Twin T circuit wanted
in [Design]
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From: John Fields on 10 Jun 2010 12:02 On Thu, 10 Jun 2010 08:06:23 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Thu, 10 Jun 2010 07:13:42 -0500, John Fields ><jfields(a)austininstruments.com> wrote: > >>On Wed, 09 Jun 2010 20:42:20 -0700, John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>>On 9 Jun 2010 18:54:01 -0700, Winfield Hill >>><Winfield_member(a)newsguy.com> wrote: >>> >>>>Jim Thompson wrote... >>>>> >>>>> John Larkin wrote: >>>>> >>>>> [snip] >>>>>> >>>>>> 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. >>>> >>>> What's the 6th part, I wonder? >>> >>>The peak detection can put a tiny flat on the negative swing of the >>>sine wave. A small resistor in series with the collector fixes that >>>and doesn't seem to do a lot of harm otherwise. >>> >>>> >>>>> I know John won't respond, but could someone, perhaps Win, tell me >>>>> how the "AGC" works? >>>> >>>> I'm sure he would, but why should I, over the years you've insulted >>>> me at least as much as him, and perhaps more aggressively? >>> >>> >>>But he didn't insult your wife, too. >>> >>> >>>> Anyway, he did explain it, SFAICT. >>>> >>>> Note the BJT is over-biased - plenty of base current, that if left >>>> unchecked would charge the base-to-ground capacitor and over-current >>>> the transistor. So the oscillator runs and examining cycle-by-cycle, >>>> the collector swings higher and higher until it goes negative with >>>> respect to the base voltage, close to saturating the transistor, >>>> and turning on the base-collector diode a bit, robbing current from >>>> the base capacitor. This process servos the BJT current to just the >>>> right level to sustain an oscillation collector-voltage level where >>>> just the right amount of current is robbed each cycle to control the >>>> base voltage. Thereby insuring that the collector goes close to the >>>> emitter on each cycle, establishing a tightly-controlled amplitude, >>>> which as John pointed out, is temperature independent to first order >>>> since Vce(sat) is relatively temperature independent. >>>> >>>> John said Vcc peak, but actually it must be closer to Vcc - Vce(sat). >>> >>> >>>Oh, picky picky. It's actually, probably, a bit more complex, since >>>the emitter is a little negative at the instant of collision, and the >>>transistor is almost saturating, so some of the stolen base current is >>>going into the collector and some is going into the emitter. It would >>>be interesting to simulate, just to see where the currents really go. >>>Adding the collector resistor changes things, too. >> >>--- >>So simulate it; I posted a working circuit list earlier if you can >>bear to use it without rancor getting in the way. >>--- >> >>>But the AGC thing does work, and the TC is close to zero. >> >>--- >>Serendipitous, no doubt, unless you intentionally designed it in. >> >>Did you? > > >Of course I did. As I noted, I needed a very amplitude-stable sinewave >source for the C5A inclinometer thing. This was a long time before DDS >and DAC versions were feasible. > >Can you think of other ways to make a very frequency and amplitude >stable sine wave using early-70s technology? I suppose that a square >wave generator and bandpass filter would work, but that's more parts. > >Got any ideas? --- Tuning fork oscillator.
From: Jim Thompson on 10 Jun 2010 12:07 On Thu, 10 Jun 2010 09:01:11 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: [snip] > >I think that most board-level designers use, and trust, Spice too >much. Given the cost and time for IC fab, it makes sense for IC >designers to use it to check their work before baking silicon. I don't >think that Widlar used Spice. > >John > Of course Widlar didn't use Spice. I didn't use Spice until around 1977 myself... on a VAX-11/780... and it was quite untrustworthy for a number of years :-) ...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 | The only thing bipartisan in this country is hypocrisy
From: BlindBaby on 10 Jun 2010 12:08 On Thu, 10 Jun 2010 11:02:49 -0500, John Fields <jfields(a)austininstruments.com> wrote: >On Thu, 10 Jun 2010 08:06:23 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Thu, 10 Jun 2010 07:13:42 -0500, John Fields >><jfields(a)austininstruments.com> wrote: >> >>>On Wed, 09 Jun 2010 20:42:20 -0700, John Larkin >>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>> >>>>On 9 Jun 2010 18:54:01 -0700, Winfield Hill >>>><Winfield_member(a)newsguy.com> wrote: >>>> >>>>>Jim Thompson wrote... >>>>>> >>>>>> John Larkin wrote: >>>>>> >>>>>> [snip] >>>>>>> >>>>>>> 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. >>>>> >>>>> What's the 6th part, I wonder? >>>> >>>>The peak detection can put a tiny flat on the negative swing of the >>>>sine wave. A small resistor in series with the collector fixes that >>>>and doesn't seem to do a lot of harm otherwise. >>>> >>>>> >>>>>> I know John won't respond, but could someone, perhaps Win, tell me >>>>>> how the "AGC" works? >>>>> >>>>> I'm sure he would, but why should I, over the years you've insulted >>>>> me at least as much as him, and perhaps more aggressively? >>>> >>>> >>>>But he didn't insult your wife, too. >>>> >>>> >>>>> Anyway, he did explain it, SFAICT. >>>>> >>>>> Note the BJT is over-biased - plenty of base current, that if left >>>>> unchecked would charge the base-to-ground capacitor and over-current >>>>> the transistor. So the oscillator runs and examining cycle-by-cycle, >>>>> the collector swings higher and higher until it goes negative with >>>>> respect to the base voltage, close to saturating the transistor, >>>>> and turning on the base-collector diode a bit, robbing current from >>>>> the base capacitor. This process servos the BJT current to just the >>>>> right level to sustain an oscillation collector-voltage level where >>>>> just the right amount of current is robbed each cycle to control the >>>>> base voltage. Thereby insuring that the collector goes close to the >>>>> emitter on each cycle, establishing a tightly-controlled amplitude, >>>>> which as John pointed out, is temperature independent to first order >>>>> since Vce(sat) is relatively temperature independent. >>>>> >>>>> John said Vcc peak, but actually it must be closer to Vcc - Vce(sat). >>>> >>>> >>>>Oh, picky picky. It's actually, probably, a bit more complex, since >>>>the emitter is a little negative at the instant of collision, and the >>>>transistor is almost saturating, so some of the stolen base current is >>>>going into the collector and some is going into the emitter. It would >>>>be interesting to simulate, just to see where the currents really go. >>>>Adding the collector resistor changes things, too. >>> >>>--- >>>So simulate it; I posted a working circuit list earlier if you can >>>bear to use it without rancor getting in the way. >>>--- >>> >>>>But the AGC thing does work, and the TC is close to zero. >>> >>>--- >>>Serendipitous, no doubt, unless you intentionally designed it in. >>> >>>Did you? >> >> >>Of course I did. As I noted, I needed a very amplitude-stable sinewave >>source for the C5A inclinometer thing. This was a long time before DDS >>and DAC versions were feasible. >> >>Can you think of other ways to make a very frequency and amplitude >>stable sine wave using early-70s technology? I suppose that a square >>wave generator and bandpass filter would work, but that's more parts. >> >>Got any ideas? > >--- >Tuning fork oscillator. Crystal oven? What did 'Tek' and HP use on their scopes for a reference?
From: BlindBaby on 10 Jun 2010 12:13 On Thu, 10 Jun 2010 09:01:11 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >I don't think so. People designed radar, magnetrons and klystrons and >waveguides and servos and all that, without computers. They did the >math. Early computers were obviously designed without help from >computers. But ALL modern radar and microwave innovations and technologies ARE computer model and simulation based, and could not be so without it. That includes things even as simple as waveguides, etc. Computer modeling has optimized every element of the entire spectrum of industries that are electronics related, much less the microwave niche. Phased array radar is a perfect example. Thanks for convicting yourself. You are guilty as charged, and your appeal will be denied, like you knew it would be.
From: Winfield Hill on 10 Jun 2010 12:17
Jim Thompson wrote... > > Why don't you study it again and report back on how it actually works? > > Hint: It IS a twist on a Rohde circuit, but Larkin can't cope with the > quick example I threw out... since it had a capacitor in the emitter > path. > > In fact I think _most_ LC _low_distortion_ oscillators work on the > real principle, NOT on "lowering the CE stage's gain". > > Sheeesh! Win! I was looking forward to having the "master" unhinge > some of Larkin's pomposity, but you blew it... BIG! What are you carrying on about? > Another Hint: It's NOT "CE". OK, yes now I see, it's a common-base signal, excuse me, of course. And let's see, ahem, a reduction of the signal current into the LC resonator that reduces the amplitude. But it's a slightly awkward mental picture about exactly how that happens, especially right before lunch. Now, what's this about a Rohde circuit? -- Thanks, - Win |