ELEC 241 Lab
Analysis
In Labs 2 and 4 we built
primitive optical communication systems
with an LED as a transmitter and a photodiode as a receiver.
The performance of these systems left much to be desired.
The waveforms were grossly distorted, and the range
was at best a few feet.
In order to produce a viable optical telephone, we need to
greatly reduce the distortion
and greatly increase the range.
Eliminating Distortion.
Fortunately, the first goal is relatively simple.
As we saw in Lab 4, the photodiode
current
is a linear function of the optical
power striking it.
In a fortunate instance of symmetry,
the optical power
produced
by a LED is an
almost
linear function of the current through it.
So just as we can linearize the response of the photodiode
by connecting it to a
transresistance amplifier
(which converts its input current to a proportional output voltage),
we can linearize the LED by driving it with a
transconductance
amplifier
(which converts its input voltage to a proportional output current).
However, we still have to solve the problem of rectification.
Remember, an LED is a diode and the current through a diode can't
be negative
(and even if it could, the power out can't be negative either).
We solve this problem
essentially
the same way we did for AM radio.
Recall, for AM:
![$x(t)=A[1+s(t)]\cos 2\pi f_c t$](img131.png)
We can do the same thing with our LED, adding a constant
offset to the signal to be transmitted so that the
modulating signal ![$[1 + s(t)]$](img132.png)
(and hence the current in the
LED) never goes negative.
Increasing Range.
As we increase the distance between the transmitter and receiver,
the size of the received signal becomes smaller.
When the signal becomes so small we can no longer hear it,
we have reached the useful range limit for our system.
We can make the signal larger by amplifying it,
but we will also amplify any noise and interference present
in the signal,
and the amplifier will introduce some additional noise of its own.
Eventually the
signal to noise ratio (SNR) limits the intelligibility of the signal
and additional amplification doesn't help.
At this point we must have either more signal or less noise.
So the things we can do to increase range are:
increase the receiver gain until SNR becomes the limiting factor,
increase the power of the received signal,
reduce or eliminate sources of noise and interference.
We already know how to increase gain,
and
the communication equation tells us how to increase
received signal power.
To reduce noise, we must identify and deal with each source
on a case by case basis.