb shape, and there is still a certain energy output at dozens or even hundreds of harmonic frequencies. The step recovery diode frequency doubler is suitable for constructing a high frequency doubler, but the amplitude does not require a large high frequency doubler and comb spectrum generator.

Doubler type

Different non-linear components can be used to form different types of frequency doublers.

Parametric frequency multiplier

Frequency multiplier composed of non-linear reactance devices. One of the most widely used non-linear reactance devices is a varactor diode. The parametric transduction effect produced by its non-linear capacitance characteristics can achieve the frequency doubling function. Theoretically, the capacitor is an ideal lossless component, and it will not consume energy when the input signal is transformed non-linearly. Therefore, the parametric frequency multiplier can convert all the input signal energy into output harmonic energy, that is, its conversion efficiency is equal to 1. In fact, varactors and filters are always dissipative, and it is not possible to filter out all unwanted components generated by non-linear capacitors. Its actual conversion efficiency is less than 1, and it tends to decrease with the increase of the number of frequency doublings. It can be seen that such parametric frequency multipliers are also impossible to achieve high frequency doubling. But compared with the triode frequency multiplier, its conversion efficiency has been greatly improved.

Frequency multiplier

In the short-wave and ultra-short-wave bands, triode frequency multipliers composed of crystal triodes are used. Because of the collector current pulse generated by the transistor under the input signal, the amplitude of each harmonic current always decreases rapidly with the increase of the harmonic order. Therefore, the higher the number of frequency doublings, the lower the frequency doubling efficiency; in order to filter out low-order harmonic components with large amplitudes, the requirements for filter out-of-band attenuation are also higher. Triode frequency multipliers can only achieve low frequency multipliers (less than five times) frequency multipliers, most of which are two or three frequency multipliers. In order to achieve high frequency multiplication, several levels of frequency multipliers can be connected in series to form a frequency multiplier link.

Phase- locked frequency multiplier

Insert a frequency divider in the phase-locked loop and change the frequency of the frequency division to achieve any multiple of the frequency. Frequency doublers are widely used in transmitters, frequency synthesizers and other information transmission and processing systems. Using a frequency doubler in the transmitter can double the lower oscillation frequency generated by the crystal oscillator to the required carrier frequency, or double the low carrier frequency and small frequency offset FM wave generated by the indirect frequency modulator to high carrier and large frequency Partial FM wave. In the frequency synthesizer, a frequency multiplier can be used to generate a stable oscillation signal of many frequencies from a stable oscillation frequency. With the development of digital signal processing technology, the frequency doubling function can be realized by software in the digital signal processor.

Frequency Multiplier Step Diode Frequency Multiplier

A frequency multiplier composed of a step recovery diode is used to achieve high frequency doubling. It is also a type of parametric multiplier. Different from varactor diodes, step recovery diodes have very steep capacitance characteristics. That is, a large capacitance appears when a forward voltage is applied; a small capacitance appears when a reverse voltage is applied. Under the action of the input signal, the large amount of charge stored in the diode during forward conduction will be quickly discharged when the reverse voltage is transferred, forming a large reverse impulse current and generating a very rich harmonic content. This is why the step diode multiplier is suitable for achieving high frequency doubler. Its frequency multiplication times can be as high as 40 or more.