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II. Analog passive filters  Examples 
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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.

N=0 
Ap=0 
Asc=0 

Butterworth_LC 
minimum order + stopband optimization 
 
passband optimization 
N, Asc > 0 ⇒ stopband optimization 
Chebyshev_LC 
minimum order + stopband optimization 
passband (ripple) optimization 
corner frequency @Ap 
N, Ap, Asc > 0 ⇒ stopband optimization 
For convenience, some schematics already have .step
commands commented out.
 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:
Zin=1 Zout=1
sim=x test=x
fc=0 BWp=0.5/pi BWs=1/pi
Asc=3.0103 As=60 N=0
 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:
Zin=1 Zout=1
sim=x test=x
fc=0 BWp={.5/pi} BWs=1k
Asc=3.0103 As=x N=10
The.TRAN
analysis shows the Nyquist impulse response (and how the impulse is affected by the input).

(either reusing the same schematic or user's choice)
 Chebyshev_LC with
3dB@100MHz
, 1dB
ripple, 50dB@95MHz
, 50Ω
input, 75Ω
output:
Zin=50 Zout=75
sim=x test=x
fc=0 BWp=100Meg BWs=95Meg
G=1 Asc=3.0103 Ap=1 As=50 N=0
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**0.11))/4)**2
(for Zin≤Zout
) or its reverse (for Zin<Zout
).
 The same example, adapted and with increased order:
Zin=50 Zout=75
sim=x test=x
fc=0 BWp=100Meg BWs=1
G=1 Asc=3.0103 Ap=1 As=0 N=27

(either reusing the same schematic or user's choice)
 Butterworth_LC,
3dB@[100Hz,10kHz], 60dB@[50Hz,20kHz], Zin=Zout=8Ω
. Some calculations need to be done first:
fc=√(100*10000)=1000
BWp=10000100=9900
BWs=2000050=19950
Zin=8 Zout=8
sim=x test=x
fc=1k BWp=9.9k BWs=19.95k
Asc=3.0103 As=60 N=0
 The same, adapted:
Zin=8 Zout=8
sim=x test=x
fc=1k BWp=9.9k BWs=9.91k
Asc=3.0103 As=1k N=20

(either reusing the same schematic or user's choice)
 Chebyshev_LC,
3dB@[300Hz,3400Hz], 60dB@[425Hz,2400Hz], 0.1dB
ripple, Zin=Zout=600Ω
. After the same calculations as above:
Zin=600 Zout=600
sim=x test=x
fc=1010 BWp=3100 BWs=1950
Asc=3.0103 Ap=0.1 As=60 N=0
The resulting order would be odd (18/2=9
), so the equal terminating impedances can be used.
 The same,
N
specified:
Zin=600 Zout=600
sim=x test=x
fc=1010 BWp=3100 BWs=1950
Asc=3.0103 Ap=0.1 As=60 N=18
A LP Butterworth_LC will be considered for all three cases. The current source in the schematic must not be left floating.
Zin=1 Zout=1
sim=x test=x
fc=0 BWp=1 BWs=2
Asc=3 As=40 N=4
Zin=1 Zout=0
sim=x test=x
fc=0 BWp=1 BWs=2
Asc=3 As=40 N=4

(reusing any of the previous schematics)
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
lowpass 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