2.1 A (very) Brief Radio Synopsis:
The Transmitters and receivers that are used in typical R/C systems operate much the same way as commercial broadcast radio stations. The transmitter consists of a Radio Frequency ("RF") oscillator that typically operates in the 72-megahertz range. A more crude explanation would say that the oscillator transitions from one electrical extreme to another at a rate of 72 million times each second. The oscillator operates in a spectrally pure range, or "narrow band". Comparing a narrow band transmitter to a wide band transmitter is like comparing a saxophone to a piccolo. The saxophone produces a mellow sound that is actually composed of several frequencies while the piccolo is quite sharp, pure, and piercing. A narrow band radio system can easily discriminate its own frequency of operation from other frequencies that are relatively close. Using this type of system, R/C radios can share many discrete frequencies within a range of frequencies that are allocated by government agencies, thus allowing several different models to be operated at the same time without interfering with each other. Using narrow band radios not only allows more users in the same allocated frequency range, but also makes the radios more reliable and less prone to interference from other sources.
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Figure 5: 3 Frames of Carrier and Baseband Signal | |
| A radio is not much good unless information can be sent and received. In commercial broadcast radios, that information takes the form of voice or music. The 72-megahertz oscillator signal discussed earlier is referred to as the "carrier signal". When no information is being sent, the carrier is a pure, periodic and repeatable sin wave signal. Information that will be sent on the carrier, such as voice or music, is referred to as the "baseband signal" and is always a much, much lower frequency than the carrier.
In an Amplitude Modulated ("AM") radio transmitter, the frequency of the carrier signal is held constant while its power or amplitude is varied as a function of the baseband signal. In a Frequency Modulated ("FM") radio, the amplitude of the carrier remains constant while the frequency of the carrier is varied as a function of the baseband signal. In either case, the baseband information is extracted from the carrier on the receiving side, amplified, and reproduced exactly as the transmitter has generated it.
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Figure 6: 1 Frame of Carrier and Baseband Signal | |
| In an R/C system, servo position information is sent in digital form from the transmitter to the receiver. I will discuss the digital servo position information and elaborate on the baseband signal in detail, but first I need to define exactly what the command signal is and how it relates to the servo position. Armed with this information, it will become clear why the pulse stream is generated the way it is.
In Figure 5 above, the top oscilloscope trace is an illustration of an actual AM RF carrier signal from the transmitter. The bottom trace is the baseband signal as seen at the receiver. The carrier appears as a "smear" across the screen because it is transitioning extremely fast relative to the baseband signal. Figure 6 zooms in on a singe frame of pulses (frames are explained later) from a 4-channel transmitter and receiver. Figure 7 shows the RF carrier referenced to the baseband signal that was reconstructed by the receiver. The carrier collapses and grows in a gradual manner in an effort to contain spectral density, or to maintain a narrow bandwidth. Modulation shaping of the carrier as well as the purity of the carrier signal itself both play large roles in the operation of a "narrow band" transmitter.
| Figure 7: AM Carrier Modulation | |
