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

[Bill Ashlock]

John A,

I wrote a nice long reply this morning but it looks like it never came 
through. My copy of Lyle's 'simple transmitter' definately indicates a 1.8nf 
capacitor, but I think it must be almost 4 years old. I'm releived in many 
ways that this has been corrected.

The filter, still, is not the best design for a 0 ohm generator and Pspice 
shows a 20% ripple in the passband when the 50 ohm resistor is changed to 1 
ohm. Increasing the filter size to a 4th order appears to fix this problem 
and this is a simple adjustment of the values and a final capacitor to gnd. 
I'll post this change after I give it a good test on the bench. I still 
haven't added the coax to the model so there could be more surprises coming.

Bill

>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 I
>posted shows 18 nF. Where did you get the 1.8 nF from?
>
>John Andrews
>
> > I spend about 2 hours tonight reviewing how these filter tables work. I
>just
> > looked at Lyle's 63uh-1.8nf-63uh filter and it looks like the 1.8nf
> > capacitor should be 18nf. Using the table and scaling equations for a 50
> > ohm, .25db ripple, N=3, Chebyshev filter, for a cutoff freq of 232k, the
> > inductors are 63uh and the capacitor between then to gnd is 18nf, not
>1.8nf.
> > I set this up in Psipe and the plotted frequency response with 50 ohm
>source
> > and load resistances and this perfectly matches the expected response.
>Then
> > I changed the capacitor to 1.8nf and the response became totally whacky!
>Now
> > it's back to the bench. Results tomorrow.
> >
> > Bill
> >
> >
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