How to design a negative-gm oscillator?

[Julian18]

negative gm oscillator

Hi, there
I am newbie in designing a oscillator, but now inevitably involved in a VCO project, I am building a negative -gm oscillator, which is shown below in the figure, but when I simulate this using Transient Analysis(Spectre), circuit simply doesn't oscillate, even after the initial condition is given, it just assumes it DC value, without any oscillating observed. I think maybe the start up condition is violated, but couldn't find how to get it satisfied. So I am stuck here.
Anyone can help me out there ? maybe some design equations specific to this circuit has to be satisfied involving the tail current and the gm of the differential pair. etc.
Thanks

 

[LvW]

negative gm

What is the purpose (and the value) of the damping resistor R2 in parallel to the tank circuit ?
Try to remove it and simulate again - and then start with very high values and reduce it successively.

 

[Blackuni]

negative gm circuits

Hi

As LvW suggested remove the damping R and try simulaiton.

U can also try varrying the current source!! But better try 2 meet all u r req. conditions based on design equations.


the necessary condition for this ckt to oscillate is the total gm of your active device shluld be 2-3 times greater than net losses(i.e loss in C & L & any othr devices)

**broken link removed**

Here is an nice example to start with.

if still problem is thr, help us with element details(size & process) I will check.

Thanks,

 

[yukane]

I am designing a negative Gm oscillator.
these days, I find a problem in test.
the frequency require 5s to settle.
Anyone can tell me the reason?

 

[FvM]

You may want to set an initial condition that involves a differential current in the oscillator.

 

[kokabanga]

Set the time step while doing transient to 1ps in Options and set conservative