John Larkin's LC oscillator
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From: John Larkin on 9 Jun 2010 23:53 On 9 Jun 2010 19:26:52 -0700, Winfield Hill <Winfield_member(a)newsguy.com> wrote: >Jon Kirwan wrote... >> >> Another way of saying all the above is that in transformers >> the flux through the core links the windings together and >> energy storage is actually parasitic, not desired. The A_L >> is basically "not wanted" and therefore minimized, but not >----------------------------------------- ^^^^^^^^^ >----------------------------------------- maximized >> designed to some specific value. Inductors are designed for >> specific values and therefore the A_L needs to be known, not >> merely minimized, and otherwise the core is actually supposed >> to provide flux linkage to the known effective gap, not two >> windings. The 'flyback transformer' is kind of a misnomer to >> me, since in reality it is really an inductor that uses a >> known A_L or gap to store energy and also is designed to link >> the flux to the gap, except that it _also_ needs to provide >> subsequent linkage from the gap to a secondary winding, as >> well. > > Yep, one winding has a well-establish inductance, not at > all like a common transformer, but the second winding is > well coupled to it, and with low leakage inductance, just > like a good transformer. > > We really need a new name for these beasts, inductors with > transformer windings. BTW, toroid cores need not apply. > Very poor AL control. The permalloy power cores are fairly good about Al. I think I did the ship fog bell with them. Can't remember for sure, after 3.5 decades. John
From: Tim Williams on 10 Jun 2010 02:23 "George Herold" <gherold(a)teachspin.com> wrote in message news:483f8a3b-8853-441e-95d7-c8c5ae780b91(a)31g2000prc.googlegroups.com... > I never use Z5U for bypass. X7R is what I like... though some X5R > creeps in via purchasing. Z5U, and to a lesser extent X5R, are so bad you have to use almost 1uF when you wanted 0.1uF. Since X7Rs are the same size and price (almost), I get X7Rs too. :) Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Tim Williams on 10 Jun 2010 02:25 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:o8o0161be50m2hg8qmdd0lvuecjkpeqc86(a)4ax.com... >> We really need a new name for these beasts, inductors with >> transformer windings. BTW, toroid cores need not apply. >> Very poor AL control. > > The permalloy power cores are fairly good about Al. I think I did the > ship fog bell with them. Can't remember for sure, after 3.5 decades. MPPs are often specified to high tolerances. Still gotta keep 'em away from DC though. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Winfield Hill on 10 Jun 2010 16:23 dagmargoodboat(a)yahoo.com wrote... > > On Jun 8, Winfield Hill wrote: >> was Re: Twin T circuit wanted >> >> John Larkin wrote... >> ftp://jjlarkin.lmi.net/Ships_Bell.JPG >> >>> On Mon, 7 Jun 2010 George Herold wrote: >> >>>> I loved your bell circuit John! I didn't quite get how the >>>> inductor was working. But still I got the idea. Thanks. >> >>> This is kind of a cute circuit. I first designed it when I needed >>> a very frequency and amplitude-stable sine wave to drive a Talyvel >>> LVDT-like inclinometer, part of the Boresight Alignment Kit for >>> the C5A. We had to measure level to arc-seconds of accuracy. >> >>> It's a transformer with a resonant tank in the collector and a >>> positive feedback drive winding into the emitter. The emitter >>> feedback is just a couple of tenths of a volt p-p. >> >>> The cool thing is that the collector swing is almost exactly 2xVcc >>> peak-to-peak. As the amplitude builds up, at the negative swing peak >>> the emitter goes a little bit negative, to get out of the way, and >>> the collector swings to just about ground. That forward-biases the >>> c-b junction and discharges the base cap, reducing transistor base >>> current hence gain. So it has a built-in peak detecting AGC amplitude >>> leveling loop with close to zero TC. All from 5 parts. Or sometimes >>> six. >> >> With your explanation, the oscillator is much more attractive. >> Our old friend, Tony Williams, R.I.P., would have been pleased. >> >> John Larkin's LC oscillator, supply-V sets amplitude [snip in favor of the improved ASCII drawing below ] > Here's a quick implementation: > > Vcc = +5v > --+-------+------+-- > | | | > | | |_ || > | | _)|| > .-. --- L1a _)|| > Rb | | C1 --- 1mH _)|| > 100k | | 1uF | _)|| > '-' | _)||7 > | | *| || > | .-+------' || > | |/ || > +---| Q1 || > | |>. 2n3904 || > | | * || > C2 --- +---------. || > 1uF --- | L1b _)|| > | | 25uH _)|| > | | | > --+---- | -------+-- GND > | > '-------------> output > > (5KHz values shown) > > collector swing ~= 2*(Vcc+0.6v) > emitter swing = Vc * sqrt(L1b/L1a) > > 1 > f(out) = ------------------- > 2*pi*sqrt(L1a * C1) > > That's a 1rst cut -- I may have left L1b a little hot... > Simulates really nicely--sine waves. Jim pointed out elsewhere, in the old Twin-T portion of the thread, that I had misunderstood how the oscillator works. He gave a hint: not CE. Yep, of course, now it becomes a bit more clear. Thought of as an amplifier, it has to be operating in common-base mode. First, given the transformer winding, the average DC voltage on the emitter always has to be zero volts. Then, if the transformer turns ratio is high enough, high enough to force class A operation, with very little signal on the emitter, the base capacitor voltage will be at roughly Vbe. Being class A, the gain includes 1/re in its terms, and the gain has some proportionality to emitter/collector current. The LC tank amplitude and collector-voltage swing can also grow, proportional to this current. As oscillation starts, with increasing current and gain, the tank amplitude grows until the collector approaches saturation on each cycle, causing two things to happen. First, the base-collector diode conducts and robs some base current, controlling the base voltage and DC current at the right value for the class A operation we're observing. Second, this base-collector diode conduction directs some of the emitter signal current to the base, away from the LC tank, reducing the common-base gain, the tank amplitude, and stabilizing the oscillator. Operating this way, it seems fair to characterize the level control as an AGC common-base amplifier gain type of control, but you can also think of it as a type of current-steering amplitude control. A high transformer turns ratio may not be a healthy way to run this oscillator, what if it's too high, and stops? So it's likely that our happy turns ratio will be well below a class-A operating limit. If so the oscillator emitter amplitude will increase, and it'll be running in a kind of switching or class C mode. In class C the resonant tank is excited by short pulses of collector current. At the maximum emitter current, at the bottom of each tank cycle, the emitter voltage will be well below ground. The forward-biased base-collector diode will get serious about stealing collector current from the tank, when it has a chance, thereby regulating the AC RMS current and the LC tank voltage amplitude. It appears Jim wants to call this ALC, not AGC, action. The regulating function may be better thought of as signal current control than as a gain control, although the right term to use seems a bit semantic to me. Unless, of course, there's some guiding literature that we should be following. I'm wondering if it wouldn't be a good idea to have an emitter resistor to help control the currents and stretch out the pulses. Maybe John had one in his old version. In a ringing bell application, as the supply voltage sags, and the amplitude drops, I imagine the circuit will move from class C back to class A operation, before stopping. -- Thanks, - Win
From: John Fields on 10 Jun 2010 17:47
On 10 Jun 2010 13:23:55 -0700, Winfield Hill <Winfield_member(a)newsguy.com> wrote: > I'm wondering if it wouldn't be a good idea to have an > emitter resistor to help control the currents and stretch > out the pulses. Maybe John had one in his old version. --- I included one in the circuit list I posted earlier, and it has a dramatic effect on the decay time. |