John Larkin's LC oscillator
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From: Phil Hobbs on 18 Jun 2010 02:19 John Larkin wrote: > On Thu, 17 Jun 2010 17:01:54 -0400, Phil Hobbs > <pcdhSpamMeSenseless(a)electrooptical.net> wrote: > >> Jim Thompson wrote: >>> On Thu, 17 Jun 2010 12:58:30 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >>> >>>> John Larkin wrote: >>>>> On Thu, 17 Jun 2010 09:44:44 -0400, Phil Hobbs >>>>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >>>>> >>> [snip] >>>>> The base voltage needs to be +1 jd for the thing to oscillate at >>>>> equilibrium. The clamp device, c-b junction or schottky, has about the >>>>> same tc, but in the opposite direction. In the c-b case (with a low >>>>> secondary swing, not some awful class C thrashing, and a big base cap, >>>>> so it's not a blocking oscillator) the collector swing that pulls >>>>> current out of the base capacitor touches zero, near where the >>>>> transistor saturates. In fact, the tc of the c-b junction is canceling >>>>> the tc of the e-b junction. Not perfectly, because they are >>>>> effectively used at different currents. Because of reverse beta, some >>>>> of the tank energy can be clamped through the emitter, which is a bit >>>>> negative at that instant. But even that current flow happens when the >>>>> c-b junction is forward biased. >>>>> >>>>> Schottky: suppose we need +0.6 on the base for stable oscillation. The >>>>> collector will dip down to, say, 0.3, the schottky will conduct and >>>>> pull charge out of the base cap, and it will stabilize there. The tc's >>>>> almost cancel. If you assume the amplitude limiting is just brute >>>>> clamping of the swing, the tc's still cancel. No saturation, since the >>>>> c-b junction doesn't get forward biased. This assumes you've chosen a >>>>> suitable base resistor, not jamming too much current into the base, >>>>> and the transistor has reasonable beta. >>>>> >>>>> Actually, the base AGC thing must be happening. With small feedback >>>>> voltage into the emitter, a sim shows that the transistor is on >>>>> throughout the cycle, "class A." So Ic ~= beta * Ib, on average. >>>>> Average Ic would be large if the current through the base resistor >>>>> were all actually going into the base. But it's not: the base cap is >>>>> being discharged at the negative swing of the collector, stealing base >>>>> current and reducing transconductance, and that is exquisitely >>>>> sensitive to p-p amplitude. Some tank energy does of course get lost >>>>> to the emitter... both mechanisms are at work. Fortunately, both >>>>> stabilize the amplitude. >>>>> >>>>> In my sim, if you short the tank, the supply current goes up about >>>>> 30x. (Hmmm, Rb could be bigger!) Output swing is 10.11 volts p-p with >>>>> a 5-volt supply. >>>>> >>>>> Several people have modeled this circuit with small base caps and low >>>>> transformer ratios and lots of base bias current. That works >>>>> differently. >>>>> >>>>> John >>>> Interesting, thanks. I agree that the Baker clamp pretty well fixes the >>>> saturation problem. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> Phil, Don't you have means to simulate Larkin's oscillator? >>> >>> If you bothered to do that you'd find some of Larkin's statements >>> above are _absolutely_ incorrect, and others are so hand-waving as to >>> be hilarious. >>> >>> But, hey! Ignorant young bucks with no knowledge of basics are good >>> for business :-) >>> >>> ...Jim Thompson >> Of course I have. >> >> I'm hip deep in measuring very small nonlinearities in InGaAs >> photodiodes just now, > > How do you do that? Summing on/off light sources? I guess the question > is, how small? > > John > Low distortion sine wave + DC driving IR LED PD goes to TIA with x64 AC gain and x1 DC gain Auxiliary light source (fibre coupled laser in this case) provides CW light Bridge circuit (like an old HP THD meter) to null out sine wave and improve dynamic range Set up IR LED to give some decent AC photocurrent like 5 uA, null carefully, move laser in and out while changing PD reverse bias voltage. Look at residual on digital scope with FFT. This allows me to see nonlinearities down to about -70 dB. It could go further, but the silly laser mode hops like mad, and it takes forever to integrate it away. Interim conclusion: InGaAs is a lot less linear than Si. The quantum efficiency improves by something like 5% for bias levels between 0 and 5 V. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: John Larkin on 18 Jun 2010 09:50 On Fri, 18 Jun 2010 02:19:29 -0400, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >John Larkin wrote: >> On Thu, 17 Jun 2010 17:01:54 -0400, Phil Hobbs >> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >> >>> Jim Thompson wrote: >>>> On Thu, 17 Jun 2010 12:58:30 -0400, Phil Hobbs >>>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >>>> >>>>> John Larkin wrote: >>>>>> On Thu, 17 Jun 2010 09:44:44 -0400, Phil Hobbs >>>>>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >>>>>> >>>> [snip] >>>>>> The base voltage needs to be +1 jd for the thing to oscillate at >>>>>> equilibrium. The clamp device, c-b junction or schottky, has about the >>>>>> same tc, but in the opposite direction. In the c-b case (with a low >>>>>> secondary swing, not some awful class C thrashing, and a big base cap, >>>>>> so it's not a blocking oscillator) the collector swing that pulls >>>>>> current out of the base capacitor touches zero, near where the >>>>>> transistor saturates. In fact, the tc of the c-b junction is canceling >>>>>> the tc of the e-b junction. Not perfectly, because they are >>>>>> effectively used at different currents. Because of reverse beta, some >>>>>> of the tank energy can be clamped through the emitter, which is a bit >>>>>> negative at that instant. But even that current flow happens when the >>>>>> c-b junction is forward biased. >>>>>> >>>>>> Schottky: suppose we need +0.6 on the base for stable oscillation. The >>>>>> collector will dip down to, say, 0.3, the schottky will conduct and >>>>>> pull charge out of the base cap, and it will stabilize there. The tc's >>>>>> almost cancel. If you assume the amplitude limiting is just brute >>>>>> clamping of the swing, the tc's still cancel. No saturation, since the >>>>>> c-b junction doesn't get forward biased. This assumes you've chosen a >>>>>> suitable base resistor, not jamming too much current into the base, >>>>>> and the transistor has reasonable beta. >>>>>> >>>>>> Actually, the base AGC thing must be happening. With small feedback >>>>>> voltage into the emitter, a sim shows that the transistor is on >>>>>> throughout the cycle, "class A." So Ic ~= beta * Ib, on average. >>>>>> Average Ic would be large if the current through the base resistor >>>>>> were all actually going into the base. But it's not: the base cap is >>>>>> being discharged at the negative swing of the collector, stealing base >>>>>> current and reducing transconductance, and that is exquisitely >>>>>> sensitive to p-p amplitude. Some tank energy does of course get lost >>>>>> to the emitter... both mechanisms are at work. Fortunately, both >>>>>> stabilize the amplitude. >>>>>> >>>>>> In my sim, if you short the tank, the supply current goes up about >>>>>> 30x. (Hmmm, Rb could be bigger!) Output swing is 10.11 volts p-p with >>>>>> a 5-volt supply. >>>>>> >>>>>> Several people have modeled this circuit with small base caps and low >>>>>> transformer ratios and lots of base bias current. That works >>>>>> differently. >>>>>> >>>>>> John >>>>> Interesting, thanks. I agree that the Baker clamp pretty well fixes the >>>>> saturation problem. >>>>> >>>>> Cheers >>>>> >>>>> Phil Hobbs >>>> Phil, Don't you have means to simulate Larkin's oscillator? >>>> >>>> If you bothered to do that you'd find some of Larkin's statements >>>> above are _absolutely_ incorrect, and others are so hand-waving as to >>>> be hilarious. >>>> >>>> But, hey! Ignorant young bucks with no knowledge of basics are good >>>> for business :-) >>>> >>>> ...Jim Thompson >>> Of course I have. >>> >>> I'm hip deep in measuring very small nonlinearities in InGaAs >>> photodiodes just now, >> >> How do you do that? Summing on/off light sources? I guess the question >> is, how small? >> >> John >> > >Low distortion sine wave + DC driving IR LED >PD goes to TIA with x64 AC gain and x1 DC gain >Auxiliary light source (fibre coupled laser in this case) provides CW light >Bridge circuit (like an old HP THD meter) to null out sine wave and >improve dynamic range > >Set up IR LED to give some decent AC photocurrent like 5 uA, null >carefully, move laser in and out while changing PD reverse bias voltage. > >Look at residual on digital scope with FFT. > >This allows me to see nonlinearities down to about -70 dB. It could go >further, but the silly laser mode hops like mad, and it takes forever to >integrate it away. So you are sort of summing light sources. I guess. If you measure the magnitude of the AC component of the PD signal, at various points of added laser light, you get the slope gain at various points, and the distortion of the LED doesn't matter. Is that the idea? LEDs are pretty limear at higher currents, but I don't know about 70 dB. Actually, it wouldn't be all that hard to find out. > >Interim conclusion: InGaAs is a lot less linear than Si. The quantum >efficiency improves by something like 5% for bias levels between 0 and 5 V. Does that imply amplitude nonlinearity at fixed bias? What about temperature? Maybe the bias voltage needs a TC correction. John
From: Phil Hobbs on 18 Jun 2010 10:00 John Larkin wrote: > On Fri, 18 Jun 2010 02:19:29 -0400, Phil Hobbs > <pcdhSpamMeSenseless(a)electrooptical.net> wrote: > >> John Larkin wrote: >>> On Thu, 17 Jun 2010 17:01:54 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >>> >>>> Jim Thompson wrote: >>>>> On Thu, 17 Jun 2010 12:58:30 -0400, Phil Hobbs >>>>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >>>>> >>>>>> John Larkin wrote: >>>>>>> On Thu, 17 Jun 2010 09:44:44 -0400, Phil Hobbs >>>>>>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >>>>>>> >>>>> [snip] >>>>>>> The base voltage needs to be +1 jd for the thing to oscillate at >>>>>>> equilibrium. The clamp device, c-b junction or schottky, has about the >>>>>>> same tc, but in the opposite direction. In the c-b case (with a low >>>>>>> secondary swing, not some awful class C thrashing, and a big base cap, >>>>>>> so it's not a blocking oscillator) the collector swing that pulls >>>>>>> current out of the base capacitor touches zero, near where the >>>>>>> transistor saturates. In fact, the tc of the c-b junction is canceling >>>>>>> the tc of the e-b junction. Not perfectly, because they are >>>>>>> effectively used at different currents. Because of reverse beta, some >>>>>>> of the tank energy can be clamped through the emitter, which is a bit >>>>>>> negative at that instant. But even that current flow happens when the >>>>>>> c-b junction is forward biased. >>>>>>> >>>>>>> Schottky: suppose we need +0.6 on the base for stable oscillation. The >>>>>>> collector will dip down to, say, 0.3, the schottky will conduct and >>>>>>> pull charge out of the base cap, and it will stabilize there. The tc's >>>>>>> almost cancel. If you assume the amplitude limiting is just brute >>>>>>> clamping of the swing, the tc's still cancel. No saturation, since the >>>>>>> c-b junction doesn't get forward biased. This assumes you've chosen a >>>>>>> suitable base resistor, not jamming too much current into the base, >>>>>>> and the transistor has reasonable beta. >>>>>>> >>>>>>> Actually, the base AGC thing must be happening. With small feedback >>>>>>> voltage into the emitter, a sim shows that the transistor is on >>>>>>> throughout the cycle, "class A." So Ic ~= beta * Ib, on average. >>>>>>> Average Ic would be large if the current through the base resistor >>>>>>> were all actually going into the base. But it's not: the base cap is >>>>>>> being discharged at the negative swing of the collector, stealing base >>>>>>> current and reducing transconductance, and that is exquisitely >>>>>>> sensitive to p-p amplitude. Some tank energy does of course get lost >>>>>>> to the emitter... both mechanisms are at work. Fortunately, both >>>>>>> stabilize the amplitude. >>>>>>> >>>>>>> In my sim, if you short the tank, the supply current goes up about >>>>>>> 30x. (Hmmm, Rb could be bigger!) Output swing is 10.11 volts p-p with >>>>>>> a 5-volt supply. >>>>>>> >>>>>>> Several people have modeled this circuit with small base caps and low >>>>>>> transformer ratios and lots of base bias current. That works >>>>>>> differently. >>>>>>> >>>>>>> John >>>>>> Interesting, thanks. I agree that the Baker clamp pretty well fixes the >>>>>> saturation problem. >>>>>> >>>>>> Cheers >>>>>> >>>>>> Phil Hobbs >>>>> Phil, Don't you have means to simulate Larkin's oscillator? >>>>> >>>>> If you bothered to do that you'd find some of Larkin's statements >>>>> above are _absolutely_ incorrect, and others are so hand-waving as to >>>>> be hilarious. >>>>> >>>>> But, hey! Ignorant young bucks with no knowledge of basics are good >>>>> for business :-) >>>>> >>>>> ...Jim Thompson >>>> Of course I have. >>>> >>>> I'm hip deep in measuring very small nonlinearities in InGaAs >>>> photodiodes just now, >>> How do you do that? Summing on/off light sources? I guess the question >>> is, how small? >>> >>> John >>> >> Low distortion sine wave + DC driving IR LED >> PD goes to TIA with x64 AC gain and x1 DC gain >> Auxiliary light source (fibre coupled laser in this case) provides CW light >> Bridge circuit (like an old HP THD meter) to null out sine wave and >> improve dynamic range >> >> Set up IR LED to give some decent AC photocurrent like 5 uA, null >> carefully, move laser in and out while changing PD reverse bias voltage. >> >> Look at residual on digital scope with FFT. >> >> This allows me to see nonlinearities down to about -70 dB. It could go >> further, but the silly laser mode hops like mad, and it takes forever to >> integrate it away. > > So you are sort of summing light sources. I guess. Exactly. Just like a two-tone tester, where you sum the tones right at the output to avoid IMD internal to the tester. > > If you measure the magnitude of the AC component of the PD signal, at > various points of added laser light, you get the slope gain at various > points, and the distortion of the LED doesn't matter. Is that the > idea? Yes. That measures the slope rather than the actual curve, which is what I'd really prefer in this instance. > > LEDs are pretty limear at higher currents, but I don't know about 70 > dB. Actually, it wouldn't be all that hard to find out. > >> Interim conclusion: InGaAs is a lot less linear than Si. The quantum >> efficiency improves by something like 5% for bias levels between 0 and 5 V. > > Does that imply amplitude nonlinearity at fixed bias? Yes, and it's worst at zero bias--maybe 2x or so, in the range I care about. Interestingly the AC output signal shows a straight-line growth with bias--effectively the quantum efficiency goes up about 5% from 0 to 5V bias. > > What about temperature? Maybe the bias voltage needs a TC correction. I hope not--this gizmo is going to have to work in a fairly harsh environment eventually. Today I'm going to do an eyeball fit to the distortion polynomial, to see if I can predict how much the output gets compressed at higher powers. It's on the order of 2% at 100 uW in a 300-um diameter detector at zero bias, which is really stinky compared with silicon. And InGaAs is the best of the infrared detector materials. Infrared is just a lot harder than visible, for a whole bunch of reasons. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: John Larkin on 18 Jun 2010 11:17 On Fri, 18 Jun 2010 10:00:30 -0400, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >Yes. That measures the slope rather than the actual curve, which is >what I'd really prefer in this instance. How about using N led's, minimum two. Turn them on and off in all possible states, measure PD current with a very good meter, and analyze the results. See how well things sum that ought to sum. You could also trim the individual LED currents. It would be reasonable to do a series of experiments that would precisely adjust the LED light outputs to be equal, or to be precise 2:1 (optical binary DAC) steps. Then you can play with combinations. LED self-heating would be the biggest error, so something clever would need to be done about that. That's the sort of thing that would be fun to do as a product, if you thought that anybody would buy it. >> >> LEDs are pretty limear at higher currents, but I don't know about 70 >> dB. Actually, it wouldn't be all that hard to find out. >> >>> Interim conclusion: InGaAs is a lot less linear than Si. The quantum >>> efficiency improves by something like 5% for bias levels between 0 and 5 V. Knowing nothing about semiconductors doesn't keep me from speculating. Maybe the high defect density causes recombinations at low drift velocities. At any rate, I should keep my PD power supplies stiff. John
From: Phil Hobbs on 18 Jun 2010 14:27
John Larkin wrote: > On Fri, 18 Jun 2010 10:00:30 -0400, Phil Hobbs > <pcdhSpamMeSenseless(a)electrooptical.net> wrote: > > >> Yes. That measures the slope rather than the actual curve, which is >> what I'd really prefer in this instance. > > > How about using N led's, minimum two. > > Turn them on and off in all possible states, measure PD current with a > very good meter, and analyze the results. See how well things sum that > ought to sum. The problem is that the photodiode is only 300 um in diameter, so there's a limit to how many LEDs you can crowd in there. I only had an afternoon to design and wire up a tester, so I used what I had lying around. My ABQ customer is a great outfit, but they have zilch prototyping supplies, so I built it at home. It's so SMT here, I was pathetically grateful to find a Radio Shack a mile away. I can do dead-bug with 0805s, but anything smaller is hard. Next time I'll bring a box of parts as well as a bag of tools. (I brought my own micromanipulators.) > You could also trim the individual LED currents. It would be > reasonable to do a series of experiments that would precisely adjust > the LED light outputs to be equal, or to be precise 2:1 (optical > binary DAC) steps. Then you can play with combinations. LED > self-heating would be the biggest error, so something clever would > need to be done about that. Back in the palmy days, I built a little tester for transistor log conformity that worked like that--it had a 10-turn pot controlling the collector current, with a switching x1-x2 amp. It used an LTC1043 low charge injection analog switch, which stored Vbe at the x1 setting and subtracted it from Vbe at the x2 setting. Looking at how the difference varied with collector current for different devices was very illuminating. > That's the sort of thing that would be fun to do as a product, if you > thought that anybody would buy it. If people knew that it was a problem, they would--but they generally have no idea, and radiometric calibration is hard to do to much better than 1%. It's mind-boggling, the number of people who wire up a photodiode and expect it to be linear to all 98 bits of their shiny new delta-sigma. So it's the sort of tester that (ideally) allows you to ship other magic products, if you're sufficiently clueful. Despite my best efforts to educate folks, a great many just wave a dead chicken over their PD and TIA, and hope that it works. Then they either sweep it under the rug or redefine what they mean by 'working'. :( >>> LEDs are pretty limear at higher currents, but I don't know about 70 >>> dB. Actually, it wouldn't be all that hard to find out. >>> >>>> Interim conclusion: InGaAs is a lot less linear than Si. The quantum >>>> efficiency improves by something like 5% for bias levels between 0 and 5 V. > > Knowing nothing about semiconductors doesn't keep me from speculating. > Maybe the high defect density causes recombinations at low drift > velocities. > > At any rate, I should keep my PD power supplies stiff. Yep. But it's primarily quantum efficiency that changes with bias, not the linearity--anywhere above half a volt or so, bias doesn't make much difference to the linearity vs photocurrent, at least not down in the sub-100uA range where I'm working. The output vs bias curves are mostly parallel straight lines, with some droop at zero bias and higher photocurrent. It's apparently important to illuminate the photodiode from the P-side. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net |