Behavioral voltage source implementation in Cadence Virtuoso

Dear Muhaisin,

Muhaisin said:

. I am trying to design a voltage-controlled resistor here. I can do that in LTSpice using a behavioral voltage source which allows me to write a function to generate the voltage. But, I cannot find a similar approach in Cadence. How should I implement it?

If your function relating the voltage across the resistor and the resistance is linear, the Spectre Linear Voltage Controlled Current Source (vccs) component can provide this functionality. Details on the use of the voltage-controlled current source as a voltage variable resistor may be found in the Spectre Components and Device Models reference manual at URL:

support.cadence.com/.../techpubDocViewerPage

In the version I am looking at (Product Version 20.1, October 2020) please refer to its description on page 79. Do you think this might allow you to model your voltage-controlled resistor adequately?

Shawn

There is also the bsource component in spectre too which can achieve this, but it would be more efficient to use the vccs that Shawn mentioned. 

I'd also ask why on earth you're using a version of Spectre from 2009? We've had 11 major versions of Spectre since then...

Andrew.

Hi, Shawn! Thanks for the feedback. Let me try out your suggestion and see. I cannot access the link that you provided because I don't have the information that it asks for (snapshot). Let me check with the IT department at my university to see if they can help me with it. In the meantime, I will check on internet to see how to configure vccs to use as a vcr. 

Hey, Andrew! Thanks for the suggestion. I wish I could answer your question. I am working with McGill University and this is the version that is provided by the university to conduct research in the lab. 

Dear Muhaisin,

Muhaisin said:

I cannot access the link that you provided because I don't have the information that it asks for (snapshot).

I might suggest you follow through with your department to get access to Cadence On-line Support as it contains a wealth of up-to-date information that will not be found in a conventional internet source. As a possible help in the interim, I have included some of the reference documents content below. I also included the result of querying an vccs instance which is defined as a voltage controlled resistor and whose current-voltage relationship is contained in a piecewise linear file. I hope this helps...

Shawn

spectre_circuit_sim_comp_device_models_ref_p79_sml_011421.pdf

You should also speak to your department about this. It's a bit odd asking you to do research when using software that is 11-12 years out of date.

Andrew

Hi, Andrew!

I tried using vccs as a voltage-controlled resistor as recommended. And, I checked whether it works like a resistor by implementing a voltage divider action, and it does as you can see in figure 01. But, I think it does something more too, which I cannot make sense of. I need help regarding that.

Figure 01

Please have a look at figure 02 and 03. My objective is to replace the 1 ohm resistor (White-marked) right after the VCVS with a vcr of equal value. After doing so, I intend to get the same transient response as I got for the circuit in figure 02. I replaced the 1 ohm resistor with the vcr in figure 03. The circuit of figure 02 is unstable (it's intentionally so), and for that, you can see the oscillation building up in the transient response when a current pulse is provided. But when I replace the 1 ohm resistor with 1 ohm vcr, keeping everything else the same, it doesn't do so. Why is that? Do you have any idea?

Figure 02

Figure 03

P.S: If the photos are not clear, please have a look at the photos from this link: storage link

Hi, Shawn!

I tried using vccs as a voltage-controlled resistor as recommended. And, I checked whether it works like a resistor by implementing a voltage divider action, and it does as you can see in figure 01. But, I think it does something more too, which I cannot make sense of. I need help regarding that.

Figure 01

Please have a look at figure 02 and 03. My objective is to replace the 1 ohm resistor (White-marked) right after the VCVS with a vcr of equal value. After doing so, I intend to get the same transient response as I got for the circuit in figure 02. I replaced the 1 ohm resistor with the vcr in figure 03. The circuit of figure 02 is unstable (it's intentionally so), and for that, you can see the oscillation building up in the transient response when a current pulse is provided. But when I replace the 1 ohm resistor with 1 ohm vcr, keeping everything else the same, it doesn't do so. Why is that? Do you have any idea?

Figure 02

Figure 03

P.S: If the photos are not clear, please have a look at the photos from this link: storage link

Dear  Muhaisin,

Muhaisin said:

The circuit of figure 02 is unstable (it's intentionally so), and for that, you can see the oscillation building up in the transient response when a current pulse is provided. But when I replace the 1 ohm resistor with 1 ohm vcr, keeping everything else the same, it doesn't do so. Why is that? Do you have any idea?

I will provide a couple of thoughts based on my viewing of your circuit topologies and waveforms...I do not have a full understanding of your circuits nor the analyses you are trying to run, so please excuse the comments if they are not relevant!

It appears the two examples you provided in your Figure 02 and Figure 03 have fundamentally different stimuli. In Figure 02, you are driving what appears to be a voltage controlled source with a pulse signal and in Figure 03 you are driving your voltage controlled source with an AC current source. However, the adjacent output waveforms are both shown as a function of time . With the circuit topology of Figure 03 with its AC driving source, I would not expect the responses to a transient pulse and a current source with zero value to provide the same circuit responses. Further, if you are wondering why the circuit topology in Figure 3 does not oscillate but that in Figure 02 does oscillate, you have not provided any noise source to result in an oscllation in the circuit topology of Figure 03 - hence the only potential source of noise to excite an oscillation is the numerical nioise of the simulator. Does this make sense?

Shawn

By the way, you don't have to give the same post twice in response to different parts of the thread. We'll all see it - so all that happens is that everyone following this got the post twice.

Can you please look at your input.scs (Simulation->Netlist->Display) and find the line (or lines) related to your vccs and paste them here? It would be useful to see the parameters you've used.

Also, you're using an old IC version (certainly no later than IC6.1.4) judging by the appearance of the waveform tool. Can you either use Help->About in the CIW and note down the version number (all the digits) or type getVersion(t) in the CIW - and post the version number here?

Thanks,

Andrew