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VCO simulation problem

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haadi20

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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..!!
 

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.
 

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
 

ads vco convergence

i think he mean tuing of the elements not the varactor

so simulation can converge

khouly
 

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

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
 

    H

    haadi20

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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
 

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