f, f. ; 
(A) (Lo) 0 RF [Amplitude | 
® | modulator 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Digital 
code 
(B) Digital A 
code y DA 
Digital A 
code y DA 
I-channel 
(C) 
Q-channel 
Digital A > 
code y DA dissi X 
  
  
  
  
  
  
  
  
  
Figure 2: Digital signal generator mechanizations: (A) RF phaseshifter 
| (B) Direct IF generator (C) Quadrature baseband modulator. 
Also the digital codes can be generated in a number of ways. One method to generate the 
quadratic phase of a linear FM is for instance to implement a digital double integration of a 
constant, [Eber and Soule,1975],[Postema,1987]. By changing the constant the FM rate can 
be varied. Alternatively the pulsecode can be generated off-line and down loaded in a fast 
buffer memory. This approach gives significantly more flexibility and the software 
generating the codes can be designed to calculate non-linear FM with specified pulselength, 
bandwidth and sidelobe level. The price paid for this flexibility is that a fairly large and very 
fast buffer memory is required. 
It is important to note that high quality signals with low sidelobes can only be obtained if the 
gain of the quadrature channels are equal and if the phase difference between the channels is 
90°. Also the characteristics of the DA converted signals (i.e. digital signals have a periodic 
spectrum, and the "box" representation of the individual numbers in the code gives a 
sinc(nf/f;) weighting of the spectrum) must be taken into account as well as other system 
components such as filters. 
The KRAS digital signal generator is based on a quadrature modulator with the converters 
running at a 200 MHz sampling rate. The low-pass filters have a cut-off frequency of 150 
MHz and a nearly linear phase characteristic. 4096 complex code words are stored in a RAM 
buffer, hence supporting pulses of a length up to 20 us. The codes are generated by a C- 
program that supports both linear and nonlinear FM codes. This program also hold the 
capacity for incorporating predicted or measured system properties, by generating 
predistorted signals, so that the signal that drives the TWT will be correct. It is also intended 
to use the calibration system described below to perform on-line measurements of transfer 
functions thereby enabling dynamic predistortion if required. How this can be implemented 
is presently subject to a Ph.D. study at the Electromagnetics Institute. 
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