Re: Oscillators in LTspice

boB <boB@K7IQ.com> wrote:

On Fri, 18 Oct 2024 10:08:15 -0700, john larkin <jl@glen--canyon.com>
wrote:
 

On Fri, 18 Oct 2024 16:33:22 -0000 (UTC), Cursitor Doom
<cd999666@notformail.com> wrote:
 

Gentlemen,
 
I've never spiced an oscillator AFAICR. Do they self-start in spice
simulators (LT in particular)? IOW, did Mike Engleheart build something
into the engine which generates wideband 'background' noise, particularly
at 'power up' as it were? I'm assuming there must be some such mechanism
and if there is, it must be present by default for all schematics one
attempts to simulate and not just oscillators?

 
Oscillators in LT Spice often need to be goosed with an injected pulse
or some initial condition setting, if they are to start in a tolerable
time, or at all.
 
Or skip the initial conditions solution, or bring up the power
supplies after sim start.
 
In a multivibrator, some tiny component value tweak can make the
asymmetry to kick things off.
 
Actually, the classic astable mv usually has a genuine hangup state,
both transistors saturated.

 
The supply coming up should be enough of a goose to start the
oscillator.
 
boB
 
 
 
 

That’s actually the problem—some circuits with lockup states will work fine
on the bench and only fail in the field, due e. g. to wimpier supplies and
brownouts.

For LC and crystal oscillators, startup problems occur when the
small-signal  (class A) gain is too low. Circuits with that problem start
up fine if the turn-on transient is large and sharp enough, but not
otherwise.

In a simulation, the very small-scale nonlinearity of rounding error can
make the circuit seem to have startup problems that the real circuit
doesn’t, sort of like how you can stand an egg on end if you’re careful.
(Or sprinkle the bar with salt first, but I digress. )

t’s perfectly legit to goose an oscillator simulation as long as it’s
firmly in the small-signal limit, i.e. the goose is << kT/e, so that it’s
always class A.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics