User manual

Code: ENIGMA III User Manual
Edition: 1 Revision: 4
Date: 19/12/2008
Page: 52/97
6.6.2 IFD matched filter
The batched filter for the IFD is a complex bandpass filter. The bandwidth of the passband has to be
matched to the pulse width.
Example: For a pulse width of 1µs you will need a bandwidth of 1Mhz
For a pulse width of 2µs you will need a bandwidth of 500kHz
The matched filter is implemented as a complex (symmetric) FIR filter. The filter coefficients need to be
symmetric and the the coefficients at both ends need to have values close to zero (causal filter). This can
be achieved by applying a tapering window to the coefficients. The recommended tapering function is the
blackman window.
Example code for the computation of the Blackman window:
double arg = 2.0*M_PI/(double)(size-1);
for (i=0; i<size; ++i)
{
window[i] = 0.42-0.5*cos(arg*i)+0.08*cos(2.0*arg*i);
}
The recommended procedure for the computation of the filter coefficients is to first compute a real
lowpass filter and then mix it up to the (folded) center frequency of the burst signal:
Example:
The sampling frequency of the ENIGMA III IFD is 76.8 MHz.
1. Assuming a IF frequency of 60 MHz the folded center frequency of the burst will be 16.8 MHz.
So the low pass filter has to be mixed up to a center frequency of 16.8 MHz. In this case the IFD
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