Twin T circuit wanted

Everybody must know the less obscure case by now.

The more obscure case: In the toob days, sometimes TV sets would have vertical bars on the screen. It was caused by Barkhausen oscillations in the horiz output tube, getting back into the RF input. Some TV techs called that "squegging."

I have no idea of the source of the word.

John

The .wav thing is cute. But the ringdown is being caused by the LC's Q, not by the transistor oscillation. The R2-C4 thing dies out pretty soon, and then the tank rings on its own. That why you need R4; at infinite Q, it would ring forever.

If you can get an LC with a Q that high, you can simulate a nice bell noise by just whacking it and letting it ring... like a real brass bell. Much simpler.

John

One ringy dingy.

[...]

Hi John,

You don't need to add the the resistor. You can just measure the emitter current by clicking on it!

--

John Devereux

...

Works fine here. version 4.07q

--- news://freenews.netfront.net/ - complaints: snipped-for-privacy@netfront.net ---

add external negative resistance.

--- news://freenews.netfront.net/ - complaints: snipped-for-privacy@netfront.net ---

the

=3D 120mV

did that

posted

So you like red beards.?

I couldn't get that to work. Maybe my aim isn't good enough.

John

I chose the wrong extension to name the file with.

John Larkin Inscribed thus:

I've always thought (a burst of oscillations at regular intervals) probably from the sound made when a radio squegs.

--
Best Regards:
                     Baron.

On Fri, 11 Jun 2010 20:04:42 -0700, John Larkin wrote:

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Speaking of real world parts and more-or-less realistic bong frequencies, L1 here is a Caddell-Burns 5.6mH 2.9 ohm choke (P/N

7200-34), C1 is 10µF with an ESR of an ohm or less, and the thing oscillates at about 675Hz.

Version 4 SHEET 1 936 680 WIRE -864 48 -1072 48 WIRE -800 48 -864 48 WIRE -576 48 -704 48 WIRE -448 48 -576 48 WIRE -352 48 -448 48 WIRE -448 64 -448 48 WIRE -352 64 -352 48 WIRE -864 96 -864 48 WIRE -576 96 -576 48 WIRE -928 144 -976 144 WIRE -448 160 -448 128 WIRE -352 160 -352 144 WIRE -352 160 -448 160 WIRE -976 176 -976 144 WIRE -1072 192 -1072 48 WIRE -752 192 -752 112 WIRE -752 192 -864 192 WIRE -448 192 -448 160 WIRE -576 240 -576 176 WIRE -512 240 -576 240 WIRE -864 272 -864 192 WIRE -752 272 -752 192 WIRE -976 288 -976 256 WIRE -576 304 -576 240 WIRE -448 320 -448 288 WIRE -352 320 -448 320 WIRE -448 336 -448 320 WIRE -352 336 -352 320 WIRE -1072 432 -1072 272 WIRE -976 432 -976 368 WIRE -976 432 -1072 432 WIRE -864 432 -864 352 WIRE -864 432 -976 432 WIRE -752 432 -752 336 WIRE -752 432 -864 432 WIRE -576 432 -576 368 WIRE -576 432 -752 432 WIRE -448 432 -448 416 WIRE -448 432 -576 432 WIRE -352 432 -352 416 WIRE -352 432 -448 432 WIRE -1072 528 -1072 432 FLAG -1072 528 0 SYMBOL ind2 -368 48 R0 SYMATTR InstName L1 SYMATTR Value 5.6e-3 SYMATTR Type ind SYMATTR SpiceLine Rser=2.9 SYMBOL ind2 -368 432 M180 WINDOW 0 36 80 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName L2 SYMATTR Value 500e-6 SYMATTR Type ind SYMATTR SpiceLine Rser=.1 SYMBOL npn -512 192 R0 SYMATTR InstName Q1 SYMATTR Value 2N3904 SYMBOL res -592 80 R0 SYMATTR InstName R1 SYMATTR Value 100k SYMBOL cap -464 64 R0 SYMATTR InstName C1 SYMATTR Value 10e-6 SYMATTR SpiceLine Rser=1 SYMBOL cap -592 304 R0 SYMATTR InstName C2 SYMATTR Value 1e-7 SYMBOL voltage -1072 176 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 3 24 104 Invisible 0 SYMATTR Value 5 SYMATTR InstName V1 SYMBOL pnp -928 192 M180 SYMATTR InstName Q2 SYMATTR Value 2N4403 SYMBOL voltage -976 272 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 3 24 104 Invisible 0 SYMATTR Value PULSE(5 0 0 1e-6 1e-6 .01) SYMATTR InstName V3 SYMBOL npn -800 112 R270 SYMATTR InstName Q3 SYMATTR Value 2N3904 SYMBOL cap -768 272 R0 SYMATTR InstName C3 SYMATTR Value 7.5e-6 SYMBOL res -880 256 R0 SYMATTR InstName R2 SYMATTR Value 2e6 SYMBOL res -992 160 R0 SYMATTR InstName R3 SYMATTR Value 1000 SYMBOL res -464 320 R0 SYMATTR InstName R4 SYMATTR Value 1000 TEXT -432 184 Left 0 !K1 L1 L2 1 TEXT -1048 464 Left 0 !.tran 2.5 uic TEXT -1048 496 Left 0 !.wave .\\bong.wav 8 10000 N008

JF

Yes, those changes to C2 and L2 cut the p-p signal at emitter from about

525 mV to 50, but the currents at c, b, and e change only a little - eg peak currents (mA) Ib 1.22 vs 1.20, Ic -5.14 vs -5.34, Ie 3.91 vs 4.15.

The FFT with YD's C2, L2 values is fairly clean, but with the C2, L2 that you suggest above, there are a lot of signal components only

20dB down from the 1.581kHz fundamental, vs at least 44dB down from the 1.578kHz fundamental in YD's.

One can click the 'Select which traces are visible' icon (just below the help button; looks like a miniature graph), or right-click in a Plot window and click 'Visible traces', and then click Ie(Q1).

Xl of 5.6 mH at 675 Hz is 23.8 ohms. So, ignoring core losses, the inductor's Q is just over 8. Your model, with the 1K across L2, has an inductor Q of about 470.

If you model with a Q of 8, it's pretty awful. And if you play with the base bias resistor, you can make something that looks fairly chaotic, sort of ratty like a superregen.

The operating mode I originally described is pretty close to class A. Most people who have modeled this have used small base caps and huge amounts of drive into the emitter, heavy class C stuff. That's probably what JT is so panty-bunched about.

ftp://jjlarkin.lmi.net/LC_YDx.gif

I suppose the emitter could be grounded, with the feedback winding in the base. The AGC effect would be similar, and the operating Q would be better. I think.

John

Late at night, by candle light, John Larkin penned this immortal opus:

Just hover the pointer over the terminal until it looks like a clampmeter. One of those things I found quite by accident, and nicely surprised.

- YD.

--
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1 - Call the cops
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Remove HAT if replying by mail.

I caught it right away. I'm a 'prober' by nature.

I know that, but I had a hard time hitting the emitter current. The resistor is a bigger target. Maybe I should buy more pixels or something.

John

Oh, I see the 2.9 ohms in the box off to the side. I must be pulling the Q down to about 4. But even at 8, the oscillation is loaded pretty bad. It's hard to get good inductors at these frequencies.

So what's R4 for?

John

--
So, what you're saying is that because of the feedback from L2, my
model refreshes the tank irrespective of its own losses and gives it
an apparent Q of about 470?

Lucky me!

That is John, 'squegging' on the group.