Multichannels receiver camera operating in 8-MM wavelength

Abstract

The receiver camera, which contains 32 sensors array and quasioptical lens antenna, is developed for scanning imaging system in 8 mm waveband. Radiometric sensors are designed as direct detection receivers with input low noise amplifiers [1] Input amplifier is realized on the basis of three monolithic amplifying ICs connected in series. The first stage is realized using low-noise IC ALH369 (TRW), the second and the third stages are realized on the basis of CHA2094 (UMS). Shottky diode HSCH-9161 (HP) was used as square-law detector. The output low frequency amplifier is realized on the basis of IC OP27GS. The use of low noise monolithic ICs has made it possible to achieve small sizes and high sensitivity of the sensors. Within the operating frequency band 31？38 GHz the sensors have temperature sensitivity better than 10 mK/Hz1/2 in total power mode. Through low pass filters and analog switchboard, signals from sensor output comes to 12-digits ADC, which converts the output signals into digital code for the following processing. Block-diagram of 32-channels array is shown in Fig. 1. The sensors have a stretched configuration that facilitates their arrangement within array structure. Waveguide input of each sensor is connected directly to its feed, which receives the signal from certain direction. The sensors are attached to two metal plates, on both sides of each plate, thus forming two-dimensional array 4×8. All sensors are packaged in such a way that its feeds are placed in focal plane of the main antenna and directed towards antenna center. Flat-convex lens was used as the main antenna. The lens has diameter 600 mm and allows to form beams with 0.9 degree width at 3 dB level. The lens is made of hard polyethylene which has dielectric constant ε = 2,38 and losses tg δ = 2-10-4. In order to minimize distortion of periphery beams the lens was designed long focal (F = 660 mm). In spite of large weight lens antennas are more suitable for array receiver system due to absence of shade effect that is especially important when arrays with big number of sensors are used. Dielectric rods were used as feeds of the main antenna. Small transverse sizes of the rods give possibility to arrange the sensors closer to each other. The feeds along with the main antenna form 4 vertical rows of beams (8 beams in each row). The distance between adjacent beams in each row is 1.4 degrees. At scanning in a horizontal plane, receiver beams register 32 horizontal strips with angular distance 0,35 degrees between the middle of adjacent strips. Receiver array with feeds and lens antenna are rigidly fixed relatively each other. Presented in Fig. 2 is the construction of designed receiver camera. The given camera allows to scan a surveyed scene 32 times faster comparing with a single receiver sensor. Thus, 32 channels receiver camera operating in 8 mm waveband is developed. During one scan the camera is capable to provide an image with good angular resolution. The further perfecting of the imaging systems require to increase the number of receiver sensors and diameter of antenna to the greatest possible levels. ？ 2003 IEEE.