Set units for LTspice signals

In the plot window, ctl-a to bring up the dialog, then plot “V(out)*1W/1K”.

The plot window is a bit of an _idiot savant_, especially in noise sims. It knows to combine noise sources in RSS fashion, but is too stupid to get the right units for input referred currents other than inoise.

In a TIA, one would like to plot, say, V(Q1)/gain, but in order to get the right units to plot on the same axes as inoise, you have to go V(Q1)*inoise/V(onoise).

Inelegant.

Cheers

Phil “I’ve a mind to demand a refund” Hobbs

Thanks Phil. That's a way to do it. I'd hoped there was some way to encode this in the schematics.

Jeroen Belleman

It does feel a little weird sometimes to have everything in volts or amps.

I do switcher efficiency calcs by having a BV compute input power, and another compute output power, with volts representing watts. Then another BV computes efficiency, 0 to 100 volts, representing per cent.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Phil Hobbs snipped-for-privacy@Electrooptical.net wrote: |---------------------------------------------| |"Phil “I’ve a mind to demand a refund” Hobbs"| |---------------------------------------------|

A lecturer told us that companies which were early users of Spice had complained to students responsible for Spice that Spice simulations are wrong, so students have answered that those companies got Spice for virtually gratis so they should not expect much. (S.

They could have hired them to fix it.

LTspice is pretty good at what it does, i.e. integrating sparse systems of nonlinear ODEs. Of course any application to a real circuit depends entirely on the quality of the device models and on how accurately the deck represents the actual circuit.

Strays, transmission line effects, and so on are the designer’s responsibility.

And of course board-level models all stink. But have a nice day. ;)

Cheers

Phil Hobbs

LT is indeed pretty good. Things that we simulate most always work as expected. And it has transmission lines.

I sometimes "design" with LT Spice, just poke parts here and there to see what happens. That works too.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Ssh don't say that when Bill is only hours away. We'll have another mega thread about why only PhDs should be allowed to design anything.

You don't have to get a Ph.D. to learn how to read a data sheet. John Larkin got his degree at Tulane, but Tulane can't be blamed for his unwillingness to read stuff and learn from it.

Messing about with a Spice simulation isn't much different from messing about with a real circuit. Both work a whole lot better if you have some idea what you are doing. At least one of the Spice simulations that you have posted here worked rather better when John May took out four transistor and marginally better than that when I added another one.

John May clearly knew what he was doing, and I went to the trouble of explaining what I thought was going on when I posted my Spice net list.

Getting a Ph.D. is basically doing a bit of scientific research and writing it up clearly enough that you could get it published (not that I ever got around to that), so it should leave you equipped to explain what you are doing in terms clear enough that other people can follow.

You certainly don't have to get a Ph.D. to acquire that particular skill, but if you have got it, getting a Ph.D. can be way of getting more money out of it.

That must be the 128735th time you've mentioned that Bill.

In any case I now have a circuit which does 150dB in a simulation and it isn't anything like the one you mention above. What it does in reality is, of course, anybody's guess and better than 90dB would surprise me. The startup characteristic should give you an orgasm because it has no overshoot, undershoot or any kind of shoot at all. No integrators or damping resistors are needed.

Probably not. It was certainly an interesting incident. You had a good idea - even if you couldn't realise it all that well - and that was unexpected.

Lots of thing surprise you.

Your conception of what's going on is unlikely to be realistic.

"Bill Sloman" snipped-for-privacy@ieee.org wrote in message news:110o3uj$4j3m$ snipped-for-privacy@dont-email.me...

I don't know your exact definition of "Lots" but please give as many examples as you can of things which surprise me.

Ah well in that case perhaps you could explain it to me.

I make no claim at all about the performance of the following circuit except that an FFT with a Blackman Harris window on a small sample near the end of the simulation measures 150dB down on harmonics. It is likely that there are reasons why a real circuit would not be able to do this but I'm not about to spend money on test equipment which can come anywhere close to measuring it.

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0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 10k SYMBOL res -256 384 R0 SYMATTR InstName R1 SYMATTR Value 510 SYMBOL diode -48 -240 R0 SYMATTR InstName D1 SYMATTR Value BAV99HY SYMBOL diode 112 -176 M180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D2 SYMATTR Value BAV99HY SYMBOL res -80 -288 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 47k SYMBOL res 320 -288 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R8 SYMATTR Value 47k SYMBOL res 848 496 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R9 SYMATTR Value 10k SYMBOL res -48 384 R0 SYMATTR InstName R10 SYMATTR Value 40k SYMBOL cap 752 96 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C2 SYMATTR Value 10n SYMATTR SpiceLine V=25 Irms=0 Rser=0.0262 Lser=534p mfg="Würth Elektronik" pn="885012008030 WCAP-CSGP 1206" type="NP0" SYMBOL cap 320 96 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR 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A sub-PPM distortion sine generator wouldn't be difficult, if you had some way to measure it.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

It's simulating slowly as I type, and the amplitude is still creeping up. It's not obvious what's going to limit it. It seems to be diode clipping at D1 and D2, which isn't going to produce a temperature stable or all that predictable output amplitude.

It's usual in these sorts of circuits to provide a mechanism that controls the amplitude at a well-defined and predictable level. I've put three terminal references into my circuits to let me do this. This does involve feedback, and stabilising a negative feedback loop to give a dead-beat response isn't rocket science. You seem to be congratulating yourself on having avoided doing any of this, but self-congratulation is one of your defining characteristics.

I could try, but I don't seem to be able to dumb down my arguments far enough to make them comprehensible for you.

Tailoring the explanation to the audience's capacity to understand it is a skill. I can dumb down stuff enough to let managers and junior engineers understand what I am saying, but I've had less practice with less-well-informed audiences. My wife wasn't all that well-informed about electronics, but she was remarkably clever (FRS, foreign member of the US academy of sciences) and probably not an ideal practice object.

The distortion consists of higher harmonics of the basic sine wave - not all that much higher. Phase sensitive detectors can pick out quite low signal levels at specific - known - frequencies in the presence of much higher amplitude signals at different frequencies.

Edward Rawde doesn't seem to have the kind of skill set needed, but that doesn't seem to inhibit his efforts.

Making the measuring gear would be as big a project as making the low-distortion oscillator, but not all that more difficult.

If you really do want to be 150dB down and measure it then you can afford to keep your diodes at constant temperature and adjust R1 for correct output level when everything is at a stable temperature.

Tell us how to measure 1 PPM distortion on an audio-range signal, and I'll tell you how to build the sub-PPM variable-frequency source.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

There are lots of other ways of doing it, and most of them are a whole lot more elegant.

Most of the circuits you have been parodying use a non-linear element to let you set up a controllable close-to-linear gain. You rectify the output to determine it's amplitude, compare that DC output with a reliable reference DC voltage, and use feedback to control the gain to get your circuit to run at a stable output. You have chosen to go for minimal clipping to control the output amplitude, which is a simpler approach, but not all that easy to get to work in practice.

I have spelled this out before, but you don't seem to have got the message.

I just did. If you could have understood the advice, your claim to be able to offer advice on building the sub-ppm distortion audio generator might be plausible.

None of the low distortion sine wave sources that have been offered here were designed to offer much variation in output frequency. Ganged Beckman ten-turn pots might get you a decade, but covering much of a frequency range does make the project more difficult.