International projectionist (Jan-Dec 1946)

all frequencies between 550 and 1600 kc. in the case of standard broadcast receivers. The output from the RF amplifier is fed into the mixer oscillator stage in the superheterodyne receiver. The Mixer-Oscillator Figure 3 shows the mixer-oscillator circuit. Reference to the block diagram of Fig. 1 shows that the output of the RF amplifier feeds into the mixer-oscillator and the output of the latter feeds into the if amplifiers. The input to the mixer comes from the RF coil which is connected to the signal-grid of the 6L7-G tube. This tube, of special design, is known as a mixer tube. In addition to the control grid, screen grid, and suppressor grids which are the normal complement of pentode tubes, the mixer tube contains an additional grid which is known as the oscillator-input grid. The output from the local oscillator is fed into this grid and is mixed within the tube with the incoming signal. The 6C5-G tube is connected in a conventional Armstrong oscillator circuit. The oscillator is tuned in the grid circuit by means of the variable condenser C. It will be seen that a variable condenser is connected in series with the tuning condenser across the secondary of the oscillator coil. This condenser is smaller than condenser C and is known as the oscillator padder condenser. It is used in order to adjust the oscillator circuit at the low-frequency end of the dial. The procedure for making this adjustment will be covered when we discuss superhet alignment. The mixing action takes place in the following manner: the incoming signal from the RF amplifier or from the antenna, as the case may be, is applied to the signal grid, while the signal from the local oscillator (pure RF) is fed into the oscillator-input grid. Both these signals are in the path of the common electron stream within the tube and they will both affect the electrons in this stream. This will result in several frequencies being present at the output of the mixer tube. The signals present at the plate of this tube are the incoming signal, the oscil FIGURE 3. Mixer-oscillator. 1st. I. F. TRANS. (input I.F.T.) 2nd I.F. TRANS. (OUTPUT I.F.T.) ■¥■ + 250V TO / DETECTOR 6L7-& FIGURE 4. IF amplifier. lator signal, the sum of the incoming and oscillator signals, the difference between the incoming and oscillator signals, and the harmonics of all of these signals. In a modern superhet receiver it is only required to make use of the difference between the oscillator and incoming signals. This means that some type of filter will be used in the plate circuit of the mixer tube. Intermediate-Frequency (IF) Amplifier The output of the mixer circuit is fed into a tuned transformer which is known as the first if transformer. Because this AUGUST QUESTIONS AND CORRECT ANSWERS 1. (Q) Why should magnetic coupling between coils be prevented? (A) To prevent interference between circuits. 2. (Q) Why are tuning condensers ganged? (A) To facilitate tuning. 3. (Q) Why is a pentode better than a triode as an R. F. amplifier? (A) It does not have a tendency to oscillate and has more gain than a triode. is a tuned circuit it will act as a filter to all frequencies except the small band which it is designed to pass. The IF transformer is tuned to only one carrier frequency. This frequency varies with different receivers; but most of the present-day receivers use a frequency of 455 or 456 kc. Some of the older receivers used an intermediate frequency of 175 kc, and some used even a lower frequency. Many modern receivers have used a frequency of 465 kc, and some St I.F.TBANS. OSCINPUT GRID 6C5-G FROM R.F. AMPLIFIER -1-I25V ~=^ +250Vi have used 262 kc. In general, a higher intermediate frequency results in less gain but better tone quality. The values of if just mentioned apply only to standard receivers and do not apply to fm and television receivers. Such receivers use much higher values for the intermediate frequency. The trimmer condensers which are shown connected across the primary and secondary windings of the if transformer are used only for alignment purposes. These condensers are adjusted only once at the factory. In some receivers the set screws from these trimmer condensers are sealed with sealing wax or with liquid solder. Figure 4 is a diagram of an if amplifier. A pentode tube is practically always used in this type of circuit because of its high gain and stability. Most of the gain of the superhet receiver takes place in the if amplifier. The large amount of selectivity and sensitivity with which the superhet is credited is due mainly to the IF amplifier stage. There is only one stage of if amplification shown in Fig. 4, although two if transformers are used. Most receivers, even the very expensive ones, use only one stage of if amplification. Some receivers, designed for special purposes, such as communication types, use two and even three such stages. Such receivers are very sensitive and selective but usually lack good tone quality: the tuning becomes very sharp as the number of if stages is increased, the result being that the circuit acts as a filter and cuts out some of the components of audio frequencies present in the if carrier. The if transformers shown in Fig. 4 are tuned by means of condensers. In recent years powdered-iron slugs have been used for this purpose. This slug increases the inductive reactance of the coil without appreciably increasing its losses. This means that the coil will have a high Q and a correspondingly high gain. The Diode Detector In previous articles the superiority of the diode detector over other types was pointed out. We learned however, that the diode detector works best when the incoming signal is of medium or large strength. This factor limits the use of INTERNATIONAL PROJECTIONIST September, 1946 21: