VCO simulation problem

[haadi20]

vco simulation

I am using spectre RF transient and PSS for VCO simulation.

Initially, i had connected the VCO directly to an output buffer (a differential amp). The output DC voltage of VCO is used to bias the buffer transistors. I also use an initial condition to start-up the oscillation. This setup works fine for me.

However, i want to isolate the VCO from the buffer. So i use de-coupling capacitor in between them and use separate biasing for the buffer. This setup starts giving me convergence problems both in transient and PSS.

Has anyone encountered a similar problem? Any suggestion/solution is welcome..

PS. Does reducing reltol and abstol help in solving this issue ? i am trying that out right now..!!

 

[Abhishekabs]

vco simulation problem

I think may be some tunning might be required.
Have u placed equivalent components of DC blocking capacitor?
I have used ADS not this software(spectra RF). but when i get this problem i tune my circuit.
It works.

 

[haadi20]

why coupling capacitor in vco

By tuning...do you mean tuning of the VCO by changing the varactor or something else?

I use an ideal capacitor with relatively large value (~1-10pF) as a DC blocking capacitor...


Regards

 

[khouly]

ads vco convergence

i think he mean tuing of the elements not the varactor

so simulation can converge

khouly

 

[haadi20]

There are so many components...its hard to modify all of them to see the effect on convergence......any other solution..??

 

[Abhishekabs]

u need to tune that component which affect the frequency most.
have u used DC blocking capacitor of correct value?
according to my knowledge buffer amplifiers have high input impedance & low output impedance.
may be there is impedance mismatch.
try matching the o/p of VCO with buffer.
Abhishekabs

 

[haadi20]

The problem was solved today. With the new setup (With De-coupling capacitors) i have to use a current pulse to start up the oscillation,
instead of mentioning an initial condition that i was using for the first setup without DC blocking capacitors...

Thanks for your suggestions...
Kind Regards