Linear modulation: AM, DSB and SSB
An AM station modulated at 100 % spends two thirds of its power on the carrier, which carries no information at all. In exchange, it can be received with a diode.
01Shifting the spectrum
In linear modulations, the complex envelope is a linear function of the message. Its effect in frequency is simple: the spectrum of the message is shifted around the carrier, without changing shape. The variants differ in whether they transmit the carrier and in how many sidebands they send. The introduction to the equipment is in analog communications, in the technician program.
02Double-sideband AM with carrier
The advantage of AM is the receiver: as long as \( m \le 1 \), the envelope reproduces the message and a diode with an RC filter recovers it. With \( m > 1 \) there is overmodulation: the envelope folds over the axis and the detector distorts. It occupies a bandwidth of twice the maximum frequency of the message.
03DSB and SSB
| Modulation | Bandwidth | Useful power | Receiver |
|---|---|---|---|
| AM | \( 2W \) | At most 33 % with a tone | Envelope detector |
| DSB-SC (suppressed carrier) | \( 2W \) | 100 % | Coherent: needs the exact carrier |
| SSB (single sideband) | \( W \) | 100 % | Coherent, tolerates small frequency errors |
| VSB (vestigial sideband) | Somewhat more than \( W \) | Almost all | Coherent; used in analog television |
DSB-SC spends no power on the carrier, but the receiver has to regenerate it with the exact frequency and phase: a phase error \( \theta \) multiplies the output by \( \cos\theta \). SSB sends only one of the two sidebands, which carry the same information, and occupies half the spectrum: it is the voice modulation of amateur radio operators and of shortwave communications.
A carrier modulated by a tone. Above, the signal and its envelope; below, its spectrum around the carrier, with the power of each line relative to the unmodulated carrier.
04How they are generated and received
AM is generated by modulating the supply voltage of a class C amplifier, or with a multiplier. DSB-SC comes from a balanced modulator, such as the MC1496 integrated circuit analyzed in a publication on this site. SSB is obtained by filtering out one of the sidebands of a DSB signal with a very selective filter, or by the phase method, which uses the Hilbert transform of the message. In reception, the superheterodyne shifts all the stations to a fixed intermediate frequency, where filtering is easier.
05In the lab
Modulate a generator in AM, measure the maximum and minimum envelopes on the oscilloscope and calculate m. Verify the sidebands on the spectrum analyzer.
Raise the index above 1 and observe the output of a diode envelope detector.
Build a balanced modulator with an MC1496 and adjust the carrier balance until the carrier is suppressed.
06Common mistakes
- Overmodulating an AM station. It distorts the audio and widens the spectrum.
- Demodulating DSB with an envelope detector. What you get is the absolute value of the message.
- Forgetting the phase error in coherent demodulation. A 90° phase shift cancels the output.
- Believing that the AM carrier carries information. It only makes reception easier.
07Self-assessment
What efficiency does AM have with m = 0.5 and a tone?
\( 0.125/1.125 \approx 11 \) %.
What bandwidth does an AM station occupy with audio up to 5 kHz? And in SSB?
10 kHz in AM; 5 kHz in SSB.
If the AM carrier is 100 W and m = 1, how much power is in each sideband?
25 W in each.
What happens if DSB is demodulated with a phase error of 60°?
The output is multiplied by \( \cos 60° = 0.5 \).
08Further reading
- B. P. Lathi and Zhi Ding. Modern Digital and Analog Communication Systems. 5th ed., Oxford University Press, 2019. Amplitude modulations with their spectra, generators and demodulators.
- Wayne Tomasi. Electronic Communications Systems: Fundamentals Through Advanced. 4th ed. (Spanish edition, “Sistemas de comunicaciones electrónicas”), Pearson, 2003. The circuits approach: AM and SSB modulators, transmitters and receivers.
- Herbert Taub and Donald L. Schilling. Principles of Communication Systems. 2nd ed., McGraw-Hill, 1986. A classic with a very clear treatment of AM, DSB, SSB and VSB.