Reply by Bill Sloman August 10, 20152015-08-10
On Monday, 10 August 2015 23:47:08 UTC+10, George Herold  wrote:
> On Saturday, August 8, 2015 at 7:16:33 PM UTC-4, Bill Sloman wrote: > > On Saturday, 8 August 2015 00:52:25 UTC+10, George Herold wrote: > > > On Thursday, August 6, 2015 at 3:29:02 PM UTC-4, Tom Del Rosso wrote: > > > > Tim Wescott wrote: > > > > > On Wed, 05 Aug 2015 12:39:09 -0400, Tom Del Rosso wrote: > > > > > > > > > >> I don't want anybody to design it for me, but in general how would > > > > >> you make an oscillator with a sine output of constant amplitude over > > > > >> a range of supply voltage (let's say the operating range of the > > > > >> op-amp). What I have in mind is a battery supply, but with no power > > > > >> wasted in a regulator. Sure the oscillator would also waste power > > > > >> but not as much. Would a square output be simpler? > > > > > > > > > > Can you get by with a squarer wave? If yes, then your easiest and > > > > > most fuel-efficient way to do this would be to regulate with a > > > > > switcher and then generate a square wave by any of the many > > > > > acceptable ways. > > > > > > > > > > Just plain easiest, and not too bad on current consumption if you > > > > > don't need any appreciable power on the output, would be a linear > > > > > regulator powering your oscillator. > > > > > > > > > > Maybe tell us what this is for so we can ground our opinions in fact > > > > > instead of spinning off into our own versions of theory-land? > > > > > > > > It's mainly for general knowledge, since my test signal doesn't have to > > > > be that constant. I just got the idea that it must be possible, so, if > > > > it was easy then I'd do it, or else just use a regulator. The Wein > > > > Bridge is easy enough to be worth a try so I'll breadboard a few and try > > > > it with a variable supply to see what happens. > > > > > > If you don't care too much about distortion the Wein bridge w/ diode > > > limiting can keep the 3rd harmonic down to the 0.1 to 1% level. > > > You can make it a bit better by taking the output (with another opamp > > > as buffer) from the non-inverting input. > > > (You get a little RC filtering "for free".) > > > > > > (DDS has taken a lot of the fun out of oscillator circuits.) > > > > Not if you want seriously low distortion - harmonic spurs more than 120dB below the fundamental. > > > I was going to ask what sort of "science" demands such low distortion. > But then I thought of one... and I have data! > > https://www.dropbox.com/s/sggrml361llcnu1/OP2p-2ndHarm.BMP?dl=0 > > Above is a sweep through an optical pumping signal > (I'd have to check the numbers.. but something like..) > RF frequency = 100 kHz > Sweep width (x-axis) is about 4 mG > The signal is transmission at different RF amplitudes. > > The peak on the right that grows linearly with amplitude > is the 2nd harmonic distortion.. > The generator (my Rigol) has some power at 2f. > The signal on the left hand side that grows as > ~RF amplitude squared (Again I'd have to look up the numbers.) > is from two photon transitions. > > The reason both signals don't sit right on top of each other > is that there is a non-linear term in the Zeeman splitting of the > ground state given (exactly) by the Breit-Rabi equation. > > OK any other science or electronics that needs such low distortion? > > (Oh you may see that the 2-photon feature is also somewhat broader.. > that's also from the quadratic term in the Zeeman energy.)
Vojtěch Janásek, http://www.janascard.cz/PDF/An%20ultra%20low%20distortion%20oscillator%20with%20THD%20below%20-140%20dB.pdf wanted it to test better-than-20-bit A/D converters. My thought is that anybody who gets serious about sub-micro-degree temperature measurement is using an A/C (probably Blumlein) bridge, which can only be balanced at one frequency. Even low-level harmonic content in the bridge drive can swamp the desired signal and saturate the high-gain amplifiers that you need to pull out the unbalance signal at the desired frequency. The short answer is that there's a small - but desperate - market out there for really low distortion sine wave sources. -- Bill Sloman, Sydney
Reply by George Herold August 10, 20152015-08-10
On Saturday, August 8, 2015 at 7:16:33 PM UTC-4, Bill Sloman wrote:
> On Saturday, 8 August 2015 00:52:25 UTC+10, George Herold wrote: > > On Thursday, August 6, 2015 at 3:29:02 PM UTC-4, Tom Del Rosso wrote: > > > Tim Wescott wrote: > > > > On Wed, 05 Aug 2015 12:39:09 -0400, Tom Del Rosso wrote: > > > > > > > >> I don't want anybody to design it for me, but in general how would > > > >> you make an oscillator with a sine output of constant amplitude over > > > >> a range of supply voltage (let's say the operating range of the > > > >> op-amp). What I have in mind is a battery supply, but with no power > > > >> wasted in a regulator. Sure the oscillator would also waste power > > > >> but not as much. Would a square output be simpler? > > > > > > > > Can you get by with a squarer wave? If yes, then your easiest and > > > > most fuel-efficient way to do this would be to regulate with a > > > > switcher and then generate a square wave by any of the many > > > > acceptable ways. > > > > > > > > Just plain easiest, and not too bad on current consumption if you > > > > don't need any appreciable power on the output, would be a linear > > > > regulator powering your oscillator. > > > > > > > > Maybe tell us what this is for so we can ground our opinions in fact > > > > instead of spinning off into our own versions of theory-land? > > > > > > It's mainly for general knowledge, since my test signal doesn't have to > > > be that constant. I just got the idea that it must be possible, so, if > > > it was easy then I'd do it, or else just use a regulator. The Wein > > > Bridge is easy enough to be worth a try so I'll breadboard a few and try > > > it with a variable supply to see what happens. > > > > > > Thanks very much to all. > > > > > > -- > > > > If you don't care too much about distortion the Wein bridge w/ diode limiting can keep the 3rd harmonic down to the 0.1 to 1% level. > > You can make it a bit better by taking the output (with another opamp > > as buffer) from the non-inverting input. > > (You get a little RC filtering "for free".) > > > > (DDS has taken a lot of the fun out of oscillator circuits.) > > Not if you want seriously low distortion - harmonic spurs more than 120dB below the fundamental. >
I was going to ask what sort of "science" demands such low distortion. But then I thought of one... and I have data! https://www.dropbox.com/s/sggrml361llcnu1/OP2p-2ndHarm.BMP?dl=0 Above is a sweep through an optical pumping signal (I'd have to check the numbers.. but something like..) RF frequency = 100 kHz Sweep width (x-axis) is about 4 mG The signal is transmission at different RF amplitudes. The peak on the right that grows linearly with amplitude is the 2nd harmonic distortion.. The generator (my Rigol) has some power at 2f. The signal on the left hand side that grows as ~RF amplitude squared (Again I'd have to look up the numbers.) is from two photon transitions. The reason both signals don't sit right on top of each other is that there is a non-linear term in the Zeeman splitting of the ground state given (exactly) by the Breit-Rabi equation. OK any other science or electronics that needs such low distortion? George H. (Oh you may see that the 2-photon feature is also somewhat broader.. that's also from the quadratic term in the Zeeman energy.)
> -- > Bill Sloman, Sydney
Reply by Bill Sloman August 8, 20152015-08-08
On Saturday, 8 August 2015 00:52:25 UTC+10, George Herold  wrote:
> On Thursday, August 6, 2015 at 3:29:02 PM UTC-4, Tom Del Rosso wrote: > > Tim Wescott wrote: > > > On Wed, 05 Aug 2015 12:39:09 -0400, Tom Del Rosso wrote: > > > > > >> I don't want anybody to design it for me, but in general how would > > >> you make an oscillator with a sine output of constant amplitude over > > >> a range of supply voltage (let's say the operating range of the > > >> op-amp). What I have in mind is a battery supply, but with no power > > >> wasted in a regulator. Sure the oscillator would also waste power > > >> but not as much. Would a square output be simpler? > > > > > > Can you get by with a squarer wave? If yes, then your easiest and > > > most fuel-efficient way to do this would be to regulate with a > > > switcher and then generate a square wave by any of the many > > > acceptable ways. > > > > > > Just plain easiest, and not too bad on current consumption if you > > > don't need any appreciable power on the output, would be a linear > > > regulator powering your oscillator. > > > > > > Maybe tell us what this is for so we can ground our opinions in fact > > > instead of spinning off into our own versions of theory-land? > > > > It's mainly for general knowledge, since my test signal doesn't have to > > be that constant. I just got the idea that it must be possible, so, if > > it was easy then I'd do it, or else just use a regulator. The Wein > > Bridge is easy enough to be worth a try so I'll breadboard a few and try > > it with a variable supply to see what happens. > > > > Thanks very much to all. > > > > -- > > If you don't care too much about distortion the Wein bridge w/ diode limiting can keep the 3rd harmonic down to the 0.1 to 1% level. > You can make it a bit better by taking the output (with another opamp > as buffer) from the non-inverting input. > (You get a little RC filtering "for free".) > > (DDS has taken a lot of the fun out of oscillator circuits.)
Not if you want seriously low distortion - harmonic spurs more than 120dB below the fundamental. -- Bill Sloman, Sydney
Reply by John Fields August 8, 20152015-08-08
On Thu, 06 Aug 2015 15:51:32 -0500, Tim Wescott
<seemywebsite@myfooter.really> wrote:


>Apparently there are (or at least were) some studio-quality mixers that >were implemented by shining variable amounts of light onto CdS cells for >low-noise, low-distortion variable resistance controlled by a DC voltage.
--- Ahh, yes. One of my all-time favorites, the VACTROL: https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=VACTROL John Fields
Reply by George Herold August 7, 20152015-08-07
On Thursday, August 6, 2015 at 3:29:02 PM UTC-4, Tom Del Rosso wrote:
> Tim Wescott wrote: > > On Wed, 05 Aug 2015 12:39:09 -0400, Tom Del Rosso wrote: > > > >> I don't want anybody to design it for me, but in general how would > >> you make an oscillator with a sine output of constant amplitude over > >> a range of supply voltage (let's say the operating range of the > >> op-amp). What I have in mind is a battery supply, but with no power > >> wasted in a regulator. Sure the oscillator would also waste power > >> but not as much. Would a square output be simpler? > > > > Can you get by with a squarer wave? If yes, then your easiest and > > most fuel-efficient way to do this would be to regulate with a > > switcher and then generate a square wave by any of the many > > acceptable ways. > > > > Just plain easiest, and not too bad on current consumption if you > > don't need any appreciable power on the output, would be a linear > > regulator powering your oscillator. > > > > Maybe tell us what this is for so we can ground our opinions in fact > > instead of spinning off into our own versions of theory-land? > > It's mainly for general knowledge, since my test signal doesn't have to > be that constant. I just got the idea that it must be possible, so, if > it was easy then I'd do it, or else just use a regulator. The Wein > Bridge is easy enough to be worth a try so I'll breadboard a few and try > it with a variable supply to see what happens. > > Thanks very much to all. > > --
If you don't care too much about distortion the Wein bridge w/ diode limiting can keep the 3rd harmonic down to the 0.1 to 1% level. You can make it a bit better by taking the output (with another opamp as buffer) from the non-inverting input. (You get a little RC filtering "for free".) (DDS has taken a lot of the fun out of oscillator circuits.) George H.
Reply by David Eather August 7, 20152015-08-07
On Fri, 07 Aug 2015 12:50:46 +1000, Bill Sloman <bill.sloman@gmail.com>  
wrote:

> On Friday, 7 August 2015 07:29:11 UTC+10, David Eather wrote: >> On Fri, 07 Aug 2015 06:51:32 +1000, Tim Wescott >> <seemywebsite@myfooter.really> wrote: >> >> > On Fri, 07 Aug 2015 06:31:16 +1000, David Eather wrote: >> > >> >> On Fri, 07 Aug 2015 06:07:39 +1000, Tim Wescott >> <tim@seemywebsite.com> >> >> wrote: >> >> >> >>> On Thu, 06 Aug 2015 15:28:56 -0400, Tom Del Rosso wrote: >> >>> >> >>>> Tim Wescott wrote: >> >>>>> On Wed, 05 Aug 2015 12:39:09 -0400, Tom Del Rosso wrote: >> >>>>> >> >>>>>> I don't want anybody to design it for me, but in general how >> would >> >>>>>> you make an oscillator with a sine output of constant amplitude >> over >> >>>>>> a range of supply voltage (let's say the operating range of the >> >>>>>> op-amp). >> >>>>>> What I have in mind is a battery supply, but with no power >> wasted >> >>>>>> in >> >>>>>> a regulator. Sure the oscillator would also waste power but not >> as >> >>>>>> much. Would a square output be simpler? >> >>>>> >> >>>>> Can you get by with a squarer wave? If yes, then your easiest and >> >>>>> most fuel-efficient way to do this would be to regulate with a >> >>>>> switcher and then generate a square wave by any of the many >> >>>>> acceptable ways. >> >>>>> >> >>>>> Just plain easiest, and not too bad on current consumption if you >> >>>>> don't need any appreciable power on the output, would be a linear >> >>>>> regulator powering your oscillator. >> >>>>> >> >>>>> Maybe tell us what this is for so we can ground our opinions in >> fact >> >>>>> instead of spinning off into our own versions of theory-land? >> >>>> >> >>>> It's mainly for general knowledge, since my test signal doesn't >> have >> >>>> to be that constant. I just got the idea that it must be possible, >> >>>> so, if it was easy then I'd do it, or else just use a regulator. >> The >> >>>> Wein Bridge is easy enough to be worth a try so I'll breadboard a >> few >> >>>> and try it with a variable supply to see what happens. >> >>>> >> >>>> Thanks very much to all. >> >>> >> >>> If you can find a suitable grain-o-wheat bulb it's fun to make a >> >>> thermally stabilized oscillator. >> >>> >> >>> The bulbs aren't easy to find these days, though -- LEDs just don't >> >>> work the same. >> >>> >> >>> >> >> J-fet >> > >> > I've done that. Distortion becomes more of an issue. >> > >> > Apparently there are (or at least were) some studio-quality mixers >> that >> > were implemented by shining variable amounts of light onto CdS cells >> for >> > low-noise, low-distortion variable resistance controlled by a DC >> voltage. >> > >> >> http://cds.linear.com/docs/en/application-note/an43f.pdf >> >> Has many ways to do it, including CDS cells or jfet (.002%) >> >> http://www.ti.com/lit/an/snoa665c/snoa665c.pdf >> >> uses a cap to block DC across the jfet for an improvement in THD they >> only >> claim .01% though. With a jfet with interchangeable D-S it does better. > > Few jfets don't have interchangeable drain and source connections.
perhaps the word symmetrical would have been better
> > Winfield Hill's AOE 3 gives and example as figure 7.22 on page 438. He > uses the drain-gate divider trick on the jfet to minimise distortion > (which Jim Williams missed in AN-43 at Fig.43).
but is in the ap note no the less.
> > The National Semiconductor LME49710 might have been a better choice of > op amp.
Reply by Bill Sloman August 6, 20152015-08-06
On Friday, 7 August 2015 05:29:02 UTC+10, Tom Del Rosso  wrote:
> Tim Wescott wrote: > > On Wed, 05 Aug 2015 12:39:09 -0400, Tom Del Rosso wrote: > > > >> I don't want anybody to design it for me, but in general how would > >> you make an oscillator with a sine output of constant amplitude over > >> a range of supply voltage (let's say the operating range of the > >> op-amp). What I have in mind is a battery supply, but with no power > >> wasted in a regulator. Sure the oscillator would also waste power > >> but not as much. Would a square output be simpler? > > > > Can you get by with a squarer wave? If yes, then your easiest and > > most fuel-efficient way to do this would be to regulate with a > > switcher and then generate a square wave by any of the many > > acceptable ways. > > > > Just plain easiest, and not too bad on current consumption if you > > don't need any appreciable power on the output, would be a linear > > regulator powering your oscillator. > > > > Maybe tell us what this is for so we can ground our opinions in fact > > instead of spinning off into our own versions of theory-land? > > It's mainly for general knowledge, since my test signal doesn't have to > be that constant. I just got the idea that it must be possible, so, if > it was easy then I'd do it, or else just use a regulator. The Wein > Bridge is easy enough to be worth a try so I'll breadboard a few and try > it with a variable supply to see what happens.
Probably not the best approach if you want to minimise current consumption. E-mail me - at bill.sloman@ieee.org - for an alternative. Tell me your preferred frequency and I might be able to tailor my solution. -- Bill Sloman, Sydney
Reply by Bill Sloman August 6, 20152015-08-06
On Friday, 7 August 2015 07:29:11 UTC+10, David Eather  wrote:
> On Fri, 07 Aug 2015 06:51:32 +1000, Tim Wescott > <seemywebsite@myfooter.really> wrote: > > > On Fri, 07 Aug 2015 06:31:16 +1000, David Eather wrote: > > > >> On Fri, 07 Aug 2015 06:07:39 +1000, Tim Wescott <tim@seemywebsite.com> > >> wrote: > >> > >>> On Thu, 06 Aug 2015 15:28:56 -0400, Tom Del Rosso wrote: > >>> > >>>> Tim Wescott wrote: > >>>>> On Wed, 05 Aug 2015 12:39:09 -0400, Tom Del Rosso wrote: > >>>>> > >>>>>> I don't want anybody to design it for me, but in general how would > >>>>>> you make an oscillator with a sine output of constant amplitude over > >>>>>> a range of supply voltage (let's say the operating range of the > >>>>>> op-amp). > >>>>>> What I have in mind is a battery supply, but with no power wasted > >>>>>> in > >>>>>> a regulator. Sure the oscillator would also waste power but not as > >>>>>> much. Would a square output be simpler? > >>>>> > >>>>> Can you get by with a squarer wave? If yes, then your easiest and > >>>>> most fuel-efficient way to do this would be to regulate with a > >>>>> switcher and then generate a square wave by any of the many > >>>>> acceptable ways. > >>>>> > >>>>> Just plain easiest, and not too bad on current consumption if you > >>>>> don't need any appreciable power on the output, would be a linear > >>>>> regulator powering your oscillator. > >>>>> > >>>>> Maybe tell us what this is for so we can ground our opinions in fact > >>>>> instead of spinning off into our own versions of theory-land? > >>>> > >>>> It's mainly for general knowledge, since my test signal doesn't have > >>>> to be that constant. I just got the idea that it must be possible, > >>>> so, if it was easy then I'd do it, or else just use a regulator. The > >>>> Wein Bridge is easy enough to be worth a try so I'll breadboard a few > >>>> and try it with a variable supply to see what happens. > >>>> > >>>> Thanks very much to all. > >>> > >>> If you can find a suitable grain-o-wheat bulb it's fun to make a > >>> thermally stabilized oscillator. > >>> > >>> The bulbs aren't easy to find these days, though -- LEDs just don't > >>> work the same. > >>> > >>> > >> J-fet > > > > I've done that. Distortion becomes more of an issue. > > > > Apparently there are (or at least were) some studio-quality mixers that > > were implemented by shining variable amounts of light onto CdS cells for > > low-noise, low-distortion variable resistance controlled by a DC voltage. > > > > http://cds.linear.com/docs/en/application-note/an43f.pdf > > Has many ways to do it, including CDS cells or jfet (.002%) > > http://www.ti.com/lit/an/snoa665c/snoa665c.pdf > > uses a cap to block DC across the jfet for an improvement in THD they only > claim .01% though. With a jfet with interchangeable D-S it does better.
Few jfets don't have interchangeable drain and source connections. Winfield Hill's AOE 3 gives and example as figure 7.22 on page 438. He uses the drain-gate divider trick on the jfet to minimise distortion (which Jim Williams missed in AN-43 at Fig.43). The National Semiconductor LME49710 might have been a better choice of op amp. -- Bill Sloman, Sydney
Reply by David Eather August 6, 20152015-08-06
On Fri, 07 Aug 2015 07:29:32 +1000, David Eather <eather@tpg.com.au> wrote:

> On Fri, 07 Aug 2015 06:51:32 +1000, Tim Wescott > <seemywebsite@myfooter.really> wrote: > >> On Fri, 07 Aug 2015 06:31:16 +1000, David Eather wrote: >> >>> On Fri, 07 Aug 2015 06:07:39 +1000, Tim Wescott <tim@seemywebsite.com> >>> wrote: >>> >>>> On Thu, 06 Aug 2015 15:28:56 -0400, Tom Del Rosso wrote: >>>> >>>>> Tim Wescott wrote: >>>>>> On Wed, 05 Aug 2015 12:39:09 -0400, Tom Del Rosso wrote: >>>>>> >>>>>>> I don't want anybody to design it for me, but in general how would >>>>>>> you make an oscillator with a sine output of constant amplitude >>>>>>> over >>>>>>> a range of supply voltage (let's say the operating range of the >>>>>>> op-amp). >>>>>>> What I have in mind is a battery supply, but with no power wasted >>>>>>> in >>>>>>> a regulator. Sure the oscillator would also waste power but not as >>>>>>> much. Would a square output be simpler? >>>>>> >>>>>> Can you get by with a squarer wave? If yes, then your easiest and >>>>>> most fuel-efficient way to do this would be to regulate with a >>>>>> switcher and then generate a square wave by any of the many >>>>>> acceptable ways. >>>>>> >>>>>> Just plain easiest, and not too bad on current consumption if you >>>>>> don't need any appreciable power on the output, would be a linear >>>>>> regulator powering your oscillator. >>>>>> >>>>>> Maybe tell us what this is for so we can ground our opinions in fact >>>>>> instead of spinning off into our own versions of theory-land? >>>>> >>>>> It's mainly for general knowledge, since my test signal doesn't have >>>>> to be that constant. I just got the idea that it must be possible, >>>>> so, if it was easy then I'd do it, or else just use a regulator. The >>>>> Wein Bridge is easy enough to be worth a try so I'll breadboard a few >>>>> and try it with a variable supply to see what happens. >>>>> >>>>> Thanks very much to all. >>>> >>>> If you can find a suitable grain-o-wheat bulb it's fun to make a >>>> thermally stabilized oscillator. >>>> >>>> The bulbs aren't easy to find these days, though -- LEDs just don't >>>> work the same. >>>> >>>> >>> J-fet >> >> I've done that. Distortion becomes more of an issue. >> >> Apparently there are (or at least were) some studio-quality mixers that >> were implemented by shining variable amounts of light onto CdS cells for >> low-noise, low-distortion variable resistance controlled by a DC >> voltage. >> > > http://cds.linear.com/docs/en/application-note/an43f.pdf > > Has many ways to do it, including CDS cells or jfet (.002%) > > http://www.ti.com/lit/an/snoa665c/snoa665c.pdf > > uses a cap to block DC across the jfet for an improvement in THD they > only claim .01% though. With a jfet with interchangeable D-S it does > better
forgot - page 6
Reply by David Eather August 6, 20152015-08-06
On Fri, 07 Aug 2015 06:51:32 +1000, Tim Wescott  
<seemywebsite@myfooter.really> wrote:

> On Fri, 07 Aug 2015 06:31:16 +1000, David Eather wrote: > >> On Fri, 07 Aug 2015 06:07:39 +1000, Tim Wescott <tim@seemywebsite.com> >> wrote: >> >>> On Thu, 06 Aug 2015 15:28:56 -0400, Tom Del Rosso wrote: >>> >>>> Tim Wescott wrote: >>>>> On Wed, 05 Aug 2015 12:39:09 -0400, Tom Del Rosso wrote: >>>>> >>>>>> I don't want anybody to design it for me, but in general how would >>>>>> you make an oscillator with a sine output of constant amplitude over >>>>>> a range of supply voltage (let's say the operating range of the >>>>>> op-amp). >>>>>> What I have in mind is a battery supply, but with no power wasted >>>>>> in >>>>>> a regulator. Sure the oscillator would also waste power but not as >>>>>> much. Would a square output be simpler? >>>>> >>>>> Can you get by with a squarer wave? If yes, then your easiest and >>>>> most fuel-efficient way to do this would be to regulate with a >>>>> switcher and then generate a square wave by any of the many >>>>> acceptable ways. >>>>> >>>>> Just plain easiest, and not too bad on current consumption if you >>>>> don't need any appreciable power on the output, would be a linear >>>>> regulator powering your oscillator. >>>>> >>>>> Maybe tell us what this is for so we can ground our opinions in fact >>>>> instead of spinning off into our own versions of theory-land? >>>> >>>> It's mainly for general knowledge, since my test signal doesn't have >>>> to be that constant. I just got the idea that it must be possible, >>>> so, if it was easy then I'd do it, or else just use a regulator. The >>>> Wein Bridge is easy enough to be worth a try so I'll breadboard a few >>>> and try it with a variable supply to see what happens. >>>> >>>> Thanks very much to all. >>> >>> If you can find a suitable grain-o-wheat bulb it's fun to make a >>> thermally stabilized oscillator. >>> >>> The bulbs aren't easy to find these days, though -- LEDs just don't >>> work the same. >>> >>> >> J-fet > > I've done that. Distortion becomes more of an issue. > > Apparently there are (or at least were) some studio-quality mixers that > were implemented by shining variable amounts of light onto CdS cells for > low-noise, low-distortion variable resistance controlled by a DC voltage. >
http://cds.linear.com/docs/en/application-note/an43f.pdf Has many ways to do it, including CDS cells or jfet (.002%) http://www.ti.com/lit/an/snoa665c/snoa665c.pdf uses a cap to block DC across the jfet for an improvement in THD they only claim .01% though. With a jfet with interchangeable D-S it does better