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vcvs Impedance Dips in Frequency

ChrisDG

Hello,

I am trying to create a near ideal voltage buffer using the components of analogLib. I have tried using vcvs and other controlled sources. All these perform the function correctly but always have sharp frequency spikes and dips in the input and output impedances. I can understand if the spikes are insignificant but some of these come to around 90% of the maximum impedance value so when placed in actual circuits it affects the performance at these frequency notches. I have tried using feedback Rs and Cs but they don't seem to help.

Attached below is one example of vcvs under 2port SP analysis where Z11 is shown.

The port settings are:

Port1: Resistance=50ohm; SourceType=Sine; PortNum=1

Port2: Resistance=50ohm; SourceType=DC; PortNum=2

Simulator: Spectre
SP Analysis: 10M 10GHz ; Single-Ended

Wondered if anyone could recommend a solution.

Thanks.

Chris

Parents
  • ChrisDG
    Hi Andrew,

    Thanks for getting back so quickly (despite being stuck at the airport).

    Since you thought it might be to do with impedance values, I have tried varying some of the C and L values and found the culprit to be C1 (decoupling cap of port1). Setting this value to 10uF I get exactly the same graph as yours above. I can change any other impedance (aside from C1) and still get the same (clean) Z11 but increasing C1 slowly brings back the artefacts and setting C1=1mF or higher is enough to recreate my Z11 (or zpm('sp 1 1) ) graph in the original post.

    So this is great as it means I have a simple fix in all my test-benches now and 10uF decoupling is more than enough for the frequencies I am working with.

    Unfortunately we haven't migrated our server to the latest tools yet but I will put in a reminder with the IT department and see where it goes. If you can think of any other settings in my current version of Spectre that might help then please let me know and I can try them out. Otherwise consider this case closed.

    Many thanks for all your help with this.

    Best Regards,
    Chris.
Reply
  • ChrisDG
    Hi Andrew,

    Thanks for getting back so quickly (despite being stuck at the airport).

    Since you thought it might be to do with impedance values, I have tried varying some of the C and L values and found the culprit to be C1 (decoupling cap of port1). Setting this value to 10uF I get exactly the same graph as yours above. I can change any other impedance (aside from C1) and still get the same (clean) Z11 but increasing C1 slowly brings back the artefacts and setting C1=1mF or higher is enough to recreate my Z11 (or zpm('sp 1 1) ) graph in the original post.

    So this is great as it means I have a simple fix in all my test-benches now and 10uF decoupling is more than enough for the frequencies I am working with.

    Unfortunately we haven't migrated our server to the latest tools yet but I will put in a reminder with the IT department and see where it goes. If you can think of any other settings in my current version of Spectre that might help then please let me know and I can try them out. Otherwise consider this case closed.

    Many thanks for all your help with this.

    Best Regards,
    Chris.
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