RF filer design Matlab

I am new to Matlab and RF Toolbox. I designed a Butterworth bandpass filter. Everything seemed to go OK, except I cannot see what the L & C values for the designed filter are. I can see S params and plots, but for this to really be a filter design tool, I need it to output the actual component values. ???

Answer 1

[Star Strider]

I don’t have the RF Toolbox (since I don’t need it, although earlier as a U.S. Amateur Extra Class Amateur Radio operator, it would likely have been useful) however your question interested me enough to go searching.

The example in the rffilter documentation gives the component values, however you have to ask for them —

Using an example from the documentation —

robj = rffilter('ResponseType','Bandpass','Implementation','LC Tee','PassbandFrequency',[950e6 2200e6],                        ...
    'StopbandFrequency',[770e6 3000e6],'PassbandAttenuation',3,'StopbandAttenuation',40);
robj.Name = 'BFCG_162W'

robj = 
  rffilter: Filter element

             FilterType: 'Butterworth'
           ResponseType: 'Bandpass'
         Implementation: 'LC Tee'
      PassbandFrequency: [950000000 2.2000e+09]
    PassbandAttenuation: 3
      StopbandFrequency: [770000000 3.0000e+09]
    StopbandAttenuation: 40
                    Zin: 50
                   Zout: 50
             DesignData: [1×1 struct]
         UseFilterOrder: 0
                   Name: 'BFCG_162W'
               NumPorts: 2
              Terminals: {'p1+'  'p2+'  'p1-'  'p2-'}

lcladder(robj)

ans = 
  lcladder: LC Ladder element

        Topology: 'bandpasstee'
     Inductances: [1.8116e-09 5.7297e-09 8.3361e-09 2.8294e-09 1.2214e-08 2.3802e-09 1.2214e-08 2.8294e-09 8.3361e-09 5.7297e-09 1.8116e-09]
    Capacitances: [6.6900e-12 2.1152e-12 1.4539e-12 4.2835e-12 9.9228e-13 5.0919e-12 9.9228e-13 4.2835e-12 1.4539e-12 2.1152e-12 6.6900e-12]
            Name: 'lcfilt'
        NumPorts: 2
       Terminals: {'p1+'  'p2+'  'p1-'  'p2-'}

The lcladder function produces them. There might also be other appropriate functions. See the Functions link for a list of all of them.

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[Michael Rojic]

Thanks for getting back to me. I didn't know about lcladder. Ill try that.

[Star Strider]

My pleasure!

[Michael Rojic]

Thanks again for the help. A followup question I have to this is how do I know what the filter looks like? In other words, with that list of capacitances and inductances, how is it constructed? What components are shunt, what are series, which is first, C or L? I thought it would generate a schematic of the filter. I have seen schematics of filters in the help sections, I thought the filter design might generate something like that.

[Star Strider]

My pleasure!

From the lcladder documentation:

it appears to be these in series. Other than this, I can’t find any schematics in the onlline documentation. (If you found others, please share the links.)

format shortE
robj = rffilter('ResponseType','Bandpass','Implementation','LC Tee','PassbandFrequency',[950e6 2200e6],                        ...
    'StopbandFrequency',[770e6 3000e6],'PassbandAttenuation',3,'StopbandAttenuation',40);
robj.Name = 'BFCG_162W';

lclobj = lcladder(robj);
L = lclobj.Inductances

L = 1×11

1.0e+00 * 1.8116e-09 5.7297e-09 8.3361e-09 2.8294e-09 1.2214e-08 2.3802e-09 1.2214e-08 2.8294e-09 8.3361e-09 5.7297e-09 1.8116e-09

C = lclobj.Capacitances

C = 1×11

1.0e+00 * 6.6900e-12 2.1152e-12 1.4539e-12 4.2835e-12 9.9228e-13 5.0919e-12 9.9228e-13 4.2835e-12 1.4539e-12 2.1152e-12 6.6900e-12

There are equal number of ‘L’ and ‘C’ elements. I can find nothing in the documentation that describes the topology of the input and output ports of the lcladder object, however it may not matter if the series inductor or parallel capacitor are the first (or last) element in the circuit. There are some GUI apps the may depict the topolgy, however I don’t have access to them and the can’t be used online here. Since you have to RF Toolbox (I don’t), you will need to explore these.

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