Note: Do not save the schematics after usage because they may be used in later examples.
Note: Opening the schematics from the links is best done with a Click+Drag over an existing opened instance of LTspice.
Setting any of the following parameters to zero allows for various optimizations. There can only be one null parameter at a time.
For convenience, some schematics already have .step commands commented out.
  1. Following the first method, suppose a minimum order for a -3dB@1rad and -60dB@2rad LP is needed for unity source/load. Choosing Butterworth_LC, the configuration looks like this:
  2. For the second method and using the same values, BWs can be 1k or even higher, as long as the conditions are satisfied. The same goes for As: it can even be null because N dictates the attenuation. More concrete:
    The.TRAN analysis shows the Nyquist impulse response (and how the impulse is affected by the input).
  1. Chebyshev_LC with -3dB@100MHz, 1dB ripple, -50dB@95MHz, 50Ω input, 75Ω output:
    The resulting order is even so the response will be distorted with the current input/output impedance settings. This could be resolved by either increasing the order or by increasing Zout to minimum tanh(asinh(1/sqrt(10**0.1-1))/4)**2 (for Zin≤Zout) or its reverse (for Zin<Zout).

  2. The same example, adapted and with increased order:
  1. Butterworth_LC, -3dB@[100Hz,10kHz], -60dB@[50Hz,20kHz], Zin=Zout=8Ω. Some calculations need to be done first:
  2. The same, adapted:
  1. Chebyshev_LC, -3dB@[300Hz,3400Hz], -60dB@[425Hz,2400Hz], 0.1dB ripple, Zin=Zout=600Ω. After the same calculations as above:
  2. The resulting order would be odd (18/2=9), so the equal terminating impedances can be used.

  3. The same, N specified:
A LP Butterworth_LC will be considered for all three cases. The current source in the schematic must not be left floating.
Since they can only be used in transient simulations, viewing them is done by switching over to the .TRAN card. For this case, using the same low-pass example, setting test=1 will enable pin 0.2 (the net named elements) and sim=50 will ensure that all the discrete values will have equal shares and will span over the entire time scale.

©Vlad, 2008 - 2015