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How can I minimize the noise of the cross-coupled devices?

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ravet

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Hello
In complementary lc vco, how can I minimize the noise of the cross-coupled devices??, I have upper limit for the channel length of them, and I reach a limit that flicker and thermal noise are nearly contributes the same noise,,
Also, when increasing the gm , the thermal noise of the tail current source increases, I thought that the gain of the noise for the tail current source increases,but as I thought the cross coupled devices represent a a 1/gm load to the tail current source which implies that increasing the gm leads to decrease the gain seen from the tail source, and this opposes what happen as mentioned above, ,,,any suggestions??

regards
 

Re: noise in lc vco

u can add a filter after the tail current , this should minimize the noise contributed from it
khouly
 
Re: noise in lc vco

khouly said:
u can add a filter after the tail current , this should minimize the noise contributed from it
khouly
Click to expand...

thanks,, as a correction to what i mentioned above , as gm increases so, resistance seen by the thermal noise current of the tail current source decreases, and so , its contribution to the output increases,,,,,,,,,,,,,,,,,so,,, about the tail current filter, it doesnt gives the expected performance in lowering the tail noise, even after adding the coil to resonate at 2fo , the improvement in phase noise isnt that much,,, anyway,,, i design my ckt so as the noise of the tail current is nearly negligable, my problem now is the noise of the cross coupled,, any suggestions?,
 
Re: noise in lc vco

In LC Oscilator, the Qulity Factor of the inductor is related with the phase noise of the oscilator.
 

Re: noise in lc vco

isaacnewton said:
In LC Oscilator, the Qulity Factor of the inductor is related with the phase noise of the oscilator.
Click to expand...

and so???!!
 

Re: noise in lc vco

So you need to design an good on-chip inductor with high Q.

Are you using an idea inductor in your simulation? Do you need to design the on-chip inductor?
 

Re: noise in lc vco

isaacnewton said:
So you need to design an good on-chip inductor with high Q.

Are you using an idea inductor in your simulation? Do you need to design the on-chip inductor?
Click to expand...

thanks but in the PDK i use , there is a pre-designed coil, and i can optimize its design parameters to reach the highest allowable Q, so , i didnt want to enhance the PN by designing another coil!! , i want to enhance it by another way , how to limit the noise of the cross-coupled flicker and thermal without increasing the current??
 

Re: noise in lc vco

check this lecture note about phase noise in VCO
**broken link removed**

it has a good discussion on how to minimize the phase noise of oscillators

khouly
 

Re: noise in lc vco

Hey,

I beleive that to reduce the phase noise, you will have to hit a tradeoff between the power consumption and also design the tank inductor with a very high q, with most of your onchip inductors you might not be able to target it any where higher than 5-6 hence the noise is also quite high !! You can also look at the tail filter predominantly something like a cap shunt to the inductor on your tail filter to the to filter out low freq noise, sometimes you can achieve atleast a db/dec drop in phase noise...there are few papers describing this by Razavi.
 

Re: noise in lc vco

ravet said:
Hello
In complementary lc vco, how can I minimize the noise of the cross-coupled devices??, I have upper limit for the channel length of them, and I reach a limit that flicker and thermal noise are nearly contributes the same noise,,
Also, when increasing the gm , the thermal noise of the tail current source increases, I thought that the gain of the noise for the tail current source increases,but as I thought the cross coupled devices represent a a 1/gm load to the tail current source which implies that increasing the gm leads to decrease the gain seen from the tail source, and this opposes what happen as mentioned above, ,,,any suggestions??

regards
Click to expand...

Basically, I think GM should be minimized to reduce phase noise.

In order to see this problem in intuitive way, the GM reflects the strength of diff-pair. If you make them stronge, then they are going to make more noise. In theory, this could be explained by terminology "Groszkowski effect".

In short, you can make thermal noise less, by making them weaker.

For flicker noise, you should reduce capacitance on tail current source drain. That is how flicker noise in diff-pair up-converted.

I hope it helps.
 

Re: noise in lc vco

the thermal noise of the cross coupled device is directly related to the gm of the device. so in order to reduce noise contribution from the cross coupled device, you need to set gm as low as possible while it can start an oscillation. to reduce flicker noise contribution from the cross coupled device, large L and W is preferred choice in device size.
 

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