By David Herres
When you modulate a signal, you are varying one or more properties of a periodic waveform, called the carrier signal, with another signal, the modulating signal, that contains information to be transmitted. Typically the modulating signal has a lower frequency than the carrier.
Modulation is applicable for combining two or more signals. The lower frequency information wave is said to modulate the higher frequency carrier wave. By this it is meant that changes in amplitude of the lower frequency wave cause the higher frequency energy to change in amplitude, frequency or phase.
Where there is modulation, there must be demodulation. (A device in a single package that is capable of performing both tasks is known as a modem, and it works well for two-way communication.) Of course, the protocols for modulation and demodulation must be agreed upon in advance by sender and receiver. This is one of the dilemmas confronting earthbound researchers who are attempting to detect extraterrestrial intelligence by means of radio signals coming our way. It may be possible to detect intelligent patterns, but decoding their meaning is a more difficult undertaking.
The first modulation method was amplitude modulation (AM). Though first devised in the early part of the twentieth century, its use is still quite common. Applications include AM radio, portable two-way radio, aircraft communication and computer modems.
A continuous radio-frequency carrier wave, fc, as modulated by an information-bearing audio or video wave can be viewed in the time domain on an oscilloscope of sufficient bandwidth such as the Tektronix MDO3104. The carrier wave appears as an envelope of varying amplitude as it moves along the X axis.
Where AM really gets interesting is when viewed in the frequency domain with a spectrum analyzer. A spectrum display shows the frequencies present and the relative amount of energy present in each one. The greatest amount of power appears at fc and in the two sidebands on either side of fc. The two sidebands are mirror images of one another. They both contain the same information, so one of them would suffice for demodulation purposes.
When only one sideband is transmitted, it is known as single-sideband transmission. In single-sideband transmission, also known as single-sideband suppressed-carrier modulation, transmitter power and bandwidth are conserved, and that is a plus. However, instrumentation and tuning complexities have conspired to prevent this protocol from becoming universal. (For one thing, the receiver must incorporate a beat-frequency oscillator to reconstitute the carrier.)
In frequency modulation (FM), the difference between the instantaneous and the base-frequency of the carrier is directly proportional to the instantaneous value of the input-signal amplitude. Thus the carrier wave amplitude is unchanged but the carrier frequency varies as it is modulated by an audio or video signal.
FM has the advantage that naturally occurring RF noise is not an issue since it does not impact the frequency of the carrier. Specifically, FM typically has a poorer signal-to-noise ratio (SNR) below a certain signal amplitude called the noise threshold. But above a higher level called the full quieting threshold, the SNR is much improved over AM. The degree of improvement depends on modulation level and deviation. FM broadcasting usually sees improvements greater than about 15 dB. Overall SNR in FM circuits can be further improved through use of such methods as pre-emphasizing higher audio frequencies with corresponding de-emphasis in the receiver. Because FM signals have constant amplitude, FM receivers normally have limiters that remove AM noise, further improving SNR.
Phase modulation is a subcategory of frequency modulation, whereby the phase angle of the carrier envelope is modified in response to changes in the signal. Phase modulation is used in many transmission applications, notably WiFi and satellite TV.
We have been discussing analog modulation schemes. A future article will take a look at the more complex topic of modulation as used in digital transmission.