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Theory of Telephone Receiver Operation

The total magnetic flux φ crossing the air gaps between the pole pieces and the iron diaphragm is composed of the constant flux φ0 produced by the magnets, and the variable flux φi caused by the voice currents passing through the coils on the soft-iron pole pieces. That is,

The air gaps tend to keep the total reluctance of the magnetic path independent of the current intensity, and it can therefore be assumed that the flux φi produced by a sine-wave test current is proportional to the current intensity, or

Combining equations 13 and 14,

As shown in most textbooks presenting magnetic theory, the force of attraction between two portions of a magnetic circuit separated by an air gap varies as the flux squared, and it can therefore be written that the force F acting on the diaphragm is

When this expression is squared it becomes

Since from trigonometry sin2 ωt = (1-cos 2ωt)/2, equation 17 can be written

Equation 18 is of importance as it indicates the forces acting on the receiver diaphragm. These are: (1) a steady pull K1φ02 produced by the permanent magnets; (2) a force 2K10Im sin ωt proportional to the product of the strength of the permanent magnets and the value of the instantaneous current flow; (3) a force (K1K2Im2)/2 which is constant; and (4) a force (K1K2Im2 cos 2ωt)/2 which has a frequency (2ωt) twice that of the impressed current. Thus, part 4 produces a double-frequency sound, causing distortion. An examination of parts 2 and 4 will show that, for good quality, the flux φ0 from the permanent magnet should be made large so that the magnetic effect of the speech currents is small in comparison with that of the permanent magnet. Then, part 2 will be large and the volume will be sufficient, but part 4 will be small and there will be little distortion.

There is also a force acting due to the eddy currents induced in the diaphragm, The voice-frequency component of the flux passes through the diaphragm, and as this flux changes it induces eddy currents. It will now be shown that these eddy currents cause distortion.

Assume that a sine-wave current i = Im sin ωt flows through the coils of the receiver. There is little hysteresis owing to the large air gaps, and therefore the flux produced is in phase with the current. This varying flux will induce a voltage in the diaphragm proportional to the rate of change of flux, but lagging 90° behind it. The eddy currents flowing in the diaphragm will be in phase with the voltage and will accordingly lag 90° behind the currents producing them. Thus, with respect to the useful current i = Imsin ωt, there will be eddy currents having the value ie = Ie(m)sin(ωt-90).

The distorting force acting on the diaphragm because of the reaction of the eddy currents and the useful voice currents is proportional to the product of the two equations just written. That is,

This can be written in the form

Since from trigonometry (sin </>£) (sin (<o£ - 90)) is equal to \ sin 2wt, equation 20 becomes

Therefore, since 2ωt is twice the frequency of ωt, the eddy currents in the diaphragm cause a double-frequency component and thus distort the original speech sounds.

This double-frequency effect can be illustrated by Fig. 24. The curve i represents the voice currents in the receiver windings, and ie represents the induced eddy currents and the resulting flux φe in the receiver diaphragm, lagging by approximately 90°. Since the force on the diaphragm is at any instant the product of the current and the flux, this force will be a double-frequency wave as indicated by the heavy line. Diaphragms of high-resistance material tend to prevent large eddy currents and thus minimize this double-frequency distortion.

Figure 24. The eddy currents in the diaphragm reacting with the voice current in the windings produce a double-frequency force F.

The degree of saturation of the receiver diaphragm has an influence on distortion. Because of hysteresis, if the diaphragm is not operated fairly high on its magnetization curve, the increase and decrease of flux with current will not be proportional, and thus the diaphragm will not follow the current variations. Since the diaphragm is thin, the desired magnetic operating point is easily reached.

The air gaps between the pole pieces and the diaphragm tend to reduce distortion by making the overall magnetization characteristics approach a straight line. Then, the rise and fall of flux in the magnetic path will closely follow current variations instead of following a hysteresis curve.

Direct current passing through the windings of a receiver not designed for it may cause distortion by opposing the flux from the permanent magnet, and thus shifting the point of operation to a non-linear portion of the magnetization curve. Also, direct current in opposing the constant magnetic flux will weaken the field, making the receiver less sensitive. Furthermore, if the current is strong enough it may clamp the diaphragm against the pole pieces or even burn out the windings.

Distortion is also caused by the mechanical resonance of the receiver diaphragm. This causes a greater response at the resonant frequencies and causes the diaphragm to tend to continue to vibrate at these frequencies. The diaphragms of the new telephone receivers are designed to minimize these effects.

Acoustic distortion of the radiated sound waves is produced by the effects of the air cavities of the receiver and of the listener's ear.



Last Update: 2011-05-18