HCT4046 VCO noise
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From: Gerhard Hoffmann on 19 Jun 2010 03:25 Bill Sloman wrote: > that the noise comes from the MOS transistors in the oscillator, > causing the random shifts in the timing between successive transitions. > > The only useful way of reducing this noise is to run the VCO faster > and use a counter to divide the output frequency down to actual > frequency that you need. > > The phase noise is still there, but it is reduced in proportion to the > divide ratio, because the voltage noise in the transistors is > unchanged but the voltage ramp that is driving the switching is > steeper in proportion to the divide ratio. With the same absolute jitter from the MOS transistors, the VCO noise sidebands will look proportionally worse at the higher frequency, and dividing down will get you where you started if you are lucky. No free lunch here. In real life you add the phase noise of the frequency divider. If you want low phase noise, you need a resonance with substantial Q. regards, Gerhard
From: Bill Sloman on 19 Jun 2010 04:50 On Jun 19, 9:25 am, Gerhard Hoffmann <use...(a)hoffmann-hochfrequenz.de> wrote: > Bill Slomanwrote: > > that the noise comes from the MOS transistors in the oscillator, > > > causing the random shifts in the timing between successive transitions. > > > > > The only useful way of reducing this noise is to run the VCO faster > > and use a counter to divide the output frequency down to actual > > frequency that you need. > > > The phase noise is still there, but it is reduced in proportion to the > > divide ratio, because the voltage noise in the transistors is > > unchanged but the voltage ramp that is driving the switching is > > steeper in proportion to the divide ratio. > > With the same absolute jitter from the MOS transistors, the VCO > noise sidebands will look proportionally worse at the higher frequency, > and dividing down will get you where you started if you are lucky. The whole point of the exercise is to reduce the absolute jitter in proportion to the divide ratio - the voltage noise in the relevant MOS transistors remains the same, but they are looking at a steeper waveform, so the absolute jitter is reduced. > No free lunch here. Wrong. > In real life you add the phase noise of the frequency divider. > If you want low phase noise, you need a resonance with substantial Q. The phase noise of the frequency divider is a lot less than the phase noise in the VCO - the transisotrs in the frequency divider are looking at much steeper-sided transistions than the transistors in the oscillator. The voltage nose level is roughly the same so the jitter is an order of magnitude or so lower. -- Bill Sloman, Nijmegen
From: Phil Hobbs on 19 Jun 2010 09:31 Gerhard Hoffmann wrote: > Bill Sloman wrote: > >> that the noise comes from the MOS transistors in the oscillator, > > causing the random shifts in the timing between successive transitions. >> >> The only useful way of reducing this noise is to run the VCO faster >> and use a counter to divide the output frequency down to actual >> frequency that you need. >> >> The phase noise is still there, but it is reduced in proportion to the >> divide ratio, because the voltage noise in the transistors is >> unchanged but the voltage ramp that is driving the switching is >> steeper in proportion to the divide ratio. > > With the same absolute jitter from the MOS transistors, the VCO > noise sidebands will look proportionally worse at the higher frequency, > and dividing down will get you where you started if you are lucky. > No free lunch here. > > In real life you add the phase noise of the frequency divider. > If you want low phase noise, you need a resonance with substantial Q. > > > regards, Gerhard The OP should really try that high value resistor to ground from the phase detector output, to pull the loop out of the dead zone. It's a lot like an LM324 that way--an ugly bug, uglier at higher frequency, easy to fix. If it's a problem with the RC VCO, it'll still be a problem with an LC VCO. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
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