Measuring input impedance and current density in Candence

[netbug]

Hello all,

I am designing a VCO running at 80 MHz. Can you help me in two very basic doubts ?

1 - Measure input impedance of the oscillator. Should I insert an AC source. Where should I have to open the loop ?

2 - Calculate the current's density of the circuit. For eg. plot a graph ID/W, that is for each value of W (1u : 10u), sweep the current (0.5 mA : 5 mA). The goal is to calculate the optimum J=ID/W.

Best regards all,

Pedro Cardoso

 

[erikl]

Re: Measuring input impedance and current density in Candenc

1 - Measure input impedance of the oscillator. Should I insert an AC source.

Yes.

Where should I have to open the loop ?

Between the control voltage (e.g. PD output) and VCO control input.

2 - Calculate the current's density of the circuit. For eg. plot a graph ID/W, that is for each value of W (1u : 10u), sweep the current (0.5 mA : 5 mA). The goal is to calculate the optimum J=ID/W.

What's your criterion for optimum? Minimum J ? Then you'd get 0.5 mA/10µm , which doesn't make much sense. I think the optimum is dictated by the economic lifetime of the chip, mainly depending on the temperature-dependent electromigration of the metal wires. Depends (a little bit) on the used process: kind of metal alloy and its thickness (height), and usually lies in the order of 1mA/µm for a max. chip operating temperature of 125°C.

 

[netbug]

Re: Measuring input impedance and current density in Candenc

Hi thanks for your reply.

At this point, I have no PLL designed, only a stand-alone oscillator. What I meant was, where to open the loop on the oscillator ?

The second question is related to current's density and how it varies according to MOSFET's dimensions and current variations. I just wanted to plot a graph to see how current varies with W.

Many thanks.

Best regards,

Pedro