Adaptive filter tuning for good pulse responseCrystal filters with good shape factors do not have very good phase response. The extreme IF filters using surplus crystals to get 20kHz bandwidth with an attenuation of 100dB 2kHz outside the 3dB points have very ugly pulse response with very long ringing after each pulse and makes this problem very clearly visible.The pulse response can be greatly improved by adding one more filter in the signal path. The optimum additional filter will correct nonlinearities in the phase response and modify the amplitude response for a good compromise between bandwidth and pulse shape.
The blanker init routineTo use this function the PC needs a pulse generator connected to both the receiver channels. The amplitude at the Soundblaster input has to be within 15dB from, but below saturation of the A/D converter.Fig 1 shows the normal receive mode spectrum when a pulse generator is connected to the receiver inputs replacing the antenna and preamplifiers.
By clicking at the box "Init" in the blanker window the operator can start the initialisation routine. The computer will collect a large number of pulses, calculate the fourier transform of each one of them. The transforms are expressed as an amplitude and as a phase slope function. These functions are independent of the phase and time of arrival of the pulse so they can be averaged over all the pulses. The average amplitude and average phase slope is an accurate evaluation of the receiver filter response. The actual pulse response of my system with extreme IF filters is shown in fig 2 which is one of the graphs in the noise blanker initialisation routine. The average transform itself is shown in fig. 3 which is another output from the initialisation routine.
Fig. 3. The average fourier transform of the pulse response is an accurate evaluation of the amplitude and phase response of the receiver. This graph shows the amplitude in linear scale and contains the same information as fig. 1 but with far higher accuracy. This graph also includes more points outside the passband. If the operator presses Y when fig 3 is shown, the initialisation routine calculates the filter required to make the total filter function equal to the desired filter function. The desired filter function has the phase=0 for all frequencies and the amplitude=1 for all frequencies within the flat region with a parabolic fall off outside. The operator has to supply the end points of the flat region and the curvature (second derivative, which is a constant for a parabola) to use at each side. Fig 4 shows how the screen looks when suitable parameters are selected.
Once the operator pressed "Y" to the graph in fig. 4, the amplitude function "Total" will be used together with the inverse phase of fig. 3. to create a filter. ("Total" is equal to "Correction" because there is no old filter function during init) The blanker "Init" function will not give a perfect correction filter, but it gives a good improvement. Still with the pulse generator connected to the antenna inputs the operator should click on the "Improve" box several times to refine the correction filter.
The improve routine is similar to the init routine, but is
includes the digital filter already established.
When running improve for the first time the output typically
looks like in fig 5,6,7 and 8.
After several runs of the blanker improve routine with appropriate changes of the flat region to avoid excessive values in the total amplitude for the digital filter the converged output from the blanker improve routine is typically as shown in figures 9 to 12. The size of the flat region and the curvature of the parabolic fall off at the sides influence the pulse response. Since the noise blanker uses 32 points when subtracting very large noise pulses it is a good idea to select parameters that make most of the oscillations stay within +/ 15 points from the pulse maximum. Smaller pulses are subtracted by use of 8 points only to save time. Therefore it is a good idea to select parameters that give a modest amplitude outside +/ 3 points also to allow the faster pulse subtraction more often.
The primary reason for running blanker improve several times is not to make
the noise blanker operation better.
It is to make the frequency response of the receiver accurately flat and
identical for both channels.
In this way the scale of the false colour graph can be made very steep
because the noise floor will have accurately the same level in the whole
20kHz frequency range.
With a very steep scale for the false colour graph signals producing
(S+N)/N of only a fraction of a dB become visible which means
that signals deep down in the noise can be seen.
Accurately identical responses in both channels means that the polarisation
calibration is accurately identical for all frequencies.
