Date post: | 22-Dec-2015 |
Category: |
Documents |
Upload: | madlyn-tucker |
View: | 221 times |
Download: | 4 times |
Power of AMThe total transmit power is given by
We can prove that, at least 50% (often close to 2/3) of the total transmitted power resides in a carrier term that is independent of the signal and thus conveys no message information. (what a waste!)
DSB Signal SpectraWe set u=1 and suppress the
unmodulated component and obtain the modulated signal:
DSB conserves power but requires complicated demodulation circuitry, whereas AM requires increased power to permit simple envelope detection.
Tone Modulation If the transmitted signal is a single tone signal,
then the tone-modulated DSB waveform is given by
Balanced ModulatorPerfect square-law devices are rare; high-
frequency DSB is obtained in practice using two AM modulation arranged in a balanced configuration to cancel out the carrier.
Ring ModulatorAnother commonly used modulator is the ring
modulator, which uses a carrier to cause the diode to switch on and off.
Switching Modulators
Efficient high-level modulators are arranged so that undesired modulation products never fully develop and need not be filtered out.
Suppressed Sideband AMConventional AM is wasteful of
both transmission power and bandwidth. Suppressing the carrier can reduce the transmit power, while suppressing one-sideband can reduce the bandwidth.
For suppressing the sideband, we have either SSB or VSB.
Generation of SSB
SSB requires perfect filter actions.But a perfect cutoff at f_c cannot be synthesized.
Fortunately, many modulating Signals of practical interest haveLittle or no low-frequency content.
V (Vestige) SBVSB achieves a tradeoff between SSB and DSB,
whose signal is passed through the following filter
Frequency ConversionThe frequency conversion starts with
multiplication by a sinusoid.
Frequency converter (mixer) Satellite transponder
Coherent Detection In coherent detection, the local oscillator of
receiver is exactly synchronized with the carrier in both phase and frequency.
We can pick off the pilot carrier by using a narrow bandpass filter, which is called homodyne detection.
Coherent Demodulation of VSBFor VSB, the sum of the vestige side band
recovers the original frequency spectrum: