Frequency converter in RF applications

Frequency converters, more commonly called mixers, are used for converting an RF signal, transmitted over the air medium, to a lower frequency signal suitable for demodulation in a receiver. Double-balanced metal-semiconductor field effect transistor (MESFET) mixers are frequently used in RF receivers. A typical frequency converter includes a digital down-converter (DDC) and a digital up-converter (DUC). The digital down-converter A/D (analog-to-digital) converts a received RF/IF (radio frequency/intermediate frequency) signal, and then down-converts the A/D-converted signal to a baseband or demodulation IF signal by digital signal processing. The digital up-converter D/A (digital-to-analog)-converts a baseband or modulation IF signal, and then up-converts the D/A-converted signal by digital signal processing in order to generate a transmission RF/IF signal. Frequency converters are capable of attenuating image signals without using a narrow band-pass filter in a receiver which receives signals of a UHF band or signals of a microwave band.

A typical frequency converter device uses a mixer to down-convert or up-convert an input signal to an output signal. Frequency conversion of signals is primarily accomplished by a frequency mixer element. The frequency mixer multiplies two or more input signals in the time domain or convolves one or more input signals in the frequency domain. For a down-conversion of a high radio frequency (RF) signal to an intermediate frequency (IF) the mixer combines the RF signal with a local oscillator (LO) signal to produce a sum and a difference. The difference gives the down-converted signal. For an up-conversion of an IF to an RF signal, the mixer combines the IF signal with the LO signal to produce a sum and a difference. The sum gives the up-converted signal. A multiplier is typically used as a mixer for frequency conversion of an intact signal, and a decimation filter and an interpolation filter are generally used for suppression of aliasing that occurs while converting a sampling frequency.

Frequency converter in electrical applications
A frequency converter is a device used to generate an adjustable alternating voltage by using a constant-frequency voltage. A frequency converter is typically used for steplessly adjusting the rotation speed of an alternating-current motor, such as a short circuit motor. The rotation speed is adjusted in the frequency converter mainly by converting the voltage supplied to the motor and the frequency of the voltage. When the frequency converter is used for controlling the motor, the motor can be controlled and adjusted from a control unit by a computer. Frequency converters are widely applied since electric motors are used in devices of many different types. The frequency converter is typically used for controlling electric motors for pumps, blowers, elevators, hoists, conveyors etc.

A frequency converter generally consists of a rectifier for rectifying a received alternating voltage, an intermediate circuit for filtering a direct voltage received from the rectifier, an inverter for changing the rectified and filtered voltage back to an alternating voltage, and a control unit for controlling the operation of the inverter on the basis of received measurement information to enable a correct frequency to be provided for the voltage to be fed to a motor. The rectification unit provides direct voltage from the supplied alternating-current voltage to the voltage intermediate circuit, to which one or more capacitors are connected to store and equalize the direct voltage depending on the power of the device. The inverter unit connects the direct voltage of the voltage intermediate circuit to the load in such a manner that the direct voltage pulses generate the desired average alternating-current voltage or a corresponding adjustable property.

The rotation speed of a short circuit motor is proportional to frequency. By steplessly changing the frequency of a motor, the rotation speed of a controlled motor can be changed correspondingly. The rotation rate adjustment is carried out in the frequency converter mainly by changing the voltage supplied to the motor and the voltage frequency. The frequency converter allows adjusting acceleration and deceleration, as well as slowing down and reversing of the motor. When the frequency converter is used for controlling the motor, it is possible to perform the adjustment and control of the motor by a computer in a control unit.