[BC] Correct URL for the Symmetra-Peak

[Robert Orban]

At 11:58 AM 8/16/2006, you wrote:
>From: Robert Meuser <Robertm at broadcast.net>
>Subject: Re: [BC] Correct URL for the Symmetra-Peak
>To: "Broadcasters' Mailing List" <broadcast at radiolists.net>
>Message-ID: <44E359B3.9000709 at broadcast.net>
>Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
>Very simplisticly, a gyrator is an opamp circuit that turns a resistor
>into an inductor. The gyrator circuit can be trimmed with a
>potentiometer, so it could actually be a variable inductor. They are the
>major component in FDNR filters found in the output of some audio
>processors, including some Orban and some CRL units.
>
>You could do the symmetra-peak circuit with 2 quad opamps.

An FDNR is not a gyrator, although FDNRs and gyrators are two specialized 
forms of "generalized immitance converter" circuits.

These circuits are particularly useful for simulating passive, 
double-terminated LC lowpass and highpass filters because the resulting 
active RC network retains much of the original LC filter's low sensitivity 
to component variations in the passband. In essence, this is because the 
passband of the filter permits maximum possible power transfer between 
source and load at the peaks of the passband ripple. Please note that this 
favorable sensitivity property does NOT apply to the stopband of a passive 
LC filter.

In general, FDNRs are useful for lowpass filters and gyrators are useful 
for highpass filters because these applications allow the GIC in question 
to have one grounded terminal. Transforming passive LC bandpass filters 
gets more complicated.

I don't recall seeing any detailed analysis of the complications that might 
arise by simulating the inductors in a lattice by floating gyrators. I'm 
not sure you could do the Kahn simulation this way, use 8 opamps, and still 
get good performance. But I don't know for sure that you couldn't.

I suspect that the minimum number of opamps required to do a 
manufacturable, accurate active-RC implementation of the Kahn is probably 
four. These would realize cascaded stages, each realizing two poles and two 
zeros, paired so that the overall response of the stage is as flat as possible.

Bob Orban