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voltage controlled oscilators

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Green_Ic

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Hi all.One question.In VCO regarding stage numbers,with increasing stage number,we decrease the output frequency,but what becomes a good with increasing stage number? Because I saw,that mostly we don't put the stage numbers in minimum. It connects with to drive the output load or maybe PSRR or SNR ?
 

Many VCOs have only one stage. I'm guessing you are
talking about ring oscillators and/or DLLs.

At very low stage counts, the edges of a RO will be more
"leisurely" and prone to translate supply noise into jitter
/ phase noise. Slow input edges stretch out the shoot-through
current interval of inverters adding to the shuttle current
(beyond simple gate charge, there is shot-through waste).
High stage counts allow edges to be more "crisp". There is
some limit to how crisp, when you are starving current, of
course.
 

In case you're talking about ring oscillators, one stage (assuming differential in and out) is never enough, because you won't accumulate enough phase around the loop to make it oscillate. This could be even problematic with two stages in the ring (again, assuming fully differential stages). THree and more should be ok to make the oscillator oscillate. It is more or less the same also for single ended ring oscillators - one stage is not going to oscillate, two stages is a latch and three stages might start oscillating. In the single ended case you'll need odd number of stages.. The more the stages, smaller the frequency and more phases you can choose from. How sharp the edges are depends on the power consumption and the size of the stage aka parasitics loading.
 

Hi I research about VCO for FM and best FM modulation with minimum harmonics is Wideband Colpitts VCO


"This wideband Colpitts VCO uses a series back-to-back connection of two SMV1232 varactors instead of a single varactor. This connection allows lower capacitance at high voltages, while maintaining the tuning ratio of a single varactor. The back-to-back varactor connection also helps reduce distortion and the effect of fringing and mounting capacitances.
The wideband Colpitts feedback capacitances C3, C4 were optimized to provide a flat power response over the wide tuning range. These values may also be re-optimized for phase noise if required.
The circuit is very sensitive to the transistor choice (tuning range and stability) due to the wide bandwidth requirement.
DC bias is provided through resistors R6 and R7, which may affect phase noise, but allows the exclusion of RF chokes. This reduces costs and the possibility of parasitic resonances which is the common cause of spurious responses and frequency instability."

I find one on internet , perhaps is professional but schematic is inquiring, im not sure it is good? can you help me to cheek and see is good? If if is god I wont tray build one for experiment .
 

2scvco.jpg

This is possible perfect , professional VCO Hi FI but i not sure it is ok... resistors and capacitors
 

Wideband Colpitts VCO


This wideband Colpitts VCO uses a series back-to-back connection of two SMV1232 varactors instead of a single varactor. This connection allows lower capacitance at high voltages, while maintaining the tuning ratio of a single varactor. The back-to-back varactor connection also helps reduce distortion and the effect of fringing and mounting capacitances.
The wideband Colpitts feedback capacitances C3, C4 were optimized to provide a flat power response over the wide tuning range. These values may also be re-optimized for phase noise if required.
The circuit is very sensitive to the transistor choice (tuning range and stability) due to the wide bandwidth requirement.
DC bias is provided through resistors R6 and R7, which may affect phase noise, but allows the exclusion of RF chokes. This reduces costs and the possibility of parasitic resonances which is the common cause of spurious responses and frequency instability.

vco2varicap.gif

Can you tell me reason why thy use 4 varicaps?
 

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