hi,
  i am designing a LC vco with 1.2v supply with nmos switching devices and pmos current source, now i juwant to know is there advantage of putting cascode rather than single one? For me there is no problem with swing.

Thanks,
Rajasekhar.

hi pancho_hideboo,
                              thanks for your reply,some one said casode will reduce phase noise but i didn't understand why it is..because cascode branch don't  effect noise. Could you please direct me any reference regading this.

Thanks,
rajasekhar.

Hi Raja,
       
     Jerry once said that Cascode will introduce much more noise than a single transistor current source, his simplest explanation being more the resistance, more it's thermal noise. So, by that logic, cascode should increase the noise pumped into the source node & hence increase phase noise, right Jerry ?

Quote:
If you want to improve you phase noise & you have room to increase your swing, do increase your swing. It should bring down phase noise...unless your vdd is highly noisy and you want better supply insensitivity than usual.
Quote:
The only way i see cascode reducing phase noise is by reducing supply sensitivity.

--
Mayank.

Hi,
    Quote:
& How do you think that ? Pls. explain.

--
mayank.

Hi,

It depends on your oscillation frequency. With high enough ones (> 50 GHz), care should be taken with the cascode device size, i.e., its total source capacitance can reduce the effective negative resistance of the whole cross-coupled transistor pair.

Hope this helps as a guideline,

DYL

Hi raja,

What I meant is the capacitance at the source of the cascode transistor. Imagine that as the bottom transistor is trying to push a current to the tank node, this capacitance will spoil the drain current from the bottom transistor, reducing the efficiency in producing negative conductance.

Hope this clarifies

DYL

If you don't use a current source, the Q of the tank is degraded. Typically the advantage of lower upconverted noise is not sufficient to overcome the Q degradation so most products out there do have the current source

you may put a resistor or even better an inductor incread of the current source. In the latter case your current will vary all over the map. I don't know how well a resistor performs as compared to using a current source.  Would welcome ideas on that. Most papers I see use a current source but you can use a resistor servoed by an amplitude control loop to set the amplitude.

Mayank,
Actually the problem is subtle
A low impedance at the common source acts to turn the rectified voltage at this node to a current which will ultimately flow easier into the tank.  
What makes up the voltage at this node?  it is the accumulation of bias current and the noise associated with it.  This just a different way of looking at a well derived phenomenon (well at least my viewpoint).

Current mirrors have Cgd and Cdb which have some value call it Cdrain.  If a resistor can be used to bias the VCO where the value of this resistor is higher than the impedance of the Cdrain at the second harmonic frequency then there should be a net improvement due to this effect.  And the effect is compounded when you consider higher harmonics where the capacitors impedance keeps reducing and a resistor remains  constant.  In addition to this the Cgd of the current source will act to lower the impedance and act to upconvert 1/f to 2f especially if the capacitor of gate bypassing cap is not sufficiently large.  

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