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VCO buffers design similar to LNA design ? - RF Cafe Forums
RF Cafe Forums closed its virtual doors in 2012 mainly due to other social media
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Below are the old forum threads, including responses to the original posts.
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Post subject: VCO buffers design similar to LNA design
? Posted: Thu Oct 30, 2008 2:42 am
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 ?
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.