Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would
be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate
that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views.
It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if
you would like to post something on RF Cafe's
page, please do.
Below are all of the forum threads, including all
the responses to the original posts.
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.