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how to measure the negative resistance of CB topology of VCO

 
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slienteyes



Joined: 17 Mar 2006
Posts: 18
Post22 Feb 2008 6:24   how to measure the negative resistance of CB topology of VCO
How to measure the negative resistance of CB topology of VCO by simulation of ADS?

I simulate for oscillation condition for my common base topology VCO in ADS with osctest. I don't know what the zo in osctest means. normally zo in osctest should be 1.1ohms. why? actually osctest is to get the S11, but why should say S11 from osctest is loopgain? Why S11 so called loop gain in nyquist plot should have magnitude larger than 1 and phase around 0 so as to have the potential to oscillate?

How can I get the negative resistance value from this ADS simulation?

My VCO schematic:


My simulation results:


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GGAPBE96



Joined: 21 Oct 2007
Posts: 15
Post19 Jul 2008 17:03   Re: how to measure the negative resistance of CB topology of
u can get input impedance by sparameter(1port) or ac simulation(a current source is required).
Negative resistance is calculated from real(Z(1,1))..

I think osctest only shows relative tendency.
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psmon



Joined: 04 Jun 2007
Posts: 108
Helped: 15
Post20 Jul 2008 18:11   Re: how to measure the negative resistance of CB topology of
Looking at Y-parameters is easier.
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khouly



Joined: 20 Oct 2003
Posts: 2421
Helped: 331
Location: EGYPT
Post20 Jul 2008 23:16   how to measure the negative resistance of CB topology of VCO
simply remove the resonator and replace it with S paramter port , and measure the impedance don't forget to use coupling cap so the 5o ohm port doesn't affect the bias point of the CB

khouly
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Abhishekabs



Joined: 12 Jan 2007
Posts: 438
Helped: 54
Location: India
Post22 Jul 2008 8:54   how to measure the negative resistance of CB topology of VCO
Follow what Khouly has said then
Also There is a Impedance measuring option in ADS on left side menu in S parameter list.
Place "Zin" Tool in schematic.
Then simulate.
When the output window opens.
add rectangular graph-> select zin->select real part
and say ok
you will see the graph freq vs impedance
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