[Lowfer] That filter can't be right!!!
Bill Ashlock
[email protected]
Thu, 05 Sep 2002 00:07:40 -0400
Jay, Jim M, and all,
These filter design variables are most intriguing! My latest discovery is
that when the I insert the 12 to 1 loop step-down transformer with coupling
coefficient of .95 in the Pspice filter model the frequency response gets
really messed up. Then, in place of the transformer I used a fairly standard
representation with a leakage inductance of 72 uh in series and the
reflected 74 ohms in parallel with a 720uh primary inductance and the plot
came out the same messed up way. Thus, it looks like this extra series
inductance (the leakage inductance) is the real villain. Maybe I can design
a filter that assumes this inductance is part of the filter. Ahh. I yern for
the simpler filterless barefoot approach!
Thanks for all the help!
.....And now to these filterless signal reports <g>
Bill
>From: Jay Rusgrove <[email protected]>
>Reply-To: [email protected]
>To: [email protected]
>Subject: Re: [Lowfer] That filter can't be right!!!
>Date: Wed, 04 Sep 2002 18:29:42 -0400
>
>Bill
>
>With the same inductor values on both the input and output arm of the 3
>pole
>filter, it is designed for the same impedance in and out. If one is trying
>to
>effect an impedance transformation from say 1 ohm up to 50 ohms you would
>want
>to use a "T" matching network instead.
>
>A "T" network for the above transformation would have the following values:
>
>
>Q XL1 XL2 XC1
>
>8 8 27.39 7.6
>
>10 10 50.5 9.17
>
>values courtesy Motorola RF Data Manual.
>
>In order to get the transformation to 1 ohm a minimum Q of 8 is required.
>The
>"T" match design has the added benefit of pretty good harmonic suppression.
>If
>further suppression is needed an additional 3 pole 50 ohm in/50 ohm filter
>out
>could be tied onto the end of the "T" network.
>
>Jay
>
>Bill Ashlock wrote:
>
> > 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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