Larkin's Oscillator Explained
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From: Jim Thompson on 13 Jun 2010 19:19 Larkin's Oscillator Explained ASCII drawing courtesy of dagmargoodboat(a)yahoo.com .... Vcc = +5v --+-------+------+-- | | | | | |_ || | | _)|| .-. --- L1a _)|| Rb | | C1 --- 1mH _)|| 100k | | 1uF | _)|| '-' | _)|| | | *| || | .-+------' || | |/ || +---| Q1 || | |>. 2n3904 || | | * || C2 --- +---------. || 1uF --- | L1b _)|| | | 25uH _)|| | | | === | === | '-------------> output Quiet lurker kevin93 <kevin(a)whitedigs.com> was close to the actual solution. Contact me via my web page mailer to claim your bottle of wine! It's really quite simple to understand. Most ALL _high_Q_ sinusoidal oscillators behave this way. Assume growing amplitude oscillation (not always a good assumption ;-) C-B junction of Q1 forward biases pulling voltage at top of C2 lower, pulling _voltage_ bias point of Q1 base lower. Since one end of L1b is tied to ground, there is less _conduction_angle_ to forward bias the Q1 emitter, When energy added (current) by the shrinking _conduction_angle_ at the emitter equals the losses in the LC (assume finite Q, otherwise you'll be forever confused) equilibrium is reached. (So the transistor is acting as a duty cycle _switch_, NOT as AGC.) I experimented with the effects of turns ratio here: www.analog-innovations.com/SED/Larkins_LC_OSC.pdf At high turns ratios some might claim class-A, but clearly the emitter CURRENT never becomes sinusoidal... it can't with the simple-minded low-impedance drive winding driving a non-linear bipolar junction characteristic. At low turns ratios (<5) I observe squegging due to over-dragging C2. I'd add at least one additional biasing resistor so that the feedback coil can't drive the emitter so hard (in series with the feedback coil). Perhaps an R in parallel with the bias cap so that node can't rise so high, possibly speeding up start-up. For a truly-AGC'd sinusoidal oscillator see the venerable MC1648. In the past year I was commissioned to design a similar architecture on a modern-day process. (With a fancier AGC so that tank losses could be accurately measured :-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: George Herold on 14 Jun 2010 15:12 On Jun 13, 7:19 pm, Jim Thompson <To-Email-Use-The-Envelope-I...(a)On-My- Web-Site.com> wrote: > Larkin's Oscillator Explained > > ASCII drawing courtesy of dagmargoodb...(a)yahoo.com .... > > Vcc = +5v > --+-------+------+-- > | | | > | | |_ || > | | _)|| > .-. --- L1a _)|| > Rb | | C1 --- 1mH _)|| > 100k | | 1uF | _)|| > '-' | _)|| > | | *| || > | .-+------' || > | |/ || > +---| Q1 || > | |>. 2n3904 || > | | * || > C2 --- +---------. || > 1uF --- | L1b _)|| > | | 25uH _)|| > | | | > === | === > | > '-------------> output > > Quiet lurker kevin93 <ke...(a)whitedigs.com> was close to the actual > solution. Contact me via my web page mailer to claim your bottle of > wine! > > It's really quite simple to understand. > > Most ALL _high_Q_ sinusoidal oscillators behave this way. > > Assume growing amplitude oscillation (not always a good assumption ;-) > > C-B junction of Q1 forward biases pulling voltage at top of C2 lower, > pulling _voltage_ bias point of Q1 base lower. > > Since one end of L1b is tied to ground, there is less > _conduction_angle_ to forward bias the Q1 emitter, > > When energy added (current) by the shrinking _conduction_angle_ at the > emitter equals the losses in the LC (assume finite Q, otherwise you'll > be forever confused) equilibrium is reached. (So the transistor is > acting as a duty cycle _switch_, NOT as AGC.) > > I experimented with the effects of turns ratio here: > > www.analog-innovations.com/SED/Larkins_LC_OSC.pdf > > At high turns ratios some might claim class-A, but clearly the emitter > CURRENT never becomes sinusoidal... it can't with the simple-minded > low-impedance drive winding driving a non-linear bipolar junction > characteristic. > > At low turns ratios (<5) I observe squegging due to over-dragging C2. Could someone tell me what squegging is? A google search gives hints of something like motor-boating, but different? Thanks George H. > > I'd add at least one additional biasing resistor so that the feedback > coil can't drive the emitter so hard (in series with the feedback > coil). Perhaps an R in parallel with the bias cap so that node can't > rise so high, possibly speeding up start-up. > > For a truly-AGC'd sinusoidal oscillator see the venerable MC1648. In > the past year I was commissioned to design a similar architecture on a > modern-day process. (With a fancier AGC so that tank losses could be > accurately measured :-) > > ...Jim Thompson > -- > | James E.Thompson, CTO | mens | > | Analog Innovations, Inc. | et | > | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | > | Phoenix, Arizona 85048 Skype: Contacts Only | | > | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | > | E-mail Icon athttp://www.analog-innovations.com| 1962 | > > I love to cook with wine. Sometimes I even put it in the food.
From: John Larkin on 14 Jun 2010 15:29 On Mon, 14 Jun 2010 12:12:17 -0700 (PDT), George Herold <gherold(a)teachspin.com> wrote: > >Could someone tell me what squegging is? A google search gives hints >of something like motor-boating, but different? > We're referring to an oscillator that oscillates in bursts, rather than continuously. A resonator connected to a negative resistance doesn't squegg, but on the other hand its amplitude builds to infinite (for certain values of infinite) at a rate determined by Q. You can think of an LC resonator as having a 1st order response, measuring oscillation envelope versus drive. Oscillators squegg when some amplitude limiting mechanism adds additional time delays or memory, more poles in the control loop, like the capacitor in the base of my circuit. A bigger cap reduces the tendency to squegg. A superregen receiver is a squegging-on-purpose RF oscillator. That's a fascinating circuit. John
From: VWWall on 14 Jun 2010 18:02 John Larkin wrote: > On Mon, 14 Jun 2010 12:12:17 -0700 (PDT), George Herold > <gherold(a)teachspin.com> wrote: > > >> Could someone tell me what squegging is? A google search gives hints >> of something like motor-boating, but different? >> > > We're referring to an oscillator that oscillates in bursts, rather > than continuously. A resonator connected to a negative resistance > doesn't squegg, but on the other hand its amplitude builds to infinite > (for certain values of infinite) at a rate determined by Q. > > You can think of an LC resonator as having a 1st order response, > measuring oscillation envelope versus drive. Oscillators squegg when > some amplitude limiting mechanism adds additional time delays or > memory, more poles in the control loop, like the capacitor in the base > of my circuit. A bigger cap reduces the tendency to squegg. > > A superregen receiver is a squegging-on-purpose RF oscillator. That's > a fascinating circuit. > The sguegging also lets you receive CW without a local beat oscillator. Here's a cute one: http://www.sm0vpo.com/rx/regenrx.htm I built one of these to receive FM using a couple of the proximity fuse vacuum tubes. The whole thing would fit in a match box. -- Virg Wall
From: John Larkin on 14 Jun 2010 18:48
On Mon, 14 Jun 2010 15:02:14 -0700, VWWall <vwall(a)large.invalid> wrote: >John Larkin wrote: >> On Mon, 14 Jun 2010 12:12:17 -0700 (PDT), George Herold >> <gherold(a)teachspin.com> wrote: >> >> >>> Could someone tell me what squegging is? A google search gives hints >>> of something like motor-boating, but different? >>> >> >> We're referring to an oscillator that oscillates in bursts, rather >> than continuously. A resonator connected to a negative resistance >> doesn't squegg, but on the other hand its amplitude builds to infinite >> (for certain values of infinite) at a rate determined by Q. >> >> You can think of an LC resonator as having a 1st order response, >> measuring oscillation envelope versus drive. Oscillators squegg when >> some amplitude limiting mechanism adds additional time delays or >> memory, more poles in the control loop, like the capacitor in the base >> of my circuit. A bigger cap reduces the tendency to squegg. >> >> A superregen receiver is a squegging-on-purpose RF oscillator. That's >> a fascinating circuit. >> >The sguegging also lets you receive CW without a local beat oscillator. > >Here's a cute one: > >http://www.sm0vpo.com/rx/regenrx.htm All those big R-L-C parts add the loop time lag to make it squegg. > >I built one of these to receive FM using a couple of the proximity fuse >vacuum tubes. The whole thing would fit in a match box. I've always wanted to score some prox fuze tubes. Something like 100 million were made during WWII. If you haven't seen them, here are some schematics: ftp://jjlarkin.lmi.net/Prox.zip And a good book: http://www.amazon.com/Deadly-Fuse-Secret-Weapon-World/dp/B001M5981O/ref=sr_1_2?ie=UTF8&s=books&qid=1276555470&sr=1-2 John |