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3.3: Early Modulation and Demodulation Technology

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    Early modulators and demodulators are considered here in part because the terms associated with the historical transmitters and receivers are still used today, but also because the early trade-offs influenced the architectures used today. Today transmitters and receivers use DSP technology, very stable LOs, and sophisticated clock recovery schemes. This was not always so. One of the early problems was demodulating a signal when the frequency of transmitter oscillators, i.e. carriers, drifted by up to 10%. Radio at first used AM and the carrier was sent with the information-carrying sidebands. With this signal, a simple single-diode rectifier circuit connected to a bandpass filter could be used, but the reception was poor. To improve performance it was necessary to lock an oscillator in the receiver to the carrier and then amplify the received signal. Here some of the early schemes that addressed these problems are discussed. There were many more variants, but the discussion covers the essential ideas.

    3.3.1 Heterodyne Receiver

    The heterodyning principle mixes a single-tone signal, the LO, with a finite bandwidth signal to produce a lower-frequency version of the information-bearing signal. With the LO frequency set appropriately, the low-frequency signal would be in the audio range. If the information-bearing signal is an AM signal, then the low-frequency version of the signal is the original audio signal, which is the envelope of the AM signal. This type of receiver is called a tuned radio frequency (TRF) receiver, and performance is critically dependent on the stability of the LO and the selectivity of the receive filter. The TRF receiver required the user to adjust a tunable capacitor so that, with a fixed inductor, a tunable bandpass filter was created. Such a filter has a limited \(Q\) and a bandwidth that is wider than the bandwidth of the radio channel.\(^{1}\) Even worse, a user had to adjust both the frequency of the bandpass filter and the frequency of the LO. The initial radios based on this principle were called audions, used a triode vacuum tube as an amplifier, and were used beginning in 1906. They were an improvement on the crystal detectors (which used a single diode with filters), but there was a need for something better.

    3.3.2 Homodyne Receiver

    The homodyne [3], syncrodyne (for synchronous heterodyne) [4], and autodyne (for automatic heterodyne) circuits were needed improvements on the audion and are based on the regenerative circuit invented by Edwin Armstrong in 1912 while he was an electrical engineering student at New York City’s Columbia University [5]. Armstrong’s circuit fed the input signal into an amplifying circuit and a portion of this signal was coupled back into the input circuit so that the signal was amplified over and over again. This is a positive feedback amplifier. A small input RF signal was amplified to such a large extent that it resulted in the amplifying circuit becoming nonlinear and consequently it rectified the amplitude modulated RF signal.


    Figure \(\PageIndex{1}\): Colebrook’s original homodyne receiver: (a) circuit with an antenna, tunable bandpass filter, and triode amplifier; and (b) triode vacuum tube.


    Figure \(\PageIndex{2}\): A common source Hartley voltage-controlled oscillator (VCO): (a) with nonlinear capacitors; and (b) with diodes which have a variable capacitance when reverse biased.

    Colebrook used this principle and developed the original homodyne receiver shown in Figure \(\PageIndex{1}\)(a). This serves to illustrate the operation of the family of regenerative receivers. The antenna shown on the left-hand side is part of a resonant circuit that is in the feedback path of a triode oscillator. The triode vacuum tube is shown in Figure \(\PageIndex{1}\)3-4(b). Here the grid coils (which control the flow of carriers between the bottom cathode\(^{2}\) and top anode) are weakly coupled to the anode circuit. When an AC signal appears at the top anode, the part within the passband of the tuned circuit is fed back to the grid and the signal is reinforced. The radio signals of the day were AM and had a relatively large carrier, so the oscillator locked on to the carrier. The AM sidebands were then successfully heterodyned down to the desired audio frequencies.

    The autodyne worked on a slightly different principle in that the oscillation frequency was tuned to a slightly different frequency from the carrier. Still, the autodyne combined the functions of an oscillator and detector in the same circuit.

    3.3.3 FM Modulator

    FM modulation can be implemented using a voltage-controlled oscillator (VCO) with the baseband signal controlling the frequency of an oscillator. A VCO can be very simple circuit and so was easily implemented in early radio. The circuits in Figure \(\PageIndex{2}\) are known as common source Hartley VCOs, where in Figure \(\PageIndex{2}\)(a) the controllable elements are the nonlinear capacitors. These are generally implemented as reverse-biased diodes, as shown in Figure \(\PageIndex{2}\)(b) where the bias, \(V_{B}\), changes the capacitance of the reverse-biased diodes. Changing the capacitance changes the resonant frequency of the feedback loop formed by the inductors and the diode capacitances (called varactors).

    3.3.4 FM Demodulator

    An FM demodulator is often implemented using a phase-locked loop with an error signal used to control the frequency of a voltage-controlled oscillator (VCO) with the loop arranged so that the VCO tracks the received signal. The desired baseband signal, i.e. the demodulated FM signal, being the loop’s error signal.

    3.3.5 Superheterodyne Receiver

    The superheterodyne receiver was invented by Edwin Armstrong in 1918 [6]. The key concept was to heterodyne down in two stages, use fixed filters, and use a tunable LO. The receiving antenna was connected to a bandpass filter that allowed several channels to pass. This relaxed the demands on the receive filter, but also filters with higher selectivity could be constructed if they did not need to be tuned. The filtered received signal is then mixed with an offset LO to produce what is called a supersonic signal—a signal above the audio range—and hence the name of this architecture. The performance of the superheterodyne (or superhet) receive architecture has only recently been achieved at cellular frequencies using direct conversion architectures.

    3.3.6 Summary

    Early radio used AM and FM modulation and both modulation and demodulation could be performed with very simple circuits. However they used a lot of spectrum for the information that was transmitted.


    [1] \(Q\) is the quality factor and is the ratio of the energy stored to the energy resistively lost in each cycle. Good frequency selectively in a filter requires high-\(Q\) components.

    [2] The cathode is heated (the heater circuit is not shown) and electrons are spontaneously emitted in a process called thermionic emission.

    This page titled 3.3: Early Modulation and Demodulation Technology is shared under a CC BY-NC license and was authored, remixed, and/or curated by Michael Steer.

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