A frequency multiplier or Frequency Doubler is an electronic circuit that generates an output signal and that output frequency is a harmonic of its input frequency. Frequency multipliers consist of a nonlinear circuit that distorts the input signal and consequently generates harmonics of the input signal.
Frequency multiplier multiplies the frequency of the input by an integer, such that the output is a multiple of the input. Frequency multipliers are generally class C amplifiers.
The circuit of the frequency doubler may be looked at from different angles. With input signals ≥1 V, T2 and T3 operate as full-wave rectifiers. This means that the fundamental frequency of the input signal is automatically doubled.
With input signals <1 V, the two anti-phase signals produced by T1 from the input signal are applied to the emitters of T2 and T3 and summed. This means that the fundamental frequency will virtually disappear, so that, because of non-linearities, only the harmonics remain: the first harmonic will then become the new fundamental frequency in the output signal. This means, of course, that the signal strength reduces appreciably: of an input of 25 mV, only 6 mV remain.
Frequency Doubler Circuit Diagram:
Assuming that the input is sinusoidal, the suppression of the fundamental frequency is optimized with P1. The operating point of T3 should be set with P2 for as near a sinusoidal output as possible in the prototype, the output signal showed a distortion of 5.5% with an input signal frequency of 1 kHz. The input frequency range is 80 Hz to well over 100 kHz.
Any tendency of T2 or T3 to oscillate may be suppressed by soldering a small ceramic capacitor (about 56 pF) between the base and collector of the transistor. The circuit draws a current of about 4 mA.
What is the purpose of the frequency multipliers?
Frequency multipliers and dividers are key components of modern radio frequency (RF) and microwave systems. They are generally used in signal generation and frequency synthesis applications.