Double balanced diode mixer
-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.
The circuit of a double balanced diode mixer
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.
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