Double balanced diode mixer circuit
-a circuit of a double balanced diode mixer and its typical applications for radio frequency, RF circuits
Radio frequency mixers such as the double balanced diode mixer are used, not for adding signals together as in an audio mixer, but rather multiplying them together. When this occurs the output is a multiplication of the two input signals, and signals at new frequencies equal to the sum and difference frequencies are produced.
Being a double balanced mixer, this type of mixer suppresses the two input signals at the output. In this way only the sum and difference frequencies are seen. Additionally the balancing also isolates the two inputs from one another. This prevents the signals from one input entering the output circuitry of the other and the resultant possibility of intermodulation.
Typical performance figures for the circuit are that isolation between ports is around 25 dB, and the conversion loss, i.e. the difference between the signal input and output levels is around 8 dB. Using typical diodes, the input level to the mixer on the local oscillator port is around 1 volt RMS or 13 dBm into 50 ohms.
The isolation between the various ports is maximised if the coils are accurately matched so that a good balance is achieved. Additionally the diodes must also be matched. Often they need to be specially selected to ensure that their properties closely match each other.
In order to obtain the optimum performance the source impedances for the two input signals and the load impedance for the output should be matched to the required impedance. It is for this reason that small attenuators are often placed in the lines of the mixer. These are typically 3 dB, and although they do reduce the signal level they improve the overall performance of the mixer.
These mixers may be constructed, but for many commercial pieces of equipment they are purchased in a manufactured form. These devices can have the required level of development and as a result their performance can be optimised. Although they are often not cheap to buy, their performance is often worth the additional expense.
By Ian Poole
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