Ge J85 Engine Manual |TOP|
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The construction of a special purpose signal conditioner for the measurement of distortion indices of a J85 turbojet engine is described. In connection with the distortion measurement, two distinct signals were applied to the engine at the same time and the same frequency: a square-wave signal with a frequency of 800 Hz was fed into the inlet orifice, and a low-frequency sinusoidal signal was connected to the exhaust ports. The signals were measured with a signal conditioner and a distortion indicator. By using the two distortion indices, namely, the distortion of the low-frequency signal (distortion 0) and that of the square-wave signal (distortion 1), the so-called instantaneous distortion level of the engine could be measured. The distortion indices of the engine were used to determine its stall margin. The special purpose signal conditioner is designed to have a current chopping function in its switch-mode power supply system. In addition, the signal characteristics of this conditioner were analyzed using the discrete Fourier transform.
Signal P1 of the noise-shaped switch-mode power supply (SW1) attenuates the high-frequency component of the square-wave signal. The distortion 1 is the product of distortion 0 by the total current of the switch-mode power supply in a period of 1/800 Hz. The distortion indicator must be sensitive enough to disclose the distortion 1, and it must also not distort the high-frequency component of the signal input as it does in the case of a HF amplifier. The distortion 0 is designed to correct distortion 1 in the product of two signals with a different frequency. The distortion 0 is defined as the ratio of the square of the distortion of the signal to that of its distortion-free complementary signal.
Conversion factors for converting distortion 0 and distortion 1 into the specific frequencies are given for different distortion levels. It is shown that the noise-shaped switch-mode power supply (SW1) attenuates the high-frequency component of the square-wave signal, and the correction of distortion 1 is achieved by the distortion 0. For a given level of distortion, a distortion indicator which measures distortion 0 and distortion 1 at the same frequency has the advantage that it does not distort the square-wave signal. d2c66b5586