[sdiy] VCR - Voltage Controlled Resistor

[Scott Gravenhorst]

harrybissell <harrybissell at prodigy.net> wrote:
>RE: LPF
>
>This structure is like one stage of a 2040 style filter... probably not directly
>applicable to SVF.  Don't be fooled by the APF (pahse shifter) example.

Pardon my niavete...  I don't know what the hell I'm doing, but I try.  I was under the
(possibly mistaken) impression that a SVF is made of 2 active LPF blocks each with a
feedback path to a summing node, signal input to the summing node and the node feeds
the first LPF block.  I sort of tried to put 1.94 and 1.83 together to get 4...  

Why is this LPF block not compatible with a SVF configuration?

>
>H^) harry
>
>Scott Gravenhorst wrote:
>
>> One comment inline.
>>
>> HOSHUYAMA <houshu at muj.biglobe.ne.jp> wrote:
>> >Hello Scott and List,
>> >
>> >Maybe I don't follow all the discussion in the weekend.
>> >But, I think I can contribute something.
>> >
>> >Scott Gravenhorst wrote:
>> >> harrybissell <harrybissell at prodigy.net> wrote:
>> >>>I suggest you hie hither to Osamu Hoshuyama's site. He has some ideas there on
>> >>>FET linearization iirc.  One of them is truly a new idea that has not been
>> >>>commonly used !
>> >>
>> >> I've been there, he has an SVF using 4069UB integrators, but with an M5222 dual
>> >> expo OTA (unobtainium), essentially as WASP type filter.  He also has a 4069UB
>> >> integrator SVF using MOSFET OTAs built from more 4069UB (but says they are tricky
>> >> to make them work).  He doesn't seem to use FET VCRs in any of his filters.
>> >> Perhaps I know why now.
>> >
>> >Actually, at the beginning of my CMOS VCF project, I tried to
>> >design some VCF structures using 4049 as shunt VCRs, but
>> >failed in those days.
>> >
>> >The reason I used inverters as OTAs in my CMOS VCF is that I was
>> >inspired by Pearl phaser PH-03, where an MC14069U is used as VCRs
>> >based on an OTA technique.  I thought the structure is the only
>> >way to obtain "smooth" and "wide-range" sweep.
>> >
>> >  http://www5b.biglobe.ne.jp/~houshu/synth/VcfMos0206.GIF
>> >  http://www5b.biglobe.ne.jp/~houshu/synth/CmosOta.GIF
>> >
>> >
>> >I've seen a schematic that uses 4007 as VCRs in the SVF.  It's an
>> >envelope-controlled filter of Electroharmonics, which may be
>> >useful, though I don't think it has sufficiently wide sweep-range
>> >for a synth VCF.
>> >
>> >If I have "matched" and "floating" FETs (MOSFETs), I'd try a 4-pole
>> >VCF based on the LPF structure in the following schematic replacing
>> >the OTA by a FET, and the opamp by a CMOS inverter.
>> >
>> >  http://www5b.biglobe.ne.jp/~houshu/synth/Vcf0210.GIF
>>
>> I saw this the other day and it's implication went whizzing right over my little
>> wooden head.
>>
>> This technique (I assume as depicted in the LPF inset) looks like a H11F1(s) could
>> be used, or perhaps a 4007UB to make a SVF from 2 of these LPF blocks.  (or am I
>> being terribly naive?)  I will not claim to understand this structure.  It is not in
>> Lancaster's Active-Filter Cookbook.  I am not at all sure exactly where the summing
>> node is!  It's almost is if there are two virtual grounds (which could explain the
>> low Vds possibility) because of the dual feedback configuration.  My guess is that
>> the source of the MOSFET would connect to the capacitor/amplifier input node.
>>
>> Would a SVF made this way have a 12 dB slope?
>>
>> >In the structure, the signal level across the FET is small
>> >thanks to negative feedback, which leads to less distortion
>> >in pass band.  My linearizing technique can also be applied.
>> >
>> >  http://www5b.biglobe.ne.jp/~houshu/synth/PhaseFet0205.GIF
>> >
>> >Linearizing the relationship between CV and the resistance of
>> >the FET is possible if master-slave technique is applied.
>> >
>> >I'm busy these days.  So, don't expect quick response please.
>> >Writing English is not easy job for me  ;-)
>> >
>> >Sam Hoshuyama
>> >Saitama, Japan
>> >
>> >
>>
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>>
>> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
>> -- Linux Rex         | RedWebMail by RedStarWare
>> -- FatMan: home1.gte.net/res0658s/FatMan/
>> -- NonFatMan: home1.gte.net/res0658s/electronics/
>> -- Autodidactic Master of Arcane and Hidden Knowledge.
>

---------------------------------------------------------
- Where merit is not rewarded, excellence fades.
- Hydrogen is pointless without solar.
- What good are laws that only lawyers understand?
- The media's credibility should always be questioned.
- The only good terrorist is a dead terrorist.
- Governments do nothing well, save collect taxes.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.gte.net/res0658s/FatMan/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- Autodidactic Master of Arcane and Hidden Knowledge.