I wonder why it requires a noiseless input source to be specified. Any why does it only analyze the noise at one node? Given that, why do I have to probe that one node? -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
LT Spice noise analysis
Started by ●February 27, 2017
Reply by ●February 27, 20172017-02-27
Am 27.02.2017 um 20:21 schrieb John Larkin:> > > I wonder why it requires a noiseless input source to be specified. > > Any why does it only analyze the noise at one node? Given that, why do > I have to probe that one node?When the simulator knows where the input is, it can weigh all noise sources in how much they impair the input signal. A strong noise source that does not make it to the output is not interesting. You can have as many components computed as you like and you can study their contribution to the result. See < https://www.flickr.com/photos/137684711@N07/33002503592/in/dateposted-public/lightbox/ > The signal "gain" is predefined and does the obvious job. The next picture shows the preamp in statu nascendi, it is still misbehaving. :-( ( 4 pairs of IF3602, they are all individuals; I'll have to spend another few hundred ��� to get 4 vaguely similar pairs.) cheers, Gerhard
Reply by ●February 28, 20172017-02-28
"Gerhard Hoffmann" wrote in message news:ehk1t7FovliU1@mid.individual.net...>Am 27.02.2017 um 20:21 schrieb John Larkin: > > >> I wonder why it requires a noiseless input source to be specified. > >> Any why does it only analyze the noise at one node? Given that, why do >> I have to probe that one node?>When the simulator knows where the input is, it can weigh all noise >sources in how much they impair the input signal. A strong noise source >that does not make it to the output is not interesting. >You can have as many components computed as you like and you can >study their contribution to the result.Not quite sure what you are saying here. The point of knowing the input source is to calculate the equivalent input noise, referred to that source. For something like a bandgap voltage reference, there is no input source to refer it to. Only the output noise matters. -- Kevin Aylward http://www.anasoft.co.uk/ - SuperSpice http://www.kevinaylward.co.uk/ee/index.html
Reply by ●February 28, 20172017-02-28
On 02/28/2017 04:16 PM, Kevin Aylward wrote:> "Gerhard Hoffmann" wrote in message > news:ehk1t7FovliU1@mid.individual.net... > >> Am 27.02.2017 um 20:21 schrieb John Larkin: >> >> >>> I wonder why it requires a noiseless input source to be specified. >> >>> Any why does it only analyze the noise at one node? Given that, why do >>> I have to probe that one node? > >> When the simulator knows where the input is, it can weigh all noise >> sources in how much they impair the input signal. A strong noise source >> that does not make it to the output is not interesting. >> You can have as many components computed as you like and you can >> study their contribution to the result. > > Not quite sure what you are saying here. > > The point of knowing the input source is to calculate the equivalent > input noise, referred to that source. For something like a bandgap > voltage reference, there is no input source to refer it to. Only the > output noise matters. >Seems as though on modern hardware it ought to be possible to keep N**2 partial derivatives, so that you could change inoise and onoise without re-doing the analysis every time. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●February 28, 20172017-02-28
On Tue, 28 Feb 2017 17:17:10 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 02/28/2017 04:16 PM, Kevin Aylward wrote: >> "Gerhard Hoffmann" wrote in message >> news:ehk1t7FovliU1@mid.individual.net... >> >>> Am 27.02.2017 um 20:21 schrieb John Larkin: >>> >>> >>>> I wonder why it requires a noiseless input source to be specified. >>> >>>> Any why does it only analyze the noise at one node? Given that, why do >>>> I have to probe that one node? >> >>> When the simulator knows where the input is, it can weigh all noise >>> sources in how much they impair the input signal. A strong noise source >>> that does not make it to the output is not interesting. >>> You can have as many components computed as you like and you can >>> study their contribution to the result. >> >> Not quite sure what you are saying here. >> >> The point of knowing the input source is to calculate the equivalent >> input noise, referred to that source. For something like a bandgap >> voltage reference, there is no input source to refer it to. Only the >> output noise matters. >> > >Seems as though on modern hardware it ought to be possible to keep N**2 >partial derivatives, so that you could change inoise and onoise without >re-doing the analysis every time. > >Cheers > >Phil HobbsIf you are clever with .STEP analysis you can do just that (I am clever >:-} Point of ease... Doesn't matter WHERE you place the AC SOURCE... V(ONOISE), at your chosen node, will not change V(INOISE) location is just a simulation subterfuge to calculate INPUT-REFERRED noise by calculating the gain from that node to the node you've designated for measuring V(ONOISE). For a bandgap, use VDD as an AC source with a DC component equal to +5V or whatever... measure V(ONOISE) at the bandgap output. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Thinking outside the box... producing elegant solutions.
Reply by ●February 28, 20172017-02-28
On Tue, 28 Feb 2017 17:42:27 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:>On Tue, 28 Feb 2017 17:17:10 -0500, Phil Hobbs ><pcdhSpamMeSenseless@electrooptical.net> wrote: > >>On 02/28/2017 04:16 PM, Kevin Aylward wrote: >>> "Gerhard Hoffmann" wrote in message >>> news:ehk1t7FovliU1@mid.individual.net... >>> >>>> Am 27.02.2017 um 20:21 schrieb John Larkin: >>>> >>>> >>>>> I wonder why it requires a noiseless input source to be specified. >>>> >>>>> Any why does it only analyze the noise at one node? Given that, why do >>>>> I have to probe that one node? >>> >>>> When the simulator knows where the input is, it can weigh all noise >>>> sources in how much they impair the input signal. A strong noise source >>>> that does not make it to the output is not interesting. >>>> You can have as many components computed as you like and you can >>>> study their contribution to the result. >>> >>> Not quite sure what you are saying here. >>> >>> The point of knowing the input source is to calculate the equivalent >>> input noise, referred to that source. For something like a bandgap >>> voltage reference, there is no input source to refer it to. Only the >>> output noise matters. >>> >> >>Seems as though on modern hardware it ought to be possible to keep N**2 >>partial derivatives, so that you could change inoise and onoise without >>re-doing the analysis every time. >> >>Cheers >> >>Phil Hobbs > >If you are clever with .STEP analysis you can do just that (I am >clever >:-} > >Point of ease... > >Doesn't matter WHERE you place the AC SOURCE... V(ONOISE), at your >chosen node, will not change > >V(INOISE) location is just a simulation subterfuge to calculate >INPUT-REFERRED noise by calculating the gain from that node to the >node you've designated for measuring V(ONOISE).But LT Spice doesn't calculate input referred noise. I just modified my circuit to add a new current source, dumping into a 1 ohm resistor, off to the side, connected to nothing else. And named that as the input reference device for the noise analysis. The gain from my new input device to the output is exactly zero. The output noise didn't change. The named input device does nothing. Besides, a circuit can have multiple inputs. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●March 1, 20172017-03-01
Am 01.03.2017 um 02:30 schrieb John Larkin: > > But LT Spice doesn't calculate input referred noise. Yes, it clearly does. Compare the 680u.png and 6u8.png pictures: Using only 6u8 for C1 (top center) does not kill the noise contribution of Q1 and D3 (5 times 1n4148 in series) for low frequencies. These are the gray and the white traces. And referred to the input, that clearly costs 30 pV/rtHz below 1KHz. It creates the bump in the top green line, which is v(onoise)/gain. One can also see that it does not pay to use more than 680u because of the 1/f noise of the FETs. No need to swim faster than the shark. Faster than the neighbour is enough. 680uF: < https://www.flickr.com/photos/137684711@N07/33029433402/in/dateposted-public/lightbox/ > 6.8uF: < https://www.flickr.com/photos/137684711@N07/33058288031/in/dateposted-public/lightbox/ > Without the active load and just a pull up, VCC noise was also a problem. That is modelled by R1=60 Ohm which delivers 1 nV/rtHz and this is multiplied to the desired noise density by E1. V3, just <> 0 was needed to avoid a div by 0 or whatever. Whithout the current source load, 2nV/rtHz on VCC was limiting performance. > I just modified my circuit to add a new current source, dumping into a > 1 ohm resistor, off to the side, connected to nothing else. And named > that as the input reference device for the noise analysis. The gain > from my new input device to the output is exactly zero. > > The output noise didn't change. The named input device does nothing. That would be OK if your previously used source also had a gain of 0. Outch, scaling to the input would require division by gain, which is problematic for gain = 0. > > Besides, a circuit can have multiple inputs. In the worst case, that would require multiple runs. A differential input would not. I happened to have this circuit in the simulator, it is in the middle of experimenting, not a finished work. But it was interesting to start with one BF862, identifying & eliminating the worst noise source and getting better by each iteration. Cheers, Gerhard
Reply by ●March 1, 20172017-03-01
>If you are clever with .STEP analysis you can do just that (I am >clever >:-}So you keep telling us. ;) But the simulator has to compute the exact same stuff over and over, which is a design wart. Cheers Phil Hobbs
Reply by ●March 1, 20172017-03-01
>But LT Spice doesn't calculate input referred noise.Sure it does--INOISE is just V(ONOISE)/GAIN. LTspice has some curiosities with units in its plots, so I usually express GAIN as V(onoise)/inoise so I can plot input-referred noise due to various components. Cheers Phil Hobbs
Reply by ●March 1, 20172017-03-01
On Wed, 1 Mar 2017 05:54:32 -0800 (PST), pcdhobbs@gmail.com wrote:>>If you are clever with .STEP analysis you can do just that (I am >>clever >:-} > >So you keep telling us. ;) > >But the simulator has to compute the exact same stuff over and over, which is a design wart. > >Cheers > >Phil HobbsTake it up with Berkeley... the method has never changed... all these years. It's not like "over and over" is a big deal... noise/AC analysis occurs about as fast as you can blink. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Thinking outside the box... producing elegant solutions.
