Frequency Multiplier

an electronic or, less frequently, electromagnetic device for delivering an output wave with a frequency that is an exact integral multiple of the input frequency. The ratio of the output frequency f_out to the input frequency f_in is called the frequency multiplication ratio m(m ≥ 2, and can be as large as several tens).

It is a characteristic property of frequency multipliers that if f_inis changed within a certain finite range, the factor m remains constant, as do some other parameters, such as the resonant frequencies of oscillatory circuits and resonators used as components of frequency multipliers. From this it follows that if for some reason the input frequency is increased by a sufficiently small increment Δf_in, the increment Δf_out will be such that Δf_in/f_in = Δf_ou/f_out. This means that the relative instability of the oscillation frequency remains unchanged upon multiplication. This important property permits frequency multipliers to be used for increasing the frequency of stable oscillations (usually obtained from a quartz driving oscillator) in various radio transmitters, radar, and measuring equipment.

The most widely used types of frequency multipliers consist of a nonlinear device (such as a transistor, varactor, varicap, coils with ferrite cores, or an electron tube) and one or several electric filters. The nonlinear device changes the waveform of input oscillations. Consequently, the oscillation spectrum at the output of the device exhibits components with frequencies that are multiples of f_in. These complex oscillations enter the input of the filter, which extracts the component that has the specified frequency and suppresses all other components. In real filters this suppression is incomplete, and unwanted, or spurious, harmonics with numbers other than m appear at the output of the frequency multiplier. This problem is alleviated if the nonlinear devices generate practically only the mth harmonic of f_in. In this case the filter may sometimes be omitted; some frequency multipliers of this type use tunnel diodes and special electron beam devices. At m > 5, it is advantageous, from the point of view of power consumption, to use multistage frequency multipliers, in which the output oscillations of one stage serve as input oscillations for the next stage.

The operation of certain frequency multipliers is based on the synchronization of self-excited oscillators. Oscillations excited in such an oscillator have the frequency f₀ ≈ mf_in, and become exactly equal to mf_in when acted upon by oscillations of a frequency f_in that enter the input of the oscillator. The disadvantage of such frequency multipliers is the comparatively narrow range of values for f_in that can be used for synchronization. In addition to the types mentioned, radio pulse frequency multipliers have also found some application. Here, the radio pulses are shaped by the input oscillations of the frequency f_in and are then fed to the input of an electrical filter.

The primary problem encountered in designing frequency multipliers is the reduction of phase instability of the output oscillations, caused by the random character of phase changes. Phase instability increases the relative frequency instability of the output as compared with the same value of the input. A rigorous design of frequency multipliers involves the integration of nonlinear differential equations.