TM-9-1290-365-24-P

TM-9-1290-365-24-P - Page 285 of 334

ARMY TM 9-1290-365-24&P

Figure D-40. Microwave Measurement Equipment Connection

D.9.2.4 SENSITIVITY MEASUREMENTS THEORY OF OPERATION. The transceiver assembly trans-

mits a low power X-band continuous wave signal. The transmitted signal strikes the outgoing projectile in

flight and the reflected RF energy is received by the antenna. The reflected signal is different from the

transmitted, due to the Doppler effect. The antenna unit detects the difference between the transmitted

and received signals, which is directly relative to the projectile velocity.

In order to achieve the simulation of the outgoing projectile, the transmitted signal is reflected and mod-

ulated, so that the received frequency is within the velocity measuring range of the transceiver assembly.

This is obtained by modulating a PIN switch in the microwave section at the appropriate frequency. The

PIN switch is modulated by a square wave that causes it to switch from off to on and vice versa. When

the PIN switch is off, the transmitted signal is reflected back to the antenna. When the PIN switch is on

the signal is absorbed by the dummy load. Because of the square wave modulation, the reflected signal

contains several spectral components. The first spectral component corresponds to the Doppler shifted

frequency.

The first spectral component power, in accordance with Fourier transform theory, is 1/II

(-9.94dB) of the

transmitted power.

Also, as a result of the on/off chopping of the signal and the absorption by the dummy load, only 1/2

(-3dB) of the transmitted power is reflected.

D-115