Re: Random thoughts on sinewave oscillators

"john larkin" <jl@glen--canyon.com> wrote in message news:qfo5hj5vqpi357bsads868ska3ap7k16rc@4ax.com...

On Sat, 19 Oct 2024 00:19:35 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:
>

On 10/18/24 23:19, Edward Rawde wrote:

"Cursitor Doom" <cd999666@notformail.com> wrote in message news:veuirv$3cmo3$10@dont-email.me...

On Fri, 18 Oct 2024 15:59:09 -0400, Edward Rawde wrote:
>

"Cursitor Doom" <cd999666@notformail.com> wrote in message
news:veucs2$3cmo3$9@dont-email.me...

On Fri, 18 Oct 2024 14:20:48 -0400, Edward Rawde wrote:
>

"Cursitor Doom" <cd999666@notformail.com> wrote in message
news:veu7kt$3cmo3$8@dont-email.me...

On Fri, 18 Oct 2024 13:47:02 -0400, Edward Rawde wrote:
>

"Cursitor Doom" <cd999666@notformail.com> wrote in message
news:veu45s$3cmo3$5@dont-email.me...

On Fri, 18 Oct 2024 11:25:19 -0400, Edward Rawde wrote:
>

"piglet" <erichpwagner@hotmail.com> wrote in message
news:vetde5$38sbk$1@dont-email.me...

Edward Rawde <invalid@invalid.invalid> wrote:

>

...

>
Without doubt, it's the trickiest aspect of the design. Definitely do-able
though. Let us know how you get on.
>

>
Ok. This simple circuit is based on the circuit you can find here.
>
https://electronics.stackexchange.com/questions/697687/how-to-control-the-amplitude-of-a-wien-bridge-oscillator
>
It produces a something wave.
I wouldn't call it sine but at least it's not clipping.
What's going on here?
>
Version 4

[Snip...]
>
You're hitting the flat portion of the Id vs. Vds curve around the
top of the wave. In that region the dynamic resistance of the FET
is very large, and therefore the gain of the opamp drops to about
one. As a result, the positive tip of the output gets sort-of
squashed.
>
There are probably ways to fix this, for example by feeding a
portion of the output signal to the FET gate, but a quick
attempt I made didn't work very well. This is why FETs aren't so
great as gain setting elements.
>
Using a lightbulb --or more generally a PTC resistor-- for R7 is
really hard to beat.
>
Jeroen Belleman

>
But tricky to Spice.
>
And a thermal device of course makes the amplitude temperature
sensitive, especially when the heating goes directly as the square of
the sine amplitude.
>
It would be better to have the native loop gain very close to 1.00 and
give the AGC mechanism a small influence, like  +-  a few per cent
maybe.

Attempting to reduce the gain control range of the FET seems to run into other issues which I've seen in other simulations.
The lower amplitue parts of the signal produced by this ciruit look nice and clean.
Now I just need a way to stop it generating an AM radio signal.
Manual adjustment of R8 is likely to be needed in any real circuit.

Version 4
SHEET 1 2196 916
WIRE -160 -224 -256 -224
WIRE -16 -224 -160 -224
WIRE 160 -224 -16 -224
WIRE 400 -224 224 -224
WIRE -160 -208 -160 -224
WIRE -256 -192 -256 -224
WIRE 400 -192 400 -224
WIRE -16 -144 -16 -224
WIRE -256 -96 -256 -128
WIRE -160 -96 -160 -128
WIRE -160 -96 -256 -96
WIRE -80 -96 -160 -96
WIRE -48 -96 -80 -96
WIRE 80 -96 48 -96
WIRE 112 -96 80 -96
WIRE 208 -96 192 -96
WIRE 272 -96 208 -96
WIRE 400 -96 400 -112
WIRE 400 -96 352 -96
WIRE -80 -32 -80 -96
WIRE -48 -32 -80 -32
WIRE 80 -32 80 -96
WIRE 80 -32 32 -32
WIRE -352 32 -480 32
WIRE 128 32 -352 32
WIRE 208 64 208 -96
WIRE 208 64 48 64
WIRE -480 80 -480 32
WIRE 128 96 128 32
WIRE -352 112 -352 32
WIRE 48 112 48 64
WIRE 96 112 48 112
WIRE 384 128 160 128
WIRE 400 128 400 -96
WIRE 400 128 384 128
WIRE 528 128 400 128
WIRE 560 128 528 128
WIRE 96 144 -80 144
WIRE -80 224 -80 144
WIRE 16 224 -80 224
WIRE 176 224 16 224
WIRE 256 224 240 224
WIRE 384 224 384 128
WIRE 384 224 336 224
WIRE -80 240 -80 224
WIRE 16 240 16 224
WIRE -480 320 -480 160
WIRE -480 320 -560 320
WIRE -560 336 -560 320
WIRE -480 336 -480 320
WIRE -352 336 -352 176
WIRE -352 336 -480 336
WIRE -336 336 -352 336
WIRE -256 336 -256 -96
WIRE -256 336 -336 336
WIRE -80 336 -80 320
WIRE -80 336 -256 336
WIRE 16 336 16 304
WIRE 16 336 -80 336
WIRE -480 464 -480 336
WIRE -336 496 -336 336
WIRE -480 656 -480 544
WIRE -336 656 -336 560
WIRE -336 656 -480 656
WIRE 128 656 128 160
WIRE 128 656 -336 656
FLAG 528 128 output
FLAG -560 336 0
DATAFLAG -432 32 "round(($)*100)/100"
DATAFLAG 64 -96 "round(($)*100)/100"
DATAFLAG 240 -96 "round(($)*100)/100"
DATAFLAG 64 -224 "round(($)*100)/100"
DATAFLAG 288 -224 "round(($)*100)/100"
SYMBOL voltage -480 64 R0
WINDOW 123 0 0 Left 0
WINDOW 39 10 135 Left 2
WINDOW 0 12 7 Left 2
WINDOW 3 15 104 Left 2
SYMATTR SpiceLine Rser=0.1
SYMATTR InstName V1
SYMATTR Value 12
SYMBOL res 352 208 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 12k
SYMBOL cap 240 208 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 15n
SYMBOL cap 32 304 R180
WINDOW 0 -33 54 Left 2
WINDOW 3 -49 18 Left 2
SYMATTR InstName C2
SYMATTR Value 15n
SYMBOL polcap -368 112 R0
SYMATTR InstName C4
SYMATTR Value 100�
SYMBOL OpAmps\LT1057 128 64 R0
SYMATTR InstName U2
SYMBOL res -176 -224 R0
SYMATTR InstName R6
SYMATTR Value 47k
SYMBOL res 208 -112 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R7
SYMATTR Value 4.7k
SYMBOL res 368 -112 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R8
SYMATTR Value 11500
SYMBOL diode 160 -208 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL res 384 -208 R0
SYMATTR InstName R9
SYMATTR Value 10k
SYMBOL njf 48 -144 R90
WINDOW 0 -38 17 VRight 2
WINDOW 3 -7 -47 VRight 2
SYMATTR InstName J1
SYMATTR Value 2N3819
SYMBOL voltage -480 448 R0
WINDOW 123 0 0 Left 0
WINDOW 39 10 135 Left 2
WINDOW 0 12 7 Left 2
WINDOW 3 15 104 Left 2
SYMATTR SpiceLine Rser=0.1
SYMATTR InstName V2
SYMATTR Value 12
SYMBOL polcap -352 496 R0
SYMATTR InstName C5
SYMATTR Value 100�
SYMBOL res -96 224 R0
SYMATTR InstName R2
SYMATTR Value 12k
SYMBOL polcap -240 -128 R180
WINDOW 0 -35 53 Left 2
WINDOW 3 -47 17 Left 2
SYMATTR InstName C3
SYMATTR Value 10�
SYMBOL res 48 -48 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R3
SYMATTR Value 1.5K
TEXT -464 352 Left 2 !.tran 5