Hi guys,
thanks for the replies. The root of the problem is that the only reference I have (the bandgap) is noisy. So if I design a current-source, I get a noisy current that gets up-converted. If I just use a resistor, the power supply noise appears directly across the resistor which is the same this as a noisy current source, so back to square 1.
Quote:Sometimes LDO's have a really large cap that serves as dominant pole compensation and provides filtering of the output voltage, is that not an available option in this case?
the problem is it would have to be a
really large cap to be effective. The noise performance is critical just outside my loop bandwidth (around 4 - 10 MHz). Beyond 10 MHz, I guess the noise shaping of the VCO makes the supply noise insignificant.
Quote:If you are designing with MOS, maybe you can try using a PMOS current source and PMOS differential pair. The PMOS current source should provide you some supply rejection.
The question is what is my current source referenced to? The noise originates from the BG reference, so if my current source is referenced to the BG reference then the situation isn't improved.
Quote:are you talking about power supply noise or thermal noise?
Its power supply noise, but because the power supply is from an LDO referenced to the bandgap, its basically the bandgap noise. My main concern is low-frequency noise being upconverted.
Quote:If you have thermal noise then large loop filter or redesigning would help
sorry I don't understand your meaning. I can't increase the loop bandwidth because its already about 1/10x the reference frequency.
Quote:whatever supply noise you have should be appeared like common mode noise, will have less impact on phase noise.
The power supply noise is affecting my current bias because the limited headroom means my current bias has poor Rout. So the low-frequency supply noise is modulating my current bias and it then gets differentially up-converted and appears as phase noise.
Quote:Please use self bias biasing rather than a constant bias or some thing else
What do you mean by self-biasing? For instance instead of a current source, use a resistor? I have tried this but it is even worse as it is even more sensitive to the power supply.
thanks,
Aaron