Automatic Frequency Correction
13.4.1. Introduction. Two effects, sideband "screech" and harmonic distortion, become very pronounced if the i.f. carrier frequency at the output of the frequency changer stage of a superheterodyne receiver is not correctly centred in the comparatively narrow pass-band of the i.e. amplifier. Sideband " screech " is characterized by high-pitched distorted reproduction, and it occurs when the i.f. signal carrier is detuned to the side of the i.f. selectivity curve. In this condition the equivalent of single sideband reception with over-accentuated high-frequency sideband components is obtained, because one set of sidebands is almost entirely eliminated, and the carrier and low-frequency sidebands are reduced by being outside the pass range. Harmonic distortion of the audio output is caused by the diode detector when one set of sidebands is removed. With normal pass-band widths (±5 kc/s) the maximum tolerable mistune is about ±1 kc/s. Automatic correction of the oscillator frequency overcomes this difficulty by reducing the error produced by inaccurate tuning, or frequency drift of the oscillator due to temperature and other effects. For example, a signal-tuning error of 5 kc/s may be reduced to an i.f. carrier error of 50 c.p.s. by this method.
The two units of the automatic frequency corrector are a discriminator or error detector, and a control device. The former translates the error in the i.f. carrier into a voltage, the magnitude and sign of which is a function of the error. The latter, operated from the discriminator voltage, provides frequency correction of the oscillator tending to reset the i.f. carrier in the centre of the i.f. amplifier pass-band. The operation of the system can be represented by an overall characteristic giving the final intermediate frequency error with different signal frequency timing settings, and this is described in Section 13.4.4. The shape of the overall control characteristic is mainly dependent upon the discriminator, but the action of the control device, especially if it has a non-linear characteristic, modifies the result.
13.4.2. The Discriminator. A typical discriminator voltage-frequency curve is shown as dashed curve ABODE in Fig. 13.6a. The accuracy of control is determined by the slope BOD, and the final frequency error after correction is least when BOD has the greatest slope. It should be noted that automatic frequency correction is similar to a.g.c., i.e., control is only exercised when there is a change in frequency, and correction can very much reduce, but not eliminate (except in special cases), the error. Two important frequencies in a.f.c. are the " pull-in " and " throw-out " points. The former is the signal-tuning setting at which a.f.c. comes into operation when approaching the required station setting ; it is governed by the outer portions AB and DE of the characteristic. The latter is the signal-tuning setting at which a.f.c. loses control when tuning away from a station ; it is mainly determined by the
distance of B and D from the horizontal axis. The " throw-out " signal-tuning setting is always greater than the " pull-in ", and it may be several channels away from the correct setting, thus causing a number of stations to be skipped when timing away from the station. For this reason during manual tuning it is usual to disconnect a.f.c. with a friction switch operated by rotation of the tuning capacitor. The actual values of the throw-out and pull-in frequencies can be calculated from the discriminator and control device curves as described in Section 13.4.4.
There are two types of discriminator, one known as the amplitude 2 and the other as the phase discriminator.4 An example of the first is shown in Fig. 13.7. Two circuits, one (No. 1) tuned to a frequency about 2 kc/s below, and the other (No. 2) to 2 kc/s above the correct i.f. carrier frequency, are transformer-coupled to the anode circuit of a valve, deriving its input voltage from a proportion of the output voltage of the last i.f. stage in the receiver. Provided stray coupling between 1 and 2 is small and the slope resistance of Vx is large compared with the maximum impedance across the primaries of 1 and 2, the frequency response of either circuit is unaffected by the other. The frequency response of each circuit relative to the response at the resonant frequency is shown by the curves 1 and 2 in Fig. 13.6a ; these curves are obtained from the generalized selectivity curve of Fig. 4.3, Part I, as described below. They are identical in shape and displaced from each other by 4 kc/s. Diode detectors (Dy and D2 in Fig. 13.7) across these
circuits, have their d.c. output voltages connected in series opposition. When there is no tuning error, the i.f. carrier voltages across 1 and 2 are equal (proportional to OF in Fig. 13.6a) and there is zero d.c. voltage across points XX' in Fig. 13.7. Mistuning to a lower i.f. carrier frequency (point G in Fig. 13.6a) increases to GK the proportional voltage applied to Dx and decreases to GH that applied to D2. Hence the d.c. output voltage at XX' is negative and proportional to HK, i.e., to GL ; its actual value can be found by multiplying GL by the product of a constant K and the voltage detection efficiency of the diodes. The constant K is a function of the input unmodulated carrier peak voltage to, and gm of, the valve Fi in Fig. 13.7, the ratio of mutual inductance to secondary tuning inductance, and the resonant or dynamic impedance of the secondary. Voltage detection efficiency, r)d in Chapter 8, Part I, is the ratio of the d.c. voltage across one of the load resistances, Bs, in Fig. 13.7 to the unmodulated carrier peak voltage output from 1 or 2. The steepness of the slope BOD (Fig. 13.6a) is fixed by the Q of the circuits 1 and 2, the intermediate frequency mid-carrier value, and the frequency separation of points B and D, so long as the stray coupling between the circuits is small. For any particular frequency separation of the points B and D, there is a value of Q which gives maximum slope to BOD at 0. Lower or higher values of Q give a smaller slope at 0 and also greater curvature to the line BOD.
Assuming that the Q values of circuits 1 and 2 are equal, the optimum Q can be calculated as follows : from Section 4.2.3, Part I, the selectivity of a single tuned circuit, i.e., its frequency response in terms of that at the resonant frequency, is shown to be equal to . 1 where F = and Af is the frequency difference Vl +Q2F2 fr
(or off-time) between the particular frequency considered and the resonant frequency fr. Thus
and the slope 8 of the selectivity curve at any off-tune frequency Af is a _ d(Sel') _ d(Sel«Q 2 dAf dF 'fr - Q2F 2
(l+Q2F2)ffr - 4Q2Af
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