Re: High purity 1kHz oscillator

On 26/10/2024 12:15 am, JM wrote:

On Fri, 25 Oct 2024 23:03:32 +1100, Bill Sloman <bill.sloman@ieee.org> wrote:
 

On 25/10/2024 7:45 pm, JM wrote:

On Fri, 25 Oct 2024 09:25:31 +0100, JM <sunaecoNoSpam@gmail.com> wrote:
>

On Fri, 25 Oct 2024 18:16:26 +1100, Bill Sloman <bill.sloman@ieee.org> wrote:
>

On 25/10/2024 3:25 pm, Bill Sloman wrote:

On 25/10/2024 4:48 am, JM wrote:

On Thu, 24 Oct 2024 16:16:49 +0100, JM <sunaecoNoSpam@gmail.com> wrote:
>

On Wed, 23 Oct 2024 02:05:52 +0100, JM <sunaecoNoSpam@gmail.com> wrote:
>

On Tue, 22 Oct 2024 01:10:41 -0400, "Edward Rawde"
<invalid@invalid.invalid> wrote:
>

But I suspect that component tolerances and mismatched FETs will
ruin it.
>
Otherwise it should be easy to get 60dB down on unwanted harmonics
with a better filter.
>
FWIW I likely won't be here for the next week.
>

>
If you actually need a low distortion oscillator look for Viktor
Mickevic's designs on diyaudio.   I've attached a schematic.
>
PK


>
It seems that I have a problem posting encoded binariesto this group
with my provider.  Attached is a link instead.
>
https://1drv.ms/u/c/1af24d72a509cd48/EakMPPRi-pdLgaAKtJ2rrwwBNMGZZsy84MV2QoH1dPcZJQ?e=o59V9a

>
And that links to an incorrect schematic - the rectified output tap is
connected to the wrong node.  Might be other errors..

>
Even so, the third harmonic is 78dB below the fundamental. The
simulation runs slowly on my computer, so it may take me a while to get
the schematic to where the designer intended it to be.
>
The choice of op amp is what I'd expect from an audio group - it does
seem to be a low distortion part.
>
I just seem to have wrecked the .raw file from the simulation, so it may
be a while before I have more to say.

>
The file wasn't wrecked, just huge (4.063 GB) and slow to load, despite
the fact that I've a solid state disk on my computer put in to hold
LTSpice .raw files and load them tolerably quickly.
>
The circuit uses a half-wave rectifier, then runs the error signal into
an integrator wrapped around U4. C4 -at 6.8u - is a biggish integrating
capacitor. R7 - at 120k - has the main purpose of stabilising the
feeback loop controlling the amplitude and also delivers around 6mV of
1kHz sinusoidal ripple into the gate of the FET. There about 12mV of
1kHz sine wave across the FET channel so this minimises any channel
modulation. Ingenious.
>
It would have been even move impressive if he'd intended to do that from
the start, but R5 and R6 make look like it was an afterthought.
>
My feeling is that an AD734 could do better. Jim Williams did better
with his FET controlled version, but I've no idea how.
>
78dB below the fundamental is respectable, but not impressive.

>
On the bench the THD in the audio band is approx. -140dB (I think - it's years since I looked into the performance of this).  Not measured by myself, but there are hundreds of measurements documented on diyaudio and other audio forums.  Viktor used to (possibly still does) sell these oscillators on ebay for a few pounds.  I have a couple and borrowed an audio precision analyser to test them - I think the AP measured to about -115dB, and the oscillators performed better than that.  That level of distortion was much better than I required so I didn't attempt the find the true value.
>
I doubt if LTSpice will give accurate distortion figures with the simulation models I provided, they have not been verified in isolation.  Very few opamp macro models provide realistic distortion results.
>
There may be a more recent schematic available as this circuit has been tweaked over the years.
>
I think when I simulated this only the 2nd or 3rd harmonic was visable in a 1 second FFT at about -120dB.  However, I didn't spend any time on it, just enough to see that it did actually oscillate, and posted it only because it's real performance is very well documented, and it may be of use to the OP.  I could spend some time validating the models and simulating in spectre but real measurments trump simulations.

>
I still had the .raw file available.  I measure the 3rd harmonic at -118dB with a blackman-harris window with a 1s FFT.

>
Here's the .asc file that I ran
>
Version 4
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FLAG -272 656 vee
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FLAG 192 208 vee
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FLAG -576 336 vee
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FLAG -1200 160 vcc
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SYMBOL OpAmps\opamp2 192 112 R0
WINDOW 3 11 165 Left 2
SYMATTR InstName U1
SYMATTR Value LME49710
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WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 15
SYMBOL voltage -272 560 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 15
SYMBOL res 208 32 R90
WINDOW 0 -18 8 VBottom 2
WINDOW 3 15 10 VTop 2
SYMATTR InstName R1
SYMATTR Value 16K
SYMBOL res 64 160 R90
WINDOW 0 -15 12 VBottom 2
WINDOW 3 19 7 VTop 2
SYMATTR InstName R2
SYMATTR Value 8K
SYMBOL res 368 176 R90
WINDOW 0 -16 11 VBottom 2
WINDOW 3 21 4 VTop 2
SYMATTR InstName R3
SYMATTR Value 600
SYMBOL cap 240 -96 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 10n
SYMBOL cap -16 144 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C2
SYMATTR Value 20n
SYMBOL OpAmps\opamp2 -208 96 R0
WINDOW 3 6 168 Left 2
SYMATTR InstName U2
SYMATTR Value LME49710
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WINDOW 0 -14 7 VBottom 2
WINDOW 3 21 6 VTop 2
SYMATTR InstName R4
SYMATTR Value 10.02K
SYMBOL cap -160 -96 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
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WINDOW 0 -15 10 VBottom 2
WINDOW 3 19 7 VTop 2
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SYMATTR Value 2.2K
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SYMBOL diode -528 656 R90
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SYMATTR Value 15K
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WINDOW 0 -15 14 VBottom 2
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SYMATTR Value 15K
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WINDOW 3 24 0 Left 2
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SYMATTR Value 1µ
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WINDOW 3 19 11 VTop 2
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SYMBOL cap -784 688 M0
SYMATTR InstName C8
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SYMBOL res -1248 352 R0
SYMATTR InstName R21
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SYMBOL pnp -1024 352 M180
WINDOW 0 18 -8 Left 2
WINDOW 3 28 48 Left 2
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SYMBOL res -944 608 M0
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SYMATTR InstName U4
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SYMBOL OpAmps\opamp2 -864 -32 M0
SYMATTR InstName U5
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TEXT -1128 864 Left 2 !.MODEL MMBF4391 NJF VTO=-4.6 BETA=0.02779
LAMBDA=0.00595 RD=1 RS=1 IS=1e-14 CGD=14p CGS=10.5p PB=1 B=1 KF=1e-18
AF=1 FC=0.5 mfg=Motorola
TEXT -1248 600 Right 2 !.tran 0 5 4.9 1e-6 startup
TEXT -1464 656 Left 2 !.lib LME49710.lib
TEXT -1464 696 Left 2 !.lib TL072.lib
>
I had to move all the resistors to get them where they were clearly
intended to be, and if Edward Rawde's experience is any guide you will
have to move them back. I haven't included the two .lib files from your
zipped folder. Why you needed to include a library for the TL072 escapes
me - it's a jelly-bean part.
>
Maybe one of the connections got messed up in the process - you did say
that one of them was misplaced.
>
I am running LTSpice 17 (XVII) and it was updated recently.
>
My FFT on V(out) had the third harmonic -78dB below the fundamental, not
-118dB.
>
Checking again over the last one 1 second and the last 10 seconds, it's
only -46dB, which is very odd.
>
Viktor Mickevic's design does look pretty good, so I suspect that
LTSpice 17 might not be performing as well as it should.

 Yes, the resistors have an offset - one of us must have a non standard symbol.

My resistor symbol  is the normal rectangular block. I don't recall doing anything to select it,

The Q1 emitter/r14 node should connect to the D3 cathode (which is the schematic error I mentioned) so that may account for the different result (I can't see any other changes that needs to be made). I used ltspice 17.1.15 - as is usual with oscillators the integration method should be trapezoidal rather than gear.

Moving the connection did make a big difference. I'm now seeing both odd and even harmonics, but 80dB below the fundamental. I collected data from 1 second rather than 4sec, and the amplitude takes most of the extra time to settle to a stable value. I'll have more of a dig tomorrow - it's 1:20am here, and I should be in bed.
I'm using LTSpice XVII (x64) (17.0.37.0) running under Windows 7.
--
Bill Sloman, Sydney