Diode single balanced mixer circuit
-a circuit of a diode single balanced mixer and its typical applications for radio frequency, RF circuits
RF mixers are widely used for radio frequency of RF applications. The mixers used in this arena multiply the two signals entering the circuit together. (note - audio mixers add signals together). The multiplier type mixers used in radio frequency applications are formed using non-linear devices. As a result the two signals entering the circuit are multiplied together - the output at any given time is proportional to the product of the levels of the two signals entering the circuit at that instant. This gives rise to signals at frequencies equal to the sum and the difference of the frequencies of the two signals entering the circuit.
One of the simpler mixer circuits is based around two diodes. This type of diode known as a single balanced diode mixer circuit provides rejection of the input signals at the output as a result of the fact that the two inputs are balanced.
The RF mixer circuit is only singly balanced and as a result it does not give isolation between the two input ports. This means that the signal from the local oscillator may leak onto the signal input line and this may give rise to intermodulation distortion. However for many applications this circuit operates quite satisfactorily. Where this may be a problem then a double balanced mixer should be used.
The RF mixer circuit has a typical conversion loss, i.e. the difference between the signal input and the output of around 8dB, although this depends upon the components used and the construction. The diodes should be as nearly matched as possible, and the transformer should be closely balanced for optimum rejection of the input signals at the output.
Where the input signals are widely spaced in frequency, it is possible to utilise a variation of the basic single balanced diode mixer to good effect. The circuit which is shown below may be used in a variety of applications, for example where an audio signal needs to be modulated onto a radio frequency, RF, carrier. In the circuit the two signals are combined using C1 as a high pass filter, and the combination of RFC and C2 as a low pass filter. In this way the leakage between the two input ports is minimised. A further refinement is that a balance control is incorporated into the balanced mixer circuit. This is used to ensure optimum balance. For example when used for modulating an RF carrier, it can be used to minimise the level of the carrier at the output, thereby ensuring only the two sidebands are produced.
Although this form of the single balanced diode mixer circuit does require a few more components, the performance is improved as the variable resistor enables much better balance to be achieved, and additionally there is some form of isolation between the two inputs.
By Ian Poole
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