MIXING & AMPLITUDE MODULATION PRINCIPLE
Ever wonder how signals were mixed and modulated? Here are some theories and explanations of some mixing techniques and modulation.
FREQUENCY CONVERSION AND MIXING
Frequency
conversion is the changing of the frequency of a carrier with its
modulation from one frequency to another. This occurs when the signal
is mixed with a second signal such as the output of an oscillator in
such a manner that the output contains products of the two signals.
One of these products will contain the sum and the difference
frequencies of the two input signals. Other components are present as
well, but all components except the desired one may be removed by
bandpass filtering. Two general methods can be used to accomplish
mixing functions, additive mixing and multiplicative mixing. The
output frequency is termed the intermediate frequency (I.F).
The
input to a mixer is the input signal voltage, which has a magnitude
of Vs at a frequency of fs. The output is usually a current component
at the IF frequency which will have a magnitude that will
be proportional to Vs.
ADDITIVE
MIXING
Additive
mixing occurs when the input signal is simply added to the output of
a local oscillator and then passed through a device with a nonlinear
transfer function such as a diode. Diodes with essentially square-law
response are usually used because this yields under certain
conditions, an almost proportional relationship between the input
signal and the output signal. The output form the mixer contains many
signal components, including the difference frequency and the sum
frequency and several harmonics of each.
Generally
this output is passed directly to an IF amplifier, which acts as a
bandpass filter just wide enough to pass modulation sidebands around
the IF, providing whatever gain is required to boost the signal to
the final detection level. An example of additive mixing is a diode
modulator which will be discussed next.
DIODE
MODULATOR
A
simple circuit for producing amplitude modulation using additive
mixing is shown in figure 3.1.
The
modulating signal, in this case audio, is applied to the top of R1
while the carrier is applied at the top of R2. The
signal at the junction of R1 & R2 is the
sum of the carrier and audio. That is, the carrier is simply riding
on the audio signal. Notice that the carrier is not amplitude
modulated at this point. It is simply added to the audio signal.
S1
is included in the circuit merely for the for explanation purposes.
Normally, it is omitted and the tank circuit composed of C1
and L1 is connected directly in parallel with R3.
The
purpose of the tank circuit, which is turned to the carrier
frequency, becomes clear when S1 is closed. Each time D1
conducts a pulse of current flows through the tank. This causes the
tank to resonate and the flywheel action of the tank produces a
negative half cycle for each positive input pulse. The high amplitude
positive pulses cause high amplitude negative pulses, and the low
positive amplitude pulses cause low amplitude pulses. Therefore, each
negative half cycle will have the same amplitude as the positive half
cycle.
As
you can see, the output is on AM wave. Thus, this simple circuit
produces amplitude modulation.
MULTIPLICATIVE
MIXING
Multiplicative
mixing occurs when the transconductance of the circuit is caused to
vary with the local oscillator voltage, so that the output current
becomes a function of the product Vo and Vs.
Figure 3.2 shows the general block diagram of a multiplicative mixer.
The
Collector Modulator
The
most commonly used AM technique is oscillator modulator or, in vacuum
tube modulators, plate modulation. In collector modulation, the
modulating signal is applied to the transistor’s collector, in
series with DC supply voltage.
A
typical collector modulation the modulating signal is applied to the
collector through transformer. With no modulating signal present,
there is approximately zero volts dropped across the secondary of the
transformer. Therefore, the entire Vcc voltage is present
at the collector of the series modulator. However, the modulation is
applied, there is an AC voltage, it is alternately in phase and out
of phase with Vcc. When it is in phase with Vcc,
a high positive potential is applied to the collector of the series
modulator. This causes an increase in collector current &
therefore higher amplitude output signal. This is the peak of the
modulation envelope. When the voltage is out of phase with Vcc,
a lower positive potential is applied to series modulator. This
results in lower collector current & hence, a lower output
amplitude. This is the trough of the modulating envelope.
While
collector modulation is the most commonly used technique, it does
have disadvantages. The most serious of these is the modulation
transformer itself. It is large, heavy, costly, and introduces
frequency distortion. That is, due to the inductance of the
transformer, some frequencies are attenuated more than the others.
One method that eliminates the modulation transformer is series
modulation.
This
technique uses a transistor in place of the transformer.
Plate
Modulator
In plate modulation
systems, the voltage delivered to the stage in change. As the power
output available as a function at the supply voltage of the output is
modulated. Its advantage is that
audio power can be applied to the stage converted into RF power.
Summary: The
principle of its modulation has its own waveform value of modulation
dependent on the value of the input and the frequency used for
modulation and the output also depends in its components had to the
modulators. The outcome when two signals mixed also depends on what
the input and output of the modulation system produced and its waveform.
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