John Larkin's LC oscillator
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From: Phil Hobbs on 17 Jun 2010 17:01 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, though, and since (unlike some folks) I have no axe to grind, there's no reason for me to do that. I just like talking about electronics. 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 17 Jun 2010 22:15 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
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 |