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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Final final 1kHz oscillator
Date: Sun, 24 Nov 2024 16:41:04 +1100
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On 24/11/2024 5:11 am, Edward Rawde wrote:
> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vhs1ea$1kbt4$1@dont-email.me...
>> On 23/11/2024 4:12 pm, Edward Rawde wrote:
>>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vhrma8$1io30$2@dont-email.me...
>>>> On 23/11/2024 3:32 am, Edward Rawde wrote:
>>>>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vhp713$12bnt$2@dont-email.me...
>>>>>> On 21/11/2024 1:00 am, Bill Sloman wrote:
>>>>>>> On 20/11/2024 2:03 pm, Edward Rawde wrote:
>>>>>>>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vhjj2v$24eu4$3@dont-email.me...
>>>>>>>>> On 20/11/2024 1:29 pm, Bill Sloman wrote:
>>>>>>>>>> On 20/11/2024 12:59 pm, Edward Rawde wrote:
>>>>>>>>>>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vhibce$1t7v2$1@dont-email.me...
>>>>>>>>>>>> On 18/11/2024 2:58 pm, Edward Rawde wrote:
>>>>>>>>>>>>> "JM" <sunaecoNoChoppedPork@gmail.com> wrote in message news:n7iijjdeqecl0kmub0bq5in0dbm60m7qam@4ax.com...
>>>>>>>>>>>>>> On Thu, 14 Nov 2024 11:14:28 -0500, "Edward Rawde"
>>>>>>>>>>>>>> <invalid@invalid.invalid> wrote:
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> "JM" <sunaecoNoChoppedPork@gmail.com> wrote in message news:t5fajjdteskfftvkf84iqsp2vc4b9ta5kj@4ax.com...
>>>>>>>>>>>>>>>> On Fri, 8 Nov 2024 15:43:41 -0500, "Edward Rawde"
>>>>>>>>>>>>>>>> <invalid@invalid.invalid> wrote:
>>>>>>>
>>>>>>> <snip>
>>>>>>>
>>>>>>>>> I've no idea why you are using the LT1994. The circuit doesn't have a common mode problem, so why are you using an op amp
>>>>>>>>> designed to deal with one?
>>>>>>>>>
>>>>>>>>> This is sci.electronics.design not sci.electronics.incremental-development.
>>>>>>>>>
>>>>>>>>> And the six diode "stabiliser string" is nuts. If you need a 3.6V reference voltage there are lots of ways to do it with
>>>>>>>>> more precision and better temperature stability.
>>>>>>>>
>>>>>>>> In that case please show a circuit with better performance.

I'm working on it. Since we don't trust the spice models to get the 
distortion exactly right, it's not a particularly well-chosen target.

>>>>>>>> It may be that some of the circuits JM posted can do better but if so then why are you using a circuit with so many
>>>>>>>> components?
>>>>>>>> My circuit has far fewer components than yours.
>>>>>>>
>>>>>>> But you don't seem to be able to tell us what they do.
>>>>>>>
>>>>>>> I think I've found my conceptual problem with my circuit. Tweaking the gain around the three-amplifier ring tweaks both
>>>>>>> amplitude
>>>>>>> and frequency - with more gain a lower frequency signal can still propagate around the ring.
>>>>>>>
>>>>>>> I've got to find a mechanism that will separate amplitude from frequency. My copy of Williams and Taylor on electronic filter
>>>>>>> design may get perused again.
>>>>>>
>>>>>> I found a simpler solution - copy the relevant arrangement in John May's circuit. It did work - after a fashion - but as I got
>>>>>> closer to getting it to a state where it could do what I wanted, the circuit got less and less willing to simulate.
>>>>>>
>>>>>> I suspect an accumulation of typo's in component values - I do try to go the through the schematic to find and purge them. But
>>>>>> the last few passes haven't shown up anything. Frustrating. My father's advice in similar situautions was to "drop it in drawer
>>>>>> for six months, then take another look". It has worked in the past.
>>>>
>>>> I found that adding a couple of 14nmH ferrite beads around the transistors and the FETs stopped the simulation dropping out
>>>> after getting stuck on a too-short time step.

Additional components in the simulation that you could probably leave 
out of a real circuit. I've found another place to put a ferrite bead 
which speeds up the simulation ever more, but it is till not all that quick.

>>>> The current version isn't simulating all that fast - I let it run over-night and the amplitude control feed back loop turned out
>>>> to have been underdamped to the point of instability - it kept on hitting the rails and overshooting back into them. The current
>>>> version - with more damping - is now on it's second millisecond.
>>>
>>> Ok well when you've got a circuit which rivals the one I posted for harmonic distortion and component count let me know.

I'm not going to be constrained by that.

>> Component count isn't all that important. If you can replace an expensive or hard to get component with a couple of cheap ones,
>> that's a win.
> 
> I've worked in plenty of places where using four times as many components as you could have used (and hence four times the cost)
> would not be seen as a win.

Four times the number of components isn't four times the cost, as I did 
point out. I got brownie points once for replacing a fifty pence 25-turn 
trimming potentiometer with a more settable trimmer with the same 
footprint at five times the cost.

What we lost on the cost of the part we more than won back in a 
half-hour drop in setting up time. The original designer should have put 
in coarse and fine pots, which would have been more parts, but still 
cheaper than my quick and dirty fix.
>> Alternative ways of doing much the same job are also interesting -
> 
> Sure. There are plenty of examples of that, such as make it a beam tetrode instead of a pentode.
> 
>> patent evasion used to be a popular sport. Once the patentable details are buried inside a programmable device it's less
>> worthwhile.
>>
>> Lawyers don't like going into technical detail - RCA won a colour TV patent case against EMI in a US Court because the judges
>> couldn't be persuaded that quadrature modulation was exactly the same thing as sine/cosine modulation.
> 
> Sony didn't care about the PAL patents. They just put a hue control on their early sets.

The EMI-RCA court battle preceded PAL versus NTSC. It was about how you 
added colour information on top of the original black and white 
brightness signal.

>>>>> I also found a simpler solution. Taking on board advice from JM and others.
>>>>>
>>>>> The circuit below does 0dB into 600 ohms and it only takes about 20 minutes to complete the simulation on my computer.
>>>>>
>>>>> When it's done, select a sample of about 1 second near the end and FFT.
>>>>> Select Use current zoom extent and Blackman-Harris window.
>>>>>
>>>>> It will say all harmonics are more than 120dB down.
>>>>>
>>>>> I'm not saying this level of performance is achievable or measurable in reality so I don't see any point simulating further.

John May claims to have observed it in reality. I trust him on that.

>>>>> The actual distortion in reality will likely be that of the op amps so choose the lowest distortion op amp you can find.
>>>>>
>>>>> Oh and if you need to know the exact function of any of the 21 components in this circuit just ask.

But you haven't actually answered when I have.

>>>>> I'd feel embarrassed to have produced a circuit using over 70 components which only claims 65dB down on harmonics.
>>>>
>>>> Of course you would. The point of producing the circuit is to find out what it can do, and change it to make it work better.
>>>> That way you get to understand what the circuit is doing and why it is doing it, which isn't your strong point.
>>>
>>> Silly me. I thought that's what I'd been doing all along.
>>
>> Perhaps.

<snip>

-- 
Bill Sloman, Sydney