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raja.cedt

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Posts: 1516
Germany

CML divider self oscilation frequency
Jan 20^th, 2015, 2:33am

Dear all,
Can any one explain how a cml divider will have self oscillation frequency, since it is a two stage circuit each stage should contribute 90 deg at oscillation frequency. But each stage can provide 90 deg phase shift at very high frequency where almost gain drops to very low(I am assuming tail node is not contributing any phase). I think I missing some thing about the cross-coupled pair, I am modelling them as -ve resistance only.

Can any one explain where i am missing/point me to useful doc

Thanks in advance,
Raj.

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: CML divider self oscilation frequency Reply #1 - Feb 6^th, 2015, 10:58am Suggestion here- please post a set of block diagrams and schematics of what you have. The question as it is right now requires too much guessing on what is going on. jerry
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: CML divider self oscilation frequency Reply #2 - Feb 6^th, 2015, 3:38pm Sorry.. Question is how can two stage cml circuit oscillates here?
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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: CML divider self oscilation frequency Reply #3 - Feb 9^th, 2015, 10:41pm the device is a cascade of amplifier - latch - amplifier - latch - back to input As the device operates in a linear manner, what is the gain and phase relationship of each stage ? At te functional level that will tell you about the oscillatory behavior of the device.
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Jeffrey987 Junior Member Offline Posts: 26	Re: CML divider self oscilation frequency Reply #4 - Mar 14^th, 2015, 2:33pm the latch has a resistance of -2/gm (like in a LC oscillator). Write down the impedance of the load (Cl//Rl//(-2/gm) and you will see when barkhausen is satisfied (Aloop = gm Load gm Load)
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: CML divider self oscilation frequency Reply #5 - Mar 16^th, 2015, 2:50am Hi jeffrey987, Thanks for your reply, in-case of RLC it is clear but here only R//(cl//-2/gm), which boils down to Cl//-ve res hence this contribute only +ve phase shift and again this will be 90 only at high frequency by that frequency gain of diff pair will very low. Correct me if I am wrong. Raj.
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