Checking Phone Transmitters

Author: J.B. Hoag

There are two methods of measuring modulation percentage by means of an oscilloscope. These are known as the wave-envelope and the trapezoid methods.

For the first of these methods, a small amount of the transmitter's output is picked up with a few turns of wire and fed by a parallel or twisted wire or concentric line to the vertical deflecting plates of the oscilloscope, as in Fig. 31 I.

Fig 31 I. Oscilloscope circuit and wave-envelope patterns for checking the modulation of a transmitter

A horizontal sweep is used at audio frequencies. R.F. harmonics can be eliminated from the vertical deflections by using a tuned circuit at the input terminals of the oscilloscope. The various patterns which will appear on the screen of the oscilloscope are shown in Fig. 31 I. A gap appears in the pattern when the transmitter is over-modulated, as shown at the bottom right of the figure. If a constant intensity tone source is operating in front of the microphone, and the sweep-frequency of the oscillator is properly adjusted, then a stationary pattern will be obtained which looks like that in Fig. 16 C. As explained earlier, one can apply an equation and compute the percentage modulation. It is to be made as nearly equal to 100 per cent as possible (see Plate Modulation for details).

Fig. 16 C. An amplitude-modulated carrier wave

The second method of checking the modulation, the so-called trapezoidal method, differs from the one just described in that, instead of the linear horizontal sweep, the beam is deflected horizontally by the audio frequency from the speech amplifier. A suitable circuit for this purpose is shown in Fig. 31 J.

Fig. 31 J. Oscilloscope circuit and trapezoidal patterns for checking the modulation of a transmitter

The sum of resistors R₁ and R₂ should be sufficiently great that only a small fraction of 1 milliampere flows through them. Roughly, their resistance should amount to one-quarter of a megohm for each 150 volts of modulator output. The blocking condenser C should be 0.1 μfd. or more. The patterns which appear on the screen are shown in the figure and should now be examined with care.

Of the two methods, the latter is easier to interpret. Sometimes the sloping slides of the trapezoid are not perfectly straight. This may be due to imperfect neutralization of the amplifiers. Or it may be caused by incorrect C-bias, or weak excitation of the modulated amplifier, or both. If the amplifier is perfectly linear, which is the correct operating condition, the sides of the trapezoids will be perfectly straight. Sometimes the patterns on the oscilloscope differ radically from those shown in the figure. In this case one should seek for stray r.f. induced into the horizontal deflecting circuits of the oscilloscope, or for some trouble in the oscilloscope circuits themselves.

If the power supplies applied to the transmitter have not been sufficiently filtered or if they have not been shielded from the r.f. circuits, an audio frequency hum will be superimposed upon the r.f. This modulation can be detected by listening with a receiver. If hum has been detected, proceed in the following manner: shut off the modulator; if hum persists, check the filtering of the power supplies of the different r.f. stages. Next, add the modulator, but not the speech amplifier. If the hum has still not been eliminated, the trouble lies in the speech amplifier or in the microphone. An oscilloscope, instead of a receiver, may be used in these tests for hum.

Unless properly shielded from each other, r.f. voltages will be induced into the speech amplifier from the r.f. circuits, with the result that an audio frequency oscillation or " howl" will be set up. This will modulate the carrier frequency. The microphone, its cord, and the speech amplifier must be shielded and grounded to prevent this r.f. pickup.