[sdiy] LTSpice problem: simulating Irwin/Gallo linearised 2164 VCA

Tom Wiltshire tom at electricdruid.net
Thu Aug 22 17:23:47 CEST 2019

Thanks Richie. I hadn’t bothered modelling the 560p/500R networks that the 2164 demands in reality, but perhaps adding those will help here. I’ll give it a try and see what happens.


       Electric Druid
Synth & Stompbox DIY

> On 22 Aug 2019, at 14:26, rburnett at richieburnett.co.uk wrote:
> Hi Tom,
> That sort of error means that the simulator has gone down to like sub-picosecond time steps in order to try to solve the matrices that describe the nodal voltages and currents in the circuit and is still failing to come up with an acceptable solution.  This often happens with feedback loops where the simulator either struggles to determine the initial conditions of the circuit at time t=0 or it struggles to re-calculate all of the nodal voltages and currents again after a rapid switching transient.
> Things you can try:
> 1. Try adding some small resistors in places around the feedback loop.  Sometimes adding a series resistor with R=0.001 ohms will magically make the simulation run.
> 2. Try putting capacitors across the feedback resistors of op-amps to limit the bandwidth.  Otherwise the simulator might assume a very large bandwidth and therefore have to work with very small time steps in order to balance its equations.
> 3. Try using a lower slew rate for edges on signals that you generate which switch from one state to another.  If things change state more slowly it is easier for the simulator to keep up!  (i.e. Don't specify pico-second rise and fall times for a pulse generator unless you are actually simulating a properly modelled microwave circuit.)
> 4. Try to include as many parasitic R's and C's as the circuit really has.  These are what can ultimately determine the rate-of-change of signals in the otherwise ideal and boundless world of simulation.
> A similarly cryptic error in Spice used to be the dreaded convergence error, where the matrices that the simulator comes up with to describe your circuit just fail to converge mathematically on a sensible answer!
> -Richie,
> On 2019-08-22 12:59, Tom Wiltshire wrote:
>> Hi All,
>> I’m having trouble simulating the Irwin/Gallo linearised 2164 VCA. The
>> error I get is: “Analysis: Time step too small; initial timepoint:
>> trouble with node n005”
>> I don’t really understand what this is trying to tell me since the
>> .tran transient analysis command I’m using is unchanged since a
>> previous typical 2164 VCA where it worked fine.
>> I can simulate other circuits using the 2164 ok, so I’m wondering if
>> it’s something particular to that circuit, and specifically whether
>> the feedback loop needs dealing with some how differently?
>> Have other people managed to sim this circuit and if so, what did you do?
>> Any pointers appreciated.
>> Thanks,
>> Tom
>> ==================
>>       Electric Druid
>> Synth & Stompbox DIY
>> ==================
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