Optical Mixer

Mixing signals at
different frequencies is common practice in many areas of electronics.
Audio systems, communications systems and radio systems are typical
application areas. With conventional frequency mixers, feedback
capacitance can cause the signal sources to be affected by the output
signal, thus making supplementary filter circuits necessary. The signals
from the individual signal sources can also affect each other. In an
optical mixer, LEDs or laser diodes are used
to first convert the signals to be mixed into optical signals. The light
beams are then aimed at a shared photo-sensor (a light-sensitive
resistor, photo-diode, photo-transistor, or photovoltaic cell). The
current in the output circuit is thus controlled by the mixed input
signals, so signal from the photo-sensor is the sum of the input


Circuit diagram

The amount of feedback capacitance can be made quite small,
depending on the construction. Another benefit is that the input and
output circuits have separate grounds, which can be electrically
connected if necessary. This operating principle directly encourages
experimentation. Additional input stages can be added to act on the
shared photo-sensor. If the receiver signal is applied to a component
with a curved characteristic, such as a diode, this produces amplitude
modulation, which can be used in a hetero-dyne receiver. If the
difference between the frequencies of the two input signals is small, a
beat effect occurs. The components must be selected according to the
frequency range that is used.

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