1. Amplitude Modulation Circuits
Chapter 3
Amplitude Modulation
Circuits

2. AMPLITUDE MODULATORS
AM is the process of multiplying the carrier by the modulating signal.
Two ways:multiply with gainor attenuation factor that varies with modulating signal or linearly mix /algebraically ad the carrier and the modulating signal,then apply the composite signal to a non liner device

3. Cont’d Non-linear mixing is another name for AM.
An amplitude modulator must multiply analog signals or mix them together in a non-linear device like a diode.

4. DIODE MODULATOR Diode provides Envelope matches multiplication
information signal
Information
Carrier
AM signal

5. HOW THE DIODE MODULATOR WORKS
The resistive network adds the carrier and modulating signals.
The diode rectifies the result producing non-linear mixing.
The parallel tuned circuit selects the carrier and sidebands and rejects the modulating signal.

6. DIFFERENTIAL AMPLIFIER MODULATOR
A differential amplifier makes a good amplitude modulator. The modulating signal is used to vary the common mode current in the transistors that amplify the carrier.
Most integrated circuit (IC) amplitude modulators are based upon the differential amplifier modulator.

7. DIFFERENTIAL AMPLIFIER(CONT’D)
ADVANTAGES:
MAKES EXCELLENT AMPLITUDE MODULATOR.
HIGH GAIN AND GOOD LINEARITY, CAN BE MODULATED 100%

8. PIN DIODE MODULATOR
A PIN diode acts as a voltage variable resistor at very high frequencies.
PIN diodes are special diodes made to be used for frequencies above 100MHz
An amplitude modulator can be made with PIN diodes if the carrier frequency is VHF, UHF, or microwaves.
The PIN diodes are connected as resistive attenuators for the carrier. The attenuation is varied by the modulating signal producing AM.

9. PIN DIODE MODULATOR Disadvantages:
-produces attenuation, thus needs of amplifying the signal back before ready for transmission.
-but widely used, because the only modulator for microwave frequencies.

10. AM LEVEL
Low level AM produces the AM signal at a very low power level. High power amplifiers increase the power to the desired level. Less efficient linear amplifiers must be used to amplify the AM signal.
High level AM is produced by amplitude modulating the final amplifier stage in a transmitter. More efficient class C amplifiers can be used.

11. AM(HIGH LEVEL)
Produce best type of AM, but needs extremely high power modulator circuit.
For 100% modulation, the modulator must have at least half the total power of class c amplifier. Say if class C amplifier has input power of 1000W, the modulator must be able to deliver 500W

12. AM DEMODULATOR
A demodulator is a circuit that recovers the original information signal from the carrier that is received.
The simplest AM demodulator is a half wave rectifier circuit called a diode detector.
The diode detector is also the simplest AM receiver. It is called a crystal radio.

13. AM DIODE DEMODULATOR (DETECTOR)
Information out
AM signal in
Diode current
(no filter)
Filtered waveform

14. BALANCED MODULATOR
A balanced modulator is an amplitude modulator that suppresses the carrier but produces both upper and lower sidebands.
A balanced modulator produces DSB.
Diode ring or diode lattice modulators are widely used.
IC balanced modulators based upon differential amplifiers are very popular.

15. DIODE LATTICE BALANCED MODULATOR
When the modulating signal is non-zero,
the diodes are unbalanced and so are the
carrier currents which produce an output.
When the modulating signal is 0, there is no
output since the currents in the output transformer
are equal and opposite (flux cancellation).
Modulating
signal
DSB
output
Carrier oscillator

16. GENERATING SSB
A balanced modulator eliminates the carrier and provides DSB.
A filter removes the undesired sideband producing SSB.
Quartz crystal filters are the most widely used sideband filters since they are very selective and inexpensive.

17. TYPICAL SSB TRANSMITTER
Antenna
DSB
signal
SSB
signal
Balanced
modulator
Carrier
oscillator
Microphone
Audio
amplifier
Sideband
filter
Linear
amplifier fC
Filter
response
curve
Lower
sidebands
Upper
sidebands

18. THE PHASING METHOD OF SSB
Another way to produce SSB uses a phase shift method to eliminate one sideband.
Two balanced modulators driven by carriers and modulating signals 90º out of phase produce DSB.
Adding the two DSB signals together results in one sideband being cancelled out.

19. DEMODULATING SSB AND DSB
The basic diode detector will not recover the information from a DSB or SSB signal.
A balanced modulator is used to recover the SSB or DSB signal.
A SSB/DSB demodulator is often called a product detector.
The incoming SSB or DSB signal is mixed with the carrier generated in the receiver to reproduce the original modulating signal.

20. DSB SIGNAL WITH SINE WAVE MODULATION
Note: the envelope is not a sine wave
The basic diode detector will produce this waveform
which is not the original information signal.

21. TYPICAL DSB OR SSB DETECTOR
Also called a
product detector
Carrier
oscillator
Balanced
modulator
Information out
DSB (or SSB) signal

22. Example of how product detector works..
Carrier=9MHz
Modulating signal=2kHz=0.002MHz
After modulation, and SSB generated, only upper sideband=9.002MHz transmitted.
In receiver, add with local oscillator at 9MHz, output is sum and difference:18.002MHz, and 0.002MHz=modulating signal.
Use filter to filter everything else