Frequency Mixer and Frequency Converter: How to Distinguish the Two

In a frequency mixer the radio frequency signal voltage if frequency and local oscillator voltage of frequency are applied to its input. The frequency mixer uses a device which presents non linear dynamic characteristics. Hence the two input voltages beat together or heterodyne with the mixer to produce output current having the components of frequency. Out of all of these terms, one of interest in the component is difference frequency. These components may be readily selected by using a tuned circuit in the output circuit tuned to this difference frequency called the intermediate frequency. Every radio frequency signal irrespective of its frequency is reduced to this standard intermediate frequency. Accordingly it is necessary that the local oscillator frequency be made to vary in such a way as to maintain difference frequency always equal to the intermediate frequency.

Several methods of frequency conversion in super heterodyne receivers are in use. In each method the collector current of the frequency mixer transistor is made to vary at a combination frequency of the signal frequency and the oscillator frequency reducing the desired intermediate frequency voltage across the tuned load. Although this basic principle of operation is the same for all types of frequency converters the difference lies in whether a separate local oscillator is used as in a frequency mixer or the same device serves both as mixer and local oscillator.

Frequency mixer uses the circuit of two transistors one as a frequency mixer and other as a local oscillator. Frequency mixer transistor is so biased that the operation takes place over the non linear region of its characteristics curve. This non linear operation results in collector current components of frequencies in addition to components of local oscillator frequency and signal frequency. The tuned circuit in the collector circuit picks up the desired frequency term being tuned to this frequency.

The radio frequency signal of frequency obtained either from the receiving antenna or from the radio frequency amplifier is fed to the base of the mixer transistor operated in common emitter configuration. The output from the local oscillator using transistor is fed to the emitter of the mixer transistor. The resulting collector current is thus influenced by both the signal voltage and the local oscillator voltage. Because of non linearity of characteristics of transistor the collector current contains the desired component of difference frequency. This difference frequency constitutes the intermediate frequency. The desired intermediate frequency voltage gets developed across the parallel tuned circuit of transistor. Capacitor feeds the oscillator voltage to the mixer but disallows the low resistance of the oscillator coil from shunting the stabilizer resistance. No bypass capacitor is connected across the stabilizing resistor otherwise the oscillator voltage fed to the emitter gets grounded.

About the Author

Tymon Hytem has worked in the electronics feild for the past 15 years. He enjoys helping people decide on electronic gadgets from telephones to XM Radio and choosing the perfect XM Satellite Radio system for their needs.