Ladder filters

[jh]

>The 4 stages of a transistor based ladder filter have "reverse isolation" a
>term used in RF circuitry.  Each section is isolated from the others, each
>section adds a single pole, all 4 poles should fall onto the same point
>(same cutoff frequency).  The diode ladder filter has no "reverse
>isolation", each section is coupled to the one next to it. The poles will
>be in different places, because each filter section has a different amount
<of loading.  The loading depends on where it is in the circuit, and if it
>is connected to the input buffer, or output buffer, or another filter
>section.
>
>Another way to look at is: the transistor ladder is equivilant to 4 RC
>lowpass sections in tandem with a buffer between each one, and the diode
>ladder is equivilant to 4 RC lowpass sections connected together in tandem,
> with no buffers between them. 

>If the filter topology is extended to the rf range, a multiple section
>filter with high isolation between sections will have a higher Q, and a
>sharper cutoff, than a filter with low isolation between sections.

Yes, shure, that's exactly how I see it.

I do not have a modeling circuit to graphically analyze this.  Has anyone
on the list run these filters on Spice or anything else?  I would be
curious to see what the lowpass functions looks like.

I have made Spice simulations of the Moog and EMS ladder some time
ago. Have to look if I find them. If they have been on my home PC, they are
gone with the HD. If they are at the office, they should still be there.
It's quite easy to model this, and to see the frequency response.
What I did not get from Spice are pole / zero plots, however.

JH.