The polyphase schematic is like this: [img][/img]

I don't know why the Q branch is heavily distorted. Can anybody tell me the reason ? Thank you very much.


James

I think you must have loaded the vco incorrectly... perhaps you should look at the input signal to make sure it looks ok.

Hi, ACWWong and aaron_do, thanks for your replies.

to ACWWong: I checked the vco output , it's not a perfect sine, but it's clean, even just before the polyphase filter, the input is still clean.

to aaron_do: could you please explain "if you connect the input as ++ -- instead of +gnd - gnd you can get a cleaner signal and the output dc level will be the same" in details ? dc level are not the same is also another question, thanks for reminding me.

Thank you very much.

hi, I just checked aaron_do's idea, now the output of polyphase is like this : [img][/img]

I think maybe I need a buffer amplifier before the polyphase, even the input looks ok, it might still load the vco output. But another thing is why it only load one branch ? the Vi_Q branch is distorted , but vi_I branch looks ok, why?

Can anybody give me some suggestion about how to design a buffer amplifier?


Best Regards
james

have you tried just driving your initial polyphase circuit with an ideal sine voltage source and no loading?

If the outputs are not nice quadrature sine waves (after a short settle time) then there must be something wrong with the R and C components you are using. At 1.9GHz your parasitic capacitance of the large resistors could come into play, as would your capacitors if they are MOS based. Perhaps try using analogLib res and cap first to make sure.

Also when designing two stage polyphase on chip, its often the case that the two stage's frequency are offset slightly to improve IQ amplitude imbalance in the presense of process variation.

Hi,

sorry i couldn't get back to you before but you connected it like i suggested anyway...

I noticed that you said Q is distorted but I is ok....actually both of them look distorted to me. Following that fact, are you positive the input signal is not distorted?

BTW you can do an AC analysis on the poly-phase filter to see if the response is ok. Input two Vsin of opposite phase and look at the 4 outputs. That way you can also see the usable bandwidth of the PPF.

Aaron

P.S if the input looks ok then you don't need a buffer...

curve6