modulator and demodulator circuit simulation on ltspice

Thread Starter

denison

Joined Oct 13, 2018
252
I have a design on ltspice for a modulator and demodulator of radio frequency signals. The multiplier was not one of the models in ltspice so I downloaded it from the analog devices website.
The balanced modulator simulation worked perfectly. However when I added the demodulator circuit I get 'time step too small'. The demodulator still has the same transmission frequency as in the modulator so this can't be the cause of the error. So the problem must be in the output of the modulator.
I have attached the ltspice schematic. I have had a lot of problems with that error and would like to know how to fix it?
 

Attachments

crutschow

Joined Mar 14, 2008
28,198
Here's the simulation.
Is that what you expected?
I changed the Integration Method (below) from the normal modified trap to Gear with the Alternate Solver which seemed to solve the "time step too small" failure.


1614574549709.png
1614573628874.png
 

Thread Starter

denison

Joined Oct 13, 2018
252
Here's the simulation.
Is that what you expected?
I changed the Integration Method (below) from the normal modified trap to Gear with the Alternate Solver which seemed to solve the "time step too small" failure.


View attachment 231718
View attachment 231716
Thanks Crutschow, that works. How did you figure it out? A lot of people get the time step error. What is different between this and 'modified trap' and the other selection for the solver?
All that is needed now is a low pass filter at the output of the demodulator to recover the original modulating frequency.
 

crutschow

Joined Mar 14, 2008
28,198
How did you figure it out? A lot of people get the time step error. What is different between this and 'modified trap' and the other selection for the solver?
I determine what to use with the old engineering trick, trial and error.
The differences are the algorithm SPICE uses (modified trapezoidal is the default Integration Method in LTspice) to arrive at a solution for the circuit matrix at each internal small time-step.
For whatever reason, sometimes a circuit will not converge with that algorithm and you get the time-step error.
Then I try various combinations of the other Integration Methods and Engine to see if that works.
Sometimes it does but other times no combination works.
In that case I will simplify the circuit into smaller chunks, to see if I can isolate the problem.

(For more info you can can try looking up Gear and Trapezoidal integration methods).
 
Top