Series DRO: How much S11 you guys got in @ds using Osctest?

[soon]

Although I got oscillation, the mag S11 is only between 1.0 and 1.1.

How to make it higher?

Thanks.
soon

 

[gecky]

Re: Series DRO: How much S11 you guys got in @ds using Oscte

You can add feedback... inductive usually... For a common source FET, i add a MLOC at the source and tune till S11 is max at my desired osc freq...

I dunno the theory behind this but seems like the bigger s11 is the better?

Hope this works for you

 

[vfone]

Re: Series DRO: How much S11 you guys got in @ds using Oscte

One thing that can improve the magnitude of S11 (once you get the oscillation) is to use higher Q for resonant components.
Also the DC bias of the active component it will affect the magnitude.

 

[soon]

Gecky,

I added the MLOC and still got S11 only 1.1.
How much did you get? 3, 10, or 50? I just would like to have an idea. Seems the output matching also affect the S11.

One paper said too high S11 could lead to unwanted spurious.


Vfone,
I mean the S11 in the linear small signal simulation before I get osillation at HB.

Yes, the DC bias also affect the magnitude. But not too much.

Soon

 

[gecky]

Re: Series DRO: How much S11 you guys got in @ds using Oscte

Hi Soon,

there is limit to how high the S11 can go... If the MLOC dun work for you, add an inductor, say, 10nH ? For my FET, max s11 is 3+...

Theoretically speaking, all you need is S11>1 (negative resistance). The rest depends on your load design... which is, i think, the point vfone is trying to put across... a high Q load will give you gamma_load~1 so no matter how near to 1 your FET S11 is, you might get osc....

Good Luck!