mystran wrote:aciddose wrote:Positive signal current flows along one path and negative signal current the other. That is why you need two capacitors, because unlike in the transistor-ladder design the positive and negative components of the signal path are at different points in the ladder.
Oh ok, that makes sense. Thanks.
edit: seems I just missed the connection from LP down on the right-hand side.. but IMHO the frequency control design is still rather weird.
Not at all weird.
To someone with electronic experience layout matters less, but layout is still extremely important. I'm able to see this variation of the layout just by glancing at it of course but that is because I understand every element of the circuit in depth.
Seeing it this way first-hand may help you:
First we have the input buffer for the signal. Simple enough. It also provides a voltage source for the mirror to operate upon.
Input buffer: (image removed, ask if you need this for some reason)
Next is the current mirror used to generate current to control the resistance of the diodes. Higher current through a diode produces lower resistance in-line with the current. That is, obviously you won't be lowering the resistance backward through the diode by forcing current forward.
We want to remove 1-n from the positive side of the diode ladder and n from the negative side so that we can control the current flowing through the diodes.
Conveniently, such a current mirror in such a configuration also has a logarithmic response to it's control current
Current mirror / differential amplifier: (image removed, ask if you need this for some reason)
Output buffer: (image removed, ask if you need this for some reason)
The feedback section: (image removed, ask if you need this for some reason)
So far these are all standard elements although you might not immediately understand how a current mirror works or how it's controlling the current through the diodes.
The next part is what I've re-arranged to help you see how this filter works more easily.
The diode 'ladder': (image removed, ask if you need this for some reason)
These little schematics are laid out the same way I lay out the circuit on actual PCB. Fits quite efficiently this way