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A frequency multiplier is a device that produces an output with a frequency that is an odd or even multiple of its input frequency. For instance, for a signal with a frequency of 100,000 cycles per second (cps) or 100 kilohertz (kHz), a frequency multiplier may produce a 200 or 300 kHz signal. The frequency multiplier is commonly used in communication circuits for producing the desired signal frequency given a reference frequency.
The main mechanism for a frequency multiplier is called non-linear signal transfer. When a frequency generator output is viewed on a spectrum analyzer, there is a single vertical line with a top end aligned with the amplitude of the signal. The level can be 1 volt (V). The horizontal axis indicates the frequency of this signal. For instance, the frequency of a 500 million hertz (Hz) or 500 megahertz (MHz) signal will be indicated on the horizontal axis of the spectrum analyzer.
When the reference frequency is fed into a non-linear device such as a class C amplifier or a diode detector, the result is a frequency spectrum with harmonics, or multiples, of the original frequency. Using a bandpass filter with a capability to pass only the desired outputs, the result is the desired harmonic. In the 500 MHz example above, a second harmonic at 1,000 MHz may be derived.
The frequency multiplier approach is used in special cases where there is only one output frequency required, due to the various combinations of bandpass filters needed to produce clean sinusoidal signals. For instance, a 10 billion cycles per second or 10-gigahertz (GHz) signal may not be possible to generate directly. It may be possible to use a 5 GHz oscillator, or frequency generator, followed by a multiplier and a bandpass filter tuned to 10 GHz.
Also known as a mixer, the signal multiplier is used in frequency modulation (FM) systems to produce an FM carrier that is shifted in frequency into the desired transmitting frequency. The mixer, unlike the frequency multiplier, requires two inputs. Although designated as an analog multiplier, the mixer output actually generates sum and differences of the input signals.
The heterostructure barrier varactor (HBV) is a special back-to-back diode arrangement that has a capacitance that varies with the voltage across the diode. A single-diode varactor uses a single diode with a capacitance that changes with the level of reverse bias on the diode. The capacitance peaks when the voltage is zero, while the capacitance drops in the positive or negative swing of the voltage across the HBV. This characteristic makes the HBV useful for frequency multiplication.
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