Conventional AM (DSBFC)

Carrier power constitutes two-thirds or more of the total transmitted power.

Utilizes twice as much bandwidth as needed with single sideband systems

Single Sideband Systems- mathematically recognized and

understood as early as 1914- first patent completed and a

successful communications link established between England and US in 1923

AM Single-Sideband Full Carrier (SSBFC) - form of amplitude modulation in which the carrier is transmitted at full power, but only one of the sidebands is transmitted.

Full-Carrier Single Sideband

SSBFC Waveform

AM Single-Sideband Suppressed Carrier(SSSC)

- a form of amplitude modulation in which the carrier is totally suppressed and one of the sidebands removed.

Suppressed-Carrier Single Sideband

SSBSC Waveform

AM Single-Sideband Reduced Carrier(SSBRC)

- a form of amplitude modulation in which one sideband is totally removed and the carrier voltage is reduced to approximately 10% of its unmodulated amplitude

- Single-Sideband Reinserted Carrier Pilot carrier

- Exalted Carrier

Reduced-Carrier Single Sideband

AM Independent Sideband (ISB)- a form of amplitude modulation in

which a single carrier frequency is independently modulated by two different modulating signals

- a form of double-sideband transmission in which the transmitter consists of two independent single-sideband suppressed carrier modulators

Independent Sideband

ISB Waveform

AM Vestigial Sideband (VSB)- a form of amplitude modulation in

which the carrier and one complete sideband are transmitted, but only part of the second sideband is transmitted.

Vestigial Sideband

Comparison of Single-Sideband Transmission to Conventional AM

Peak Envelope Power (PEP)- the rms power

developed at the crest of the modulation envelope

Advantages: Power Conservation Bandwidth Conservation Selective Fading Noise Reduction Disadvantages: Complex Receivers Tuning Difficulties

Conventional AM vs Single Sideband

Example:For a 500W carrier modulated

to a depth of 80%, find the total power in each of the following forms of AM: (a) A3E (b) J3E (c) H3E (d) DSB.

Recall:Forms of Amplitude Modulation

A3E – Double Sideband Full CarrierH3E – Single Sideband Full CarrierB8E – Independent SidebandJ3E – Single Sideband Suppressed CarrierR3E – Single Sideband Reduced CarrierC3F – Vestigial Sideband

Mathematical Analysis of Suppressed-Carrier AM

Carrier

Vam(t) = [ 1 + m sin (2fmt)] [ Ec sin (2fct)]

AM Modulator

Modulating Signal

Vam(t)

Constant + modulating signal Unmodulated carrier

Vam(t) = [ m sin (2fmt ) ] [ Ec sin (2fct) ]

Vam(t) = -mEc/2 cos [2(fc+fm)t] + mEc/2 cos [ 2(fc-fm)t]

Upper sideband frequency component

Lower sideband frequency component

Single-Sideband Generation- most SSB system has either

suppressed carrier or reduced carrier Notch Filters – remove carrier from the

modulated wave or reduce its amplitude - do not have sufficient Q-

factor to remove the carrier without also removing a portion of the sideband

Double-Sideband Suppressed-Carrier Modulators

- modulator circuits that inherently remove the carrier during the modulation process

- Balanced Modulator

Balanced Ring Modulator- Balanced Lattice Modulator- Balanced Modulator- made up of diodes and transformers

Semiconductor Diodes – ideally suited for use in balanced modulator circuits because they are stable, requires no external power source, have long life and require virtually no maintenance

Balanced Ring Modulator

2 Inputs: single carrier frequency and modulating signal

Circuit OperationD1 to D4 – electronic switches that control whether the modulating signal is passed from the input transformer to output transformer as is or with a 180 phase shift

Carrier Polarity: D1 and D2 – FB

D3 and D4 – RB

* The modulating signal is transferred across the closed switches from T1 to T2 without phase reversal.

Carrier Polarity Reverses: D1 and D2 – RB

D3 and D4 – FB

* Modulating signal undergoes a 180 phase reversal before reaching T2

Carrier current flows from its source to the center taps of T1 and t2 where it splits and goes in opposite directions through the upper and lower halves of the transformers.

Thus, their magnetic field cancel on the secondary windings of the transformers and the carrier is suppressed.

Carrier Leak – the small carrier component always present in the output signal

- 40 dB- 60dB is the amount of carrier suppression

Modulating signal

Carrier Signal

Output Waveform Before Filtering

Output Waveform After Filtering

Output: Consists of a series of RF pulses whose repetition rate is determined by the RF carrier switching frequency.

: The amplitude is controlled by the level of the modulating signal.

: It takes the shape of the modulating signal, except with the alternating positive and negative polarities that correspond to the polarity of the carrier signal.

Example:For a Balanced Ring Modulator, a

carrier input frequency fc= 400kHz and a modulating signal frequency range fm= 0 kHz to 4 kHz; determine (a) output frequency spectrum

(b) output frequency for a single-frequency input fm= 2.8 kHz

FET Push-Pull Balanced ModulatorFET – a non-linear device that exhibits

square-law properties and produce only second-order cross-product frequencies

FET Modulator – product modulator and produces only the

sidebands at its output and suppresses the carrier

FET Push-Pull Balanced Modulator

Circuit Operation Carrier is fed into the circuit in such a way that

it is applied simultaneously and in phase to the gates of both FET amplifier (QA and QB).

Carrier produces currents in both the top and bottom halves of the output transformer T3 that are equal in magnitude but 180 out of phase.

Therefore, they cancel and no carrier component appears in the output waveform.

Circuit Operation Modulating signal is applied to the circuit

in such a way that it is applied simultaneously to the gate of the two FETs 180 out of phase.

Modulating signal causes an increase in the drain current in one FET and a decrease in the drain current in the other FET.

In-phase sum of dc and carrier currents

IqaIdaIqb

Idb

ItaItb

Out-of-Phase sum of dc and carrier currents

Ida

Iqb Iqa

ItaItb

Idb

Sum of dc, carrier, and modulating signal current

Ita

Ima

IqaIqb IdaIdb

Itb

Imb

Balanced Bridge Modulator- operation is completely

dependent on the switching action of the diodes D1 through D4 under the influence of the carrier and modulating signal voltages

- carrier voltage controls the on and off conditions of the diodes

Balanced Bridge Modulator

How does the carrier signal being suppressed?

Operation:

Diode biased off Diode biased on

Output Waveform

Linear Integrated-Circuit Balanced Modulators

- available up to 100MHz- LM 1496/1596- can provide carrier suppression of 50dB

at 100MHz- 65 dB at 500 kHz- a double-balanced

modulator/demodulator that produces an output signal that is proportional to the product of its input signals.

Differential Amplifier

AM DSBSC modulator using the LM1496/1596 LIC

Assignment: Research on the methods on how

single-sideband is being generated.