In radio and television receivers, some means must exist to convert the incoming radio-frequency (RF) signal to audio or some other usable form. That is, the radio wave must be demodulated, and the device or circuit that performs this function is called a detector (also known as a demodulator).
The name is an old one, going back to the earliest days of radio. At that time, most radio receiving sets were little more than a longwire antenna of considerable length, a tuning coil or two, a detector, and some headphones. The detectors were crude; in most cases they consisted of a piece of spring wire (the cat’s whisker) affixed to a small handle, which was used to probe the surface of a galena crystal. The radio operator moved the cat’s whisker around the galena until signals were heard – thus detecting the radio signal. Some more elaborate receiving systems used a coherer, a small glass tube filled with metal filings, sealed at both ends and fitted with electrodes. The coherer worked well enough, but the filings tended to bunch up at one end or the other, requiring the operator to tap the coherer to restore operation.
When it was discovered around 1910 that a simple single-plate single-cathode diode vacuum tube could be used as a detector, it immediately superseded all other devices used as detectors. The vacuum tube reigned supreme in commercial receivers until the advent of solid-state devices, with which better demodulation circuits could be built.
In modern electronic practice, a number of detector circuits have become standard over the last few decades:
Envelope Detector: The generic term for the diode detector as used in amplitude-modulation (AM) receivers. This detector works in much the same was as an ordinary half-wave rectifier circuit and consists, in its simplest form, of a diode and a capacitor. Its major virtue is its simplicity, but it suffers from a tendency toward distortion of the audio waveform and is prone to overloading. The early detectors cited above were envelope detectors.
Regenerative Detector: Used mostly in experimental and hobby receivers. In circuit form, it resembles an ordinary oscillator circuit in that a portion of the output is fed back to the input, which if unchecked would normally result in oscillations. However, the circuit is not allowed to break into oscillation; through the use of a regeneration control (usually a variable resistor), it is instead held just at the point of oscillation. This builds up the received signal and makes reception of weaker signals possible. A refined form, the superregenerative detector, includes a separate oscillator to assist in holding the circuit at the point of detection.
Foster-Seeley Discriminator: This was the first practical circuit used in commercial receivers to demodulate frequency-modulation (FM) signals. The frequency variations of the incoming signal are sent through a double-tuned transformer to convert them to amplitude variations. At that point, the signal can be demodulated in a manner similar to the envelope detector, and the original audio thus recovered. Since the Foster-Seeley discriminator is affected by unwanted amplitude variations (such as noise), a limiter circuit is required to remove any such components of the incoming signal.
Ratio Detector: The ratio detector circuit is also used to demodulate FM signals, and does a better job of it than the Foster-Seeley discriminator. The circuit is similar to that of the discriminator, with a third coil added to the double-tuned transformer. The ratio detector circuit is not affected by amplitude variations on the incoming signal, and can function with less input signal than is required for the discriminator. Also, less noise will appear on the output due to the circuit’s inherent filtering action.
Product Detector: The product detector is used in communications receivers to demodulate single sideband and continuous-wave (usually morse code) signals, which typically arrive at the receiver without a carrier wave. It requires a separate local oscillator, known as the beat-frequency oscillator (BFO). In operation the product detector functions as a mixer, combining the received signal with the BFO signal. The BFO supplies the missing carrier wave, and makes demodulation possible through mixing action on the two signals. The product of the two signals is the recovered audio.
Synchronous Detector: Similar to the product detector, the synchronous detector circuit is found in high-performance receivers as it provides demodulation, free of fading and distortion, of AM signals. The circuit resembles the product detector in that it mixes the received signal and a locally provided oscillator signal. The difference is that the frequency of the local oscillator must be held to the same value as the incoming signal’s frequency, and it is also phase-locked to the received signal. The result is clean demodulation without the presence of any audio beat note or noise.
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