* The Pole placer / synthesizer *

This is a visual demonstration showing the major steps in the process of designing a filter using the new all- Windows version of S/FILSYN.

The first step, after establishing the required performance specifications, is to take a guess at the design cutoff frequencies, and enter these into the program, along with other parameters. This is accomplished using a Windows- type menu, with multiple choices and places for numeric entry, as in this screen-shot:

More details of the stopband specifications, corners or template, and specific infinite rejection frequencies (if desired), can be entered on the next screen, as "detailed parameters".

After entering all the information, the program proceeds to compute a transfer function, and optimize, moving the design parameters to equalize the "safety factors" in all stopband regions. This can be repeated and parameters changed until satisfactory performance is achieved, as seen in this screen shot:

At this point, the program is told to proceed with synthesis, and it selects a feasible topology, and synthesizes a filter. ALKDEM1 (seen below) is the filter as synthesized by S/FILSYN, with no direction or assistance. It can be analyzed, in S/FILSYN, and will be found to meet its specifications, but it is not in a form, or with element values, suitable for building. The termination is not 50 ohms, there are 'hot' nodes with no accomodation for stray capacitances, and we cannot yet determine whether the component values will be graceful.

    1   |---R---|       50.000000  ohm
    3   |---C---|       22.145111 pF
        |     ,-',
    4   |     L  C      80.057065 nH      res.frequency
        |     `-,'      21.393992 pF      121.611427 MHz
    5   |---C---|       11.181378 pF
        |     ,-',
    6   |     L  C     707.430739 nH      res.frequency
        |     `-,'       2.783256 pF      113.423128 MHz
    8   |       C      807.550316 fF
    9   |--L-C--|      168.771711 nH      res. frequency
        |       |       30.344368 pF       70.328484 MHz
   11   |---C---|       29.691067 pF
        |     ,-',
   12   |     L  C     272.778047 nH      res.frequency
        |     `-,'       7.629688 pF      110.321973 MHz
   14   |       C        1.970483 pF
   15   |---C---|        1.363709 pF
   17   |---R---|      521.389450  ohm

The first step is to do a Norton transformation, and to interchange a few branches, to see what the element values become when we have 50 ohm output. In ALKDEM2, we see that the last section has an inductor of only 26 nanohenries, which we feel might be inconveniently low. No exact transformation can alleviate this deficiency.

ALKDEM2
    1   |---R---|       50.000000  ohm
    3   |---C---|       22.145111 pF
        |     ,-',
    4   |     L  C      80.057065 nH      res.frequency
        |     `-,'      21.393992 pF      121.611427 MHz
    5   |---C---|       11.181378 pF
        |     ,-',
    6   |     L  C     707.430739 nH      res.frequency
        |     `-,'       2.783256 pF      113.423128 MHz
    8   |       C      807.550316 fF
    9   |--L-C--|      168.771711 nH      res. frequency
        |       |       30.344368 pF       70.328484 MHz
   11   |---C---|       25.298450 pF
   12   |       C        6.363100 pF
   13   |---C---|       14.184678 pF
        |     ,-',
   14   |     L  C      26.158762 nH      res.frequency
        |     `-,'      79.560782 pF      110.321973 MHz
   15   |---C---|       14.220471 pF
   17   |---R---|       50.000000  ohm

The next schematic, ALKDEM3, shows the result of several operations, all of which are made by simple point and click strokes right on the screen. We have moved the 110 MHz peak from the end of the filter and put the 113 MHz peak there. This is an exact transformation, and results in that inductor increasing to 40 nanohenries, which we will deem to be an acceptable value, for the purposes of this example. Note that there are no longer any 'hot' nodes.

ALKDEM3

    1   |---R---|       50.000000  ohm
    3   |---C---|       22.145111 pF
        |     ,-',
    4   |     L  C      80.057065 nH      res.frequency
        |     `-,'      21.393992 pF      121.611427 MHz
    5   |---C---|        6.078869 pF
    6   |       C        6.088803 pF
    7   |---C---|       78.318170 pF
    9   |--L-C--|       53.421024 nH      res. frequency
        |       |       95.866207 pF       70.328484 MHz
   10   |       C       27.138332 pF
   11   |---C---|       22.910529 pF
        |     ,-',
   12   |     L  C      40.411541 nH      res.frequency
        |     `-,'      51.500425 pF      110.321973 MHz
   13   |---C---|        8.357900 pF
   14   |       C        6.277423 pF
   15   |---C---|       10.829628 pF
        |     ,-',
   16   |     L  C      40.411541 nH      res.frequency
        |     `-,'      48.722730 pF      113.423128 MHz
   17   |---C---|       17.661198 pF
   19   |---R---|       50.000000  ohm

The final schematic, ALKDEM4, shows a buildable filter. Here we see the same filter after several more transformations, all done by simple operations on the screen. We have converted the inconvenient series resonant three-element branch to one in which a capacitor appears across the coil. We have made a few Norton transformations to make the two internal coil inductances the same as the end 80 nanohenry coil, but they could be the same as the 40 nH coil, or values in between if desired.

ALKDEM4
    1   |---R---|       50.000000  ohm
    3   |---C---|       22.145111 pF
        |     ,-',
    4   |     L  C      80.057065 nH      res.frequency
        |     `-,'      21.393992 pF      121.611427 MHz
    5   |---C---|        7.862239 pF
    6   |       C        4.305434 pF
        |-L-,   |       80.057065 nH
    7   |   |-C-|       55.374117 pF      res. frequency
        |-C-'   |        8.596138 pF      191.853007 MHz
    9   |---C---|       30.392276 pF
   10   |       C       13.633929 pF
   11   |---C---|       11.629945 pF
        |     ,-',
   12   |     L  C      80.057065 nH      res.frequency
        |     `-,'      25.996600 pF      110.321973 MHz
   13   |---C---|        2.927686 pF
   14   |       C        4.459994 pF
   15   |---C---|       12.647057 pF
        |     ,-',
   16   |     L  C      40.411541 nH      res.frequency
        |     `-,'      48.722730 pF      113.423128 MHz
   17   |---C---|       17.661198 pF
   19   |---R---|       50.000000  ohm

Every operation shown, every result achieved, was done right at the keyboard, and screen, inside the S/FILSYN program, without the use of external programs, calculators or any other devices. Below, just as S/FILSYN would display it, is the final schematic and analysis:


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