Superhet Receiver

separated. Such screening is also effective in preventing some noise pick-up at the receiver's input circuit.

A tuned antenna circuit or radio-frequency amplifier not only improves the image ratio, but is also effective in improving the signal-to-noise ratio of the receiver. Some compromise is necessary in reconciling the two considerations of image suppression and sensitivity, however. Image suppression will generally be better as the coupling between antenna and input circuit is looser, while signal-to-noise ratio will be better with closer coupling. The ultimate limit on sensitivity is the noise originating in the first circuit of the receiver, as was pointed out earlier in this chapter. It is therefore important to make the signal voltage in the first tuned circuit as large as possible, to compete with the thermal agitation voltage; and to obtain the best amplification possible in the first stage, to make the signal voltage as large as possible in comparison with the tube-noise voltages in the plate circuit of the first stage. A radio-frequency amplifier has more effective gain than a first detector, as a general rule, which makes the r.f. pre-selector stage advantageous in overcoming tube noise. Thermal agitation noise is greater at the lower frequencies, where the tuned-circuit impedance is higher, and falls off at the higher frequencies. The tendency, therefore, is for tube noise to predominate over thermal agitation noise in our high-frequency receivers.

A tremendous improvement in the signal-noise ratio also can be obtained by use of a directional receiving antenna system such as the rhombic and other types described in Chapter Sixteen. With such systems improvements of 10 db and greater can be accomplished, making weak signals which are undiscernible on an ordinary antenna completely intelligible.

Frequency Converter Circuits

• The frequency converter is the heart of the superhet receiver and on its operation depends largely the performance of the whole set. Since the intermediate-frequency value adopted for short-wave supers represents a considerable difference between the signal and local oscillator frequencies, it is not feasible to use a simple autodyne detector having one tuned circuit as in the autodyne regenerative receivers used for beat-note c.w. reception. Separate circuits must be used, that of the first detector input being tuned to the signal frequency and that of the oscillator being tuned higher or lower by an amount equal to the intermediate frequency. Because of circuit convenience and other factors, it is general practice to have the oscillator tuning intermediate frequency higher than the first detector input circuit.

With the two tuned circuits, oscillator and first detector, two separate tubes may be used; or there may be a single tube designed to provide separate sets of elements for oscillator and detector circuits. Arrangements of both types are shown in Figs. 612, 613, 614, 615 and 616. These figures show standard types of oscillator-detector arrangements. In the grid injection system of Fig. 612, the signal input circuit LiCi is tuned to the incoming signal and the oscillator circuit L2C2 is tuned intermediate-frequency higher. The oscillator is of the electron-coupled type, its output being coupled to the control grid of the first detector through a small capacitance. The 100,000-ohm plate load resistor of the oscillator may be replaced by a high-frequency r.f. choke in some instances, the operation being equivalent. The essential feature of this arrangement is that both the signal and oscillator voltages are impressed on the same grid. The conversion gain (ratio of i.f. voltage output to signal voltage input) and input selectivity are generally good, so long as the sum of the two voltages impressed on the grid does not exceed the grid bias and run the grid positive. Since the i.f. voltage produced is the product of the signal and oscillator voltages, it is desirable to make the oscillator voltage as high as possible without exceeding this limitation. In practice, with the circuits tuning over a number of bands and therefore likely to give wide fluctuations in oscillator output, oscillator r.f. voltage is made considerably less than the maximum limit.

The circuits of Fig. 613 are considerably less critical in this respect, since the signal and oscillator voltages are applied to separate grids. The circuit at 613-A uses a combined detector-oscillator tube having internal electron coupling between the two sets of elements, such a tube being known as a pentagrid converter. Quite high conversion efficiency can be obtained as well as good input selectivity. The tube is not a particularly desirable one for high-frequency work when used in this way, however, because the output of the oscillator drops off as the frequency is raised and because the two sections of the tube are not well enough isolated to prevent space-charge coupling and "pulling," or the tendency of the detector tuning to affect the oscillator frequency. An arrangement which overcomes these defects to a considerable extent is shown at Fig. 613-B. In this circuit the oscillator grid (No. 1) of the pentagrid converter is used as the mixing element, but is fed from a separate oscillator. The better performance of the 56 or 76 tubes as contrasted with the oscillator section of the 2A7 or 6A7 at high frequencies results in more uniform output over the high-fre-quency range. In the circuits of Fig. 613 the oscillator voltage is not critical, so long as enough is supplied, and the grid-current limitation of the circuit of Fig. 612 is absent.

A third type of first-detector-oscillator coupling is given in Fig. 614. In these diagrams the suppressor grid of a pentode-type detector is used as the means for introducing the oscillator voltage into the detector circuit to beat with the incoming signal. Suppressor-grid coupling offers the same advantages as the circuit of Fig. 613-B, but usually will require a greater oscillator voltage because of the lesser control factor of the suppressor grid as compared to the inner grid of a pentagrid converter tube. The oscillator voltage is not critical, however, and does not affect the input selectivity of the detector. Since the suppressor must be maintained at an average voltage considerably negative with respect to the cathode, the plate impedance of the first detector is reduced. This tends to lower the gain out of the first detector, compared to the gain the same tube would give with its suppressor maintained at cathode potential as is usual in amplifier applications. The suppressor must have negative bias, it should be emphasized, since otherwise the oscillator would be ineffectual in modulating the first-detector space current.

A circuit which utilizes screen-grid injection in the first detector, with a separate oscillator, is shown in Fig. 615. This arrangement requires somewhat more power from the oscillator, since the screen-grid circuit of the detector has a relatively low resistance compared to the grids used in other methods. The oscillator voltage swing required is also considerable for strict screen-grid modulation. However, it permits the use of a pentode type first detector and operates with a higher plate impedance than a pentode with suppressor injection. The latter feature tends to keep up the gain at intermediate frequency, where a high-impedance transformer circuit is the

6a7 Superhet
x -B +B

ZA7oR 6A7

ZA7oR 6A7

FIG. 613 — THESE FREQUENCY-CONVERTER CIRCUITS ARE FOR USE WITH PENTAGRID TUBES The circuit at A shows how the tube is used as o combined detector-oscillator. A better arrangement for high-frequency work, making use of a separate oscillator with the pentagrid tube as detector or "mixer," is shown at B.

FIG. 613 — THESE FREQUENCY-CONVERTER CIRCUITS ARE FOR USE WITH PENTAGRID TUBES The circuit at A shows how the tube is used as o combined detector-oscillator. A better arrangement for high-frequency work, making use of a separate oscillator with the pentagrid tube as detector or "mixer," is shown at B.

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