Antenna 
Steering 
Penn hen f, = 200 MHz 
: Antenna 
: T/R TWT power Up-converter |. | I/O Modulator [^7] I/Q Code & 
: ; switch amplifier E- & Driver La—] DA converters 
le I i I 
T i 
pe : I 
INU 1 1 i i 
Calibration || + | Reference LL pp Timing Unit | | 
Switch a : Oscillators 
i 1 y 
I I i 
i I i 
y 1 T 
; i ! ! 
Receiver [ lo Down- P IQ E AD converters d Digital 
front-end Converter demodulator & Data Buffer Preprocessor 
f, = 100 MHz 
Serial bus A 
| Y 
a) Computer pomme 
: Serial bus B 
Figure 1: Block diagram of KRAS radar. 
The Control Computer is based on a VME data bus. The main control CPU s are based on 
the Motorola 68010 and 68020 processors. The Control Computer is linked to the individual 
subunits through two serial busses A and B. Bus A connects all "sensitive" units and is 
always shot down during echo reception. The Control Computer also includes IEEE-488, 
RS-232 and MIL-STD-1553 interfaces. Most of the radars subunits are also equipped with a 
number of BITE (Build In Test Equipment) facilities, which the Control Computer monitors. 
The Control Computer can initiate radar operation by down loading the code to be 
transmitted. The code can be up to 4096 samples long and each sample consist of 8 bit I and 
8 bit Q data. 
The digital codes are converted to analog signals at a 200 MHz sampling rate in dual DA 
converters and the analog I and Q channels are transformed to a 300 MHz IF signal in the 
quadrature modulator. In the up-converter the signal is converted to 5.3 GHz and the signal 
is amplified to a level sufficient to drive the TWT tube ( 22 dBm). 100 MHz, 300 MHz and 
5 GHz local oscillator signals are generated in a reference oscillator unit, while 200 MHz is 
derived in the timing unit. Following the driver the signal is injected into the TWT power 
amplifier and a sample is directed into the Calibration Switch. The power level of the drive 
signal is controlled by a variable attenuator in a closed control loop. The power amplifier is a 
2 kW Traveling Wave Tube amplifier with low phase noise. Following the TWT the signal 
is guided to the antenna. Both the power level of the signal injected into the antenna, and the 
signal reflected from the antenna is monitored using a -60 dB cross coupler. The reflected 
signal will shut down the TWT Electronic Power Conditioner ( EPC) if the level is 
excessive. 
The slotted waveguide array antenna consist of four separate panels to provide large 
bandwidth. Seven waveguides are stacked in elevation and the elevation pattern is shaped to 
give a modified cosecant squared illumination over a 40? sector, with sidelobes suppressed 
24 dB. This is of major importance to insure that reflections from fuselage and wings will 
not give rise to two-way propagation, and thereby interference fringes. 
The received signal is, after passing a solid state receiver protection unit, amplified by a low 
noise amplifier ( LNA) with a noise figure of 2 dB, and following bandpass filtering a 
sensitivity time control ( STC) is applied before the signal is down-converted to IF, hence 
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