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Fig. 53 shows a non-syrnetricalsignal due to a nonlinear amplification stagein the modulation circuits of an AM transmitter. The curvature of the top and bottom portions of the trapezoid are due to the non-linear amplification. An extreme caseof non-linear amplifica-
Fig. i2-Trapezoidal shift.
pattern showing signs of phase
for these connections.When a signal is applied to the transmitter, the waveformshown in Fig. 51 results.If a phase shift like that shown in Fig. 52 is evident, the scope'shorizontal input is not connectedto the modulation line. Examine the transmitter schematicdiagram to make sure there are no inductors or capacitorsbetween the modulation line and scope connection that may causesuch a phase-shift.
Fig. |4-TYapezoidal pattern showing seuere mse of non-linear modulation. tion is shown in Fig. 54 . The next waveforms in Fig. 55 show the three signals used to produce the waveform in Fig. 54 . Fig. A showsthe modulation signal being applied to the collectorsof the RF transistors. Fig. B shows the resulting RF output. The audio input signal is shown in Fig. C.
If the transmitter output is unmodulated,a vertical line should be present on the scope.Applying modulation forms the trapezoid. The formulas for determining modulation using the trapezoidal method of waveform analysis are the same as for standard modulation. The amplitude of the unmodulated carrier (the vertical line just mentioned) is used as the reference.The maximum and minimum points of the trapezoid are then used for determining positive and negativemodulation.
Fig.
ii-Signals used to produce waueform in Figure 54.
If the transmitter is showing signs of negative overmodulation, the waveform shown in Fig. 56 results. The "tail" at the left of the waveform is a result of the clipped portion of the modulation envelope.
Ftg. |?-Tfapezoidal pattern showing slight amount of non-linear modulation.
45
Fig. i2-Trapezoidal shift.
pattern showing signs of phase
for these connections.When a signal is applied to the transmitter, the waveformshown in Fig. 51 results.If a phase shift like that shown in Fig. 52 is evident, the scope'shorizontal input is not connectedto the modulation line. Examine the transmitter schematicdiagram to make sure there are no inductors or capacitorsbetween the modulation line and scope connection that may causesuch a phase-shift.
Fig. |4-TYapezoidal pattern showing seuere mse of non-linear modulation. tion is shown in Fig. 54 . The next waveforms in Fig. 55 show the three signals used to produce the waveform in Fig. 54 . Fig. A showsthe modulation signal being applied to the collectorsof the RF transistors. Fig. B shows the resulting RF output. The audio input signal is shown in Fig. C.
If the transmitter output is unmodulated,a vertical line should be present on the scope.Applying modulation forms the trapezoid. The formulas for determining modulation using the trapezoidal method of waveform analysis are the same as for standard modulation. The amplitude of the unmodulated carrier (the vertical line just mentioned) is used as the reference.The maximum and minimum points of the trapezoid are then used for determining positive and negativemodulation.
Fig.
ii-Signals used to produce waueform in Figure 54.
If the transmitter is showing signs of negative overmodulation, the waveform shown in Fig. 56 results. The "tail" at the left of the waveform is a result of the clipped portion of the modulation envelope.
Ftg. |?-Tfapezoidal pattern showing slight amount of non-linear modulation.
45
