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LC Oscillators startup

MTP3

Hi all!

I a trying to simulate a cross coupled LC oscíllator using the components from the pdk provider (ST microelectronics) in my case. Ths issue I am facing is kind of confusing me that is the oscillator does not start oscillating if I attach a series resistor with the caps(the resistor is to model the loses in the caps, which themselves are not ideal in this case). Even if I use a 1 Ohm series resitor the oscillator would not start oscillating.

 So far I have tried the following to get around the issue without success

1) Using the different errprest as suggested by Mr. Andrew Beckett does not help.

2) I have also tried forcing initial conditions on the circuit like tying the complementary nodes to vss and vdd during initial conditions but still no.

3) Adding noise contribution from mos devices and turing on the transient noise. 

4) Injecting an ideal sine-wave of the same frequency but smaller amplitude into the osillating nodes (complementary ofcourse) as suggested by Ken Kundert in one of documents but no help.

5) Ramping up the supplies isn't helpful either.

6) Setting the Tmax to 25 times the oscillation period as suggested in some documents doesn't help either.  

Can anyone please suggest some alternate ways or a work around this issue. The inductor has a Q of 13 where as the capaciotrs have a Q of around 80, So I guess adding a 1 ohm series resistance should be that disastrous.

I am using IC6.1.5-64b.72 with MMSIM10.1. 

 
Parents

  • Hi again!

    Many thanks for the advice tkhan and Tawna. So some updates about the circuits. I have manged to startup the oscillator with some redesigning of the circuit (I basically had to increase the negative resistance contribution of the feedback transistors). I think the issue(probably) was that the loses in the varactors were not negligible.

    So , I started to other varactor structures and this is where the new issue pops up. I am testing a pmos varactor (two pmos transistors with Source=Bulk=Drain, shown at link below ). The gates are  connected to the oscillator nodes and control bit is connected to S-B-D terminal. When I try to measure the series resitance of the structure it comes out tp be very very small in the order of uOhm (even nOhm in some cases). I am not sure as to why I see this behaviour. It would be great it you could advise is my setup correct or am I doing something wrong. 

    http://www.freeimagehosting.net/igmxt 

    Regards,

    MTP3 

    P.S. The serires resistance is measured by placing a DC source and OscP and keeping the OscN at the same DC level(with ac level set to 1 at OscN) do an AC simulation and get the impedenace and then simply taking its real part should give the resistance as by definition Z=R+jXc. I must point out that the sp analysis gives similar results.

     

Reply

  • Hi again!

    Many thanks for the advice tkhan and Tawna. So some updates about the circuits. I have manged to startup the oscillator with some redesigning of the circuit (I basically had to increase the negative resistance contribution of the feedback transistors). I think the issue(probably) was that the loses in the varactors were not negligible.

    So , I started to other varactor structures and this is where the new issue pops up. I am testing a pmos varactor (two pmos transistors with Source=Bulk=Drain, shown at link below ). The gates are  connected to the oscillator nodes and control bit is connected to S-B-D terminal. When I try to measure the series resitance of the structure it comes out tp be very very small in the order of uOhm (even nOhm in some cases). I am not sure as to why I see this behaviour. It would be great it you could advise is my setup correct or am I doing something wrong. 

    http://www.freeimagehosting.net/igmxt 

    Regards,

    MTP3 

    P.S. The serires resistance is measured by placing a DC source and OscP and keeping the OscN at the same DC level(with ac level set to 1 at OscN) do an AC simulation and get the impedenace and then simply taking its real part should give the resistance as by definition Z=R+jXc. I must point out that the sp analysis gives similar results.

     

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