[Lowfer] That filter can't be right!!!

[Bill Ashlock]

John A,

>Not sure how bleary you were when typing that message, but the diagram >I 
>have of Lyle's transmitter shows .018 uF, which is 18 nF. The >diagram 
>posted shows 18 nF. Where did you get the 1.8 nF from?

Wow. Am I glad to find out 1.8nF is the wrong value!!  The schematic that 
I've been using for about four years originally came from Lyle's Web site. 
This copy has been in my original, well worn, Lowfer notebook all this time 
and clearly shows 1.8nF (and believe me, I looked at this about 10 times 
last night, just to be sure). Well, at least I'm a bit more versed in the 
filter area for my efforts.

BTW: This N=3 filter is not a real good match for a 0 ohm output PA like we 
are using. All odd-numbered filters including this N=3 are supposed to have 
the same input and output resistance, IE: 50 ohms, in and out, for the 
Lyle's values. In my Pspice simulation last night I replaced the 50 ohm 
generator resistor with a 1 ohm  resistor and this produced a 20% ripple in 
the pass band. This ripple is not a serious problem unless you are running 
experiments at different Lowfer frequencies and need a constant output. An 
N=4 or other even-numbered filter with a final capacitor to gnd is a better 
choice for this application. The basic three component values will need to 
be adjusted slightly as well. Even an N=2 (just a 63uh inductor and a 8.7nf 
capacitor) filter can be used if your stop band attenuation needs are not 
too great.

Bill


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