"Paul Burridge" schrieb im Newsbeitrag
...
Hi all,

On page 57 of RF circuit Design, Chris Bowick sets out a filter design
example. I've posted this to a.b.s.e under the same subject header. He
claims that the filter in question - a low pass Butterwoth - matches
50 ohms source to 500 ohms load.

Hello Paul,
to be honest, the circuit filters exactly as written. It is designed for
an input impedance of 50 Ohm and an output impedance of 500 Ohm.

However, having checked this out with
the aid of a Smith Chart, it appears there is some capacitive
reactance present that would require the addition of a shunt inductor
to neutralize. However, this would of course totally screw up the
filter's characteristics. Upon closer examination, it appears
impossible that this type of arrangement could ever be designed
without introducing some reactance into the signal path. Or am I nuts?

You are wrong here. I assume that the book doesn't claim to do an
impedance match to 50 Ohm input resistance for max. output power.
It's just designed as a passive lowpass filter with different source
and load resistor having a flat amplitude response. Nothing more.
The input resistance of this filter is for example 5 Ohm at f=12MHz.
According to the seven reactive parts, it has 3 notches and 3 resonances
for the input resistance over the frequency band from 0 to 200Mhz and
an additional zero at infinity frequency.


I'd always thought of these kind of filters as being purely resistive
overall at Fo but is that really the case? It don't look like it...


There is always a phase shift at the 3dB corner frequency
of any lowpass filter.

Design criteria:

Centre frequency: 35Mhz
This the -3dB corner frequency!
Response -60dB at 105Mhz
zero ripple(!)
Rs 50 ohms
Rl 500 ohms


Paul's filter from a.b.s.e.
---------------------------
A netlist file for the old SPICE people.

* Butter50_500_35MHz.asc
Rs N003 in 50
L1 in N001 152n
L2 N001 N002 323n
L3 N002 out 414n
RL out 0 500
C4 out 0 143p
C3 N002 0 153p
C2 N001 0 97p
C1 in 0 21p
V1 N003 0 AC 1
..ac dec 100 10k 200MEG
..end

Best Regards,
Helmut


PS: Such filters are analyzed with the .AC command in (LT)-SPICE.

The simulation type: .AC DEC 100 10k 1G
The SPICE voltage source: V1 0 AC 1

V(in) is the node after the source resistor Rs.
The frequency response: Logarithmic Bode Plot V(out)
The input impedance: Logarithmic Bode Plot V(in)/I(Rs)

If you want the output impedance, then you have to add a source in
series with the load RL and stimulate from the output side only.
The output impedance: Logarithmic Bode Plot: V(out)/I(RL)


For the newcomers to this group:
--------------------------------
LTSPICE is free SPICE with graphical GUI from www.linear.com/software
There is a special newsgroup for LTSPICE:
http://groups.yahoo.com/group/LTspice/




This is the schematic file of this filter for LTSPICE.
Save it in a file named "Butter50_500_35MHz.asc".


Version 4
SHEET 1 880 708
WIRE 0 176 32 176
WIRE 208 176 240 176
WIRE 368 176 400 176
WIRE 512 176 560 176
WIRE 688 336 560 336
WIRE 688 336 688 304
WIRE 400 224 400 176
WIRE 400 176 432 176
WIRE 240 224 240 176
WIRE 240 176 288 176
WIRE 64 224 64 176
WIRE 64 176 128 176
WIRE 64 288 64 336
WIRE 64 336 -80 336
WIRE 240 288 240 336
WIRE 240 336 64 336
WIRE 400 288 400 336
WIRE 400 336 240 336
WIRE 560 288 560 336
WIRE 560 336 400 336
WIRE 560 224 560 176
WIRE 560 176 688 176
WIRE 688 176 688 224
WIRE -80 176 -224 176
WIRE -224 176 -224 224
WIRE -80 368 -80 336
WIRE -80 336 -224 336
WIRE -224 336 -224 304
WIRE 32 176 64 176
FLAG -80 368 0
FLAG 688 176 out
FLAG 32 176 in
SYMBOL res -96 192 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 0 56 VBottom 0
SYMATTR InstName Rs
SYMATTR Value 50
SYMBOL ind 112 192 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 5 56 VBottom 0
SYMATTR InstName L1
SYMATTR Value 152n
SYMBOL ind 272 192 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 5 56 VBottom 0
SYMATTR InstName L2
SYMATTR Value 323n
SYMBOL ind 416 192 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 5 56 VBottom 0
SYMATTR InstName L3
SYMATTR Value 414n
SYMBOL res 672 208 R0
SYMATTR InstName RL
SYMATTR Value 500
SYMBOL cap 544 224 R0
SYMATTR InstName C4
SYMATTR Value 143p
SYMBOL cap 384 224 R0
SYMATTR InstName C3
SYMATTR Value 153p
SYMBOL cap 224 224 R0
SYMATTR InstName C2
SYMATTR Value 97p
SYMBOL cap 48 224 R0
SYMATTR InstName C1
SYMATTR Value 21p
SYMBOL voltage -224 208 R0
WINDOW 123 21 106 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value ""
SYMATTR Value2 AC 1
TEXT -192 24 Left 0 !.ac dec 100 10k 200MEG