VCO buffers design similar to LNA design ? - RF Cafe Forums
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Post subject: VCO buffers design similar to LNA design ? Posted: Thu Oct 30, 2008 2:42 am
Joined: Thu Oct 30, 2008 2:16 am
(glad to be member of this well known place)
Having designed a 5GHz differential Pmos cross-coupled negative-gm VCO, I checked the output impedance with a sp analysis (assuming no buffers are connected), it has a real and imaginary part. THEREFORE:
- Should we arrange a matching network (just like in LNA design) at the input of the buffer ?
- In case a reduction of the output amplitude of the VCO is desired, can't we make an impedance MISmatch instead of passive scaling in the VCO core ? in that case, the reflected wave could disturb the VCO right ?
Since noise is also a concern (since phase noise is attempted to be minimize), a VCO buffer start looking pretty much like an LNA (with lower gain thought)
Your comment about these unclear issues are most welcomed. Thanks.
Post subject: Posted: Mon Dec 15, 2008 11:36 am
Joined: Thu Oct 19, 2006 6:02 pm
If the loading output of the VCO significantly changes the VCO, do not do conjugate match. You reduce the signal in the loop and degrade the phase noise that you worked so hard for. The consequence of lightly coupling the buffer to the VCO is broad band flat noise. There is usually more room for flat noise degredation than for close in phase noise degredation.