Hello Community, for my Bachelorthesis i need to modify the DC1858A-Unit for different frequencies and amplitudes.For verification i want to simulate the results in LTSpice. I recreated the circuit from the schematic but cant find a model for the VTL5C10 optocoupler. So at the moment my oscillator is working but none of the AGC.
Can you help me out with this?
I have made a model of the VTL5C10. See my uploaded test circuit.
You can directly place it in your schematic or you can make a subcircuit of it.
Possible improvements: One could make the time constant dependent of the LED-current.
Supplement: You should change the formula in BVT1 to avoid any zero or negative current.
wow thank you, that was a fast reply. I made a subsystem out of the model and placed it in my circuit. It seems to be working quite well.
I have attached my model and the manual for the board.
When I change R22 on the real board it simply varys the output amplitude like shown in the manual, I don´t need to make any more adjustments. But when I change R22 in my model i get more like the modulated amplitude mentioned in the manual and I have to find a new R4 value for a steady output. Is there something more I could try for a more real world behavior?
1. When your circuit has reached steady state, the curernt through the LED is zero. This means you don't regulate the amplitude. You can monitor the LED-current by plotting I(R26). Please rotate R26 by 180° to get a positive current in the waveform viewer. This non-regulation is the reason why the amplitude changed, when you changed the resistance "pot".
Change "pot" to 200 Ohm to get 2mA LED-current for the VACTROL.
.param pot 200
The current of the VACTROL will now be 2mA.
2. Reduce the proportional gain of the PI-regulator circuit by decreasing R23 to 10kOhm. The regulator has oscillated with a resistance of 36kOhm.The stability of the control loop depends on the PI-regulator (U8, R23, C24, C25), the lowpass (R25, C26) and the time constant and gain of the VACTROL (model).
I have attached the file with my modifications.
this is working a bit better, but I still cant get the simple gain control working every time with just changing R22 like in the manual and reality. I see that decreasing R23 helps with oscillation but why didnt I need this on the real board?
I see you changed the simulation setup to .tran 0 6 0 100u startup form my .tran 6 startup and it produces different results. Can you tell me whats the difference here?the schematic says, that the "pot" should be adjusted so that the voltage over R26 is about 5V ->~10mA LED current. I cant come near that in my model.
Do you think the inconsistencies between the component models and reality causes the different behaviours?
I changed the resistance of the pot now to 232 Ohm. This change increased the control current of the Vactrol to nearly 10mA as intended.The change has also allowed to set R23 back to 36kOhm as used in the demo board. Now the simulated schematic fully agree with the schematic of the demo board and the simulation works as expected.