Single-Sideband Communications Systems
Transcript of Single-Sideband Communications Systems
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.