Functional Principle of Radio Receivers

Around 1888 the physicist Heinrich Hertz experimentally verified the existence of electromagnetic waves and Maxwell’s theory. At the time his transmitting system consisted of a spark oscillator serving as a high frequency generator to feed a dipole of metal plates. Hertz could recognize the energy emitted by the dipole in the form of sparks across a short spark gap connected to a circular receiving resonator that was located at some distance. However, this rather simple receiver system could not be used commercially.

The road to commercial applications opened only after the Frenchman Branly was able to detect the received high-frequency signal by means of a coherer, also known as a fritter. His coherer consisted of a tube filled with iron filings and connected to two electrodes. The transfer resistance of this setup decreased with incoming high-frequency pulses, producing a crackling sound in the earphones. When this occurred the iron filings were rearranged in a low-resistance pattern and thus insensitive to further stimulation. To keep them active and maintain high resistance they needed to be subjected to a shaking movement. This mechanical shaking could be produced by a device called a Wagner hammer or knocker. A receiving system comprising of a dipole antenna, a coherer as a detector, a Wagner hammer with direct voltage source and a telephone handset formed the basis for Marconi to make radio technology successful world-wide in the 1890s.

The components of this receiver system had to be modified to meet the demands of wider transmission ranges and higher reliability. An increase in the range was achieved by replacing the simple resonator or dipole by the Marconi antenna. This featured a high vertical radiator as an isolated structure or an expanded fan- or basket-shaped antenna Functional blocks of the detector receiver. The demodulator circuit shown separately represents the actual detector. With the usually weak signals received the kink in the characteristic curve of the demodulator diode is not very pronounced compared to the signal amplitude. The detector therefore has a nonlinear characteristic. It is also known as a square-law detector.

Particularly in military use and in air and sea traffic, wireless telegraphy spread rapidly. With the invention of the electron tube and its first applications as a rectifier and RF amplifier came the discovery, in 1913, of the feedback principle, another milestone in the development of receiver technology. The use of a triode or multi-grid tube, known as the audion, allowed circuit designs that met all major demands for receiver characteristics.

For the first time it was possible to amplify the high-frequency voltage picked up by the antenna several hundred times and to rectify the RF signal simultaneously. The unique feature, however, was the additional use of the feedback principle, which allowed part of the amplified high frequency signal from the anode to be returned in the proper phase to the grid of the same tube. The feedback was made variable and, when adjusted correctly, resulted in a pronounced undamping of the frequency-determining grid circuit. This brought a substantial reduction of the receive bandwidth (Section III.6.1) and with it a considerable improvement of the selectivity. Increasing the feedback until the onset of oscillation offered the possibility of making the keyed RF voltage audible as a beat note. In 1926, when there were approximately one million receivers Germany, the majority of designs featured the audion principle, while others used simple detector circuits.

The nomenclature for audion circuits used ‘v’, derived from the term ‘valve’ for an electron tube. Thus, for example, 0-v-0 designates a receiver without RF amplifier and without AF amplifier; 1-v-2 is an audion with one RF amplifier and two AF amplifier stages. Improvements in the selective power and in frequency tuning as well as the introduction of direct-voltage supply or AC power adapters resulted in a vast number of circuit variations for industrially produced receiver models. The general interest in this new technology grew continuously and so did the number of amateur radio enthusiasts who built their devices themselves. All these various receivers had one characteristic in common: They always amplified, selected and demodulated the desired signal at the same frequency. For this reason they were called tuned radio frequency (TRF) receivers (Fig. I.2).

The superheterodyne receiver essentially consists of RF amplifier, mixer stage, intermediate frequency amplifier (IF amp), demodulator with AF amplification, and tunable oscillator (Fig. I.4). The high-frequency signal obtained from the receiving antenna is increased in the preamplifier stage in order to ensure that the achieved signal-to-noise ratio does not deteriorate in the subsequent circuitry. In order to process a wide range from weak to strong received signals it is necessary to find a reasonable compromise between the maximum gain and the optimum signal-to-noise ratio (Section III.4.8). 

Previously, the mixer stage (Section V.4) was designed as an additive mixer using a triode tube. This was later replaced by a multiplicative mixer using a multi-grid tube like a hexode (in order to increase the signal stability some circuit designs made use of beam-reflection tubes as mixers). With the continued progress in the development of semiconductors, field-effect transistors were used as additive mixers. These feature a distinct square characteristic and are clearly superior to the earlier semiconductor mixers using bipolar transistors. Later developments led to the use of mixers with metal oxide field-effect transistors (FETs). The electric properties of such FETs with two control electrodes correspond to those of cascade systems and enable improved multiplicative mixing. High oscillator levels result in acceptable large-signal properties (Section III.12).

Symmetrical circuit layouts suppressing the interfering signal at the RF or IF gate are still used today in both simple- and dual-balanced circuit designs with junction FETs. Only with the introduction of Schottky diodes for switches did it become possible to produce simple low-noise mixers with little conversion damping in large quantities as modules with defined interface impedances. Measures such as increasing the local oscillator power by a series arrangement of diodes in the respective branch circuit resulted in high-performance mixers with a very wide dynamic range, which are comparatively easy to produce. Today, they are surpassed only by switching mixers using MOSFETs as polarity switches and are controlled either by LO injection signals of very high amplitudes or by signals with extremely steep edges from fast switching drivers [1]. With modern switching mixers it becomes particularly important to terminate all gates with the correct impedance and to process the IF signal at high levels and with low distortion.

The first IF amplifiers used a frequency range between about 300 kHz and 2 MHz. This allowed cascading several amplifier stages without a significant risk of self-excitation, so that the signal voltage suitable for demodulation could be derived even from signals close to the sensitivity limit (Section III.4) of the receiver. Initially, the necessary selection was achieved by means of multi-circuit inductive filters. Later on the application of highly selective quartz resonators was discovered, which soon replaced the LC filters. The use of several quartz bridges in series allowed a bandwidth adapted to the restrictions of the band allocation and the type of modulation used. Since quartz crystals were costly, several bridge components with switchable or variable coupling were used instead. This enabled manual matching of the bandwidth according to the signal density, telegraphy utilization or radiotelephony. Sometime later, optimum operating comfort was obtained by the use of several quartz filters with bandwidths matched to the type of modulation used. Replacing the quartz crystals by ceramic resonators provided an inexpensive alternative.

 The characteristics of mechanical resonators were also optimized to suit high performance IF filters. Electro-mechanical transducers, multiple mechanical resonators and so-called reverse conversion coils could be integrated into smaller housings, making them fit for use in radio receivers. The high number of filter poles produced with utmost precision were expensive, but their filter properties were unsurpassed by any other analog electro-mechanical system.

The mixer stage of a superheterodyne receiver satisfies the mathematical condition for generating an intermediate frequency from the heterodyne signal with two different receive frequencies (III.5.3). Both the difference between the receive frequency (fRX) and the LO frequency (fLO) and the difference between the LO frequency and a second receive frequency generate the same intermediate frequency (fIF). The two receive frequencies form a mirror image relative to the frequency of the oscillator, both separated by the IF. The unwanted receive frequency is therefore called the image frequency. The frequency of any such signal is equal to the IF and directly affects the wanted signal or, in extreme cases, covers it altogether. To avoid this, the image frequency must be suppressed. This is usually done by preselection, i.e. by means of the resonance circuits of the RF preamplifier or the preselector. At the beginning of the superhet era the near selection (Section III.6), responsible for the selectivity by filtering the useful signal from the adjacent signals, was possible only with high-quality multi-circuit bandpass filters having a low frequency.

From the actual image frequency it is obvious that, for a low IF, it can be suppressed only with a considerable amount of filtering. Especially with receivers designed for several frequency ranges, the reception of high-frequency signals was strongly affected by an insufficiently suppressed image frequency (Section III.5.3). It was therefore necessary to find a compromise between image frequency suppression and selectivity, based on the intermediate frequency.

This problem was solved by twofold heterodyning. To reject the image frequency the first IF was made as high as possible; the higher the IF the lower the effort to suppress the image frequency (see Fig. III.36). A second mixer converted to a second IF so low that good near selection was possible at an acceptable cost (Fig. I.6). But the second mixer again produces both a useful frequency and an image frequency. The second image frequency must also be suppressed as far as possible by means of a filter operating on the first IF. In the era of coil filters this required very careful selection of the frequency.