Continue to Site

# Is the VCO duty cycle always 50%?

Status
Not open for further replies.

#### dhasmana

##### Member level 5
Hi,

Is it true that the duty cycle of every VCO is 50% (assuming no variation because of PVT)? If we add inverters in the VCO for getting rail to rail output then they will add jitter due to threshold variations and thus duty cycle will drift away from 50%.Please comment in this regard.

Regards,
Jitendra.

VCO Duty cycle

Some times, a VCO does not have a 50% duty cycle precisely.
Adding inverters after the VCO, duty cycle will drift, I agree.
but I don't think the jitter is added by inverters, jitter is produce by VCO itself.

Ryan

VCO Duty cycle

Don't you think that drift from 50% duty cycle is same as jitter?This drift in duty cycle is a random process or a deterministic?I mean the duty cycle varies from cycle to cycle or just stays around a value away from 50%?
What is the difference between deterministic jitter and random jitter?I mean in terms of the origin /reason for their existence?

Regards,
Jitendra.

VCO Duty cycle

jitter does not mean drift in duty cycle, it means the variations in the zero crossings of the output clock, if you plot all the zero crossings at a single point you will gent to know the jitter.

amarnath

VCO Duty cycle

Can you please elaborate on "variations in the zero crossings of the output clock"
I did not understand this point.Do you mean the variation in the clock edge timing , i.e when is the edge apearing?

VCO Duty cycle

basically if your system is getting clocked onj postivie edge, then take the positive edge as a reference and then measure the varioations of the zero crossings with reference to this edge.

amarnath

### dhasmana

Points: 2
VCO Duty cycle

Ok.This is what I understood.Now if the +ve edge is reference and -ve edge is varying with time, then it also means that the duty cycle (from one cycle to other) will vary.These two phenomena are not independent I guess.Jitter will translate in duty cycle variation also?Please comment.

Re: VCO Duty cycle

Hi,

Is it true that the duty cycle of every VCO is 50%
If you look at datasheets, you will see that duty cycle are specified between 45% - 55% (sometime 40% - 50%).

If we add inverters in the VCO for getting rail to rail output then they will add jitter
Adding any component will increase the jitter. This is due to the thermal noise, flicker noise etc. of the component itself.

due to threshold variations and thus duty cycle will drift away from 50%
Jitter is not a drift, it is a variation (pseudo oscillation) from the ideal position. Your duty cycle will change 50% +/- jitter, but the mean value will be 50%.

### dhasmana

Points: 2
Re: VCO Duty cycle

dhasmana said:
Ok.This is what I understood.Now if the +ve edge is reference and -ve edge is varying with time, then it also means that the duty cycle (from one cycle to other) will vary.These two phenomena are not independent I guess.Jitter will translate in duty cycle variation also?Please comment.

Jitter will not translate in duty cycle variation as you thought. Jitter is said in one clock period, if +ve edge is reference, -ve edge change, and if we consider two -ve edge as one period, then the period will not change, because the -ve edge has changed the same. and jitter is generated. But the duty cycle has change, because the duty cycle is considering between +ve and -ve edge.

Ryan

VCO Duty cycle

how to simuliate jitter?

Re: VCO Duty cycle

The duty cycle variation is another term called DCD(Duty Cycle Distortion) jitter. Since DCD is bounded, it is a kind of DJ(deterministic jitter). In deed, DJ can be decomposed in to ISI, DDJ, DCD and PJ(periodic jitter).
As you may know, jitter from VCO output is unbounded, which means it is random jitter.
Therefore, unless you add extra logics(or circuits) in proper way, they may introduce some jitter, which may look like DJ. However, in general, RJ is dominant term in clock-like signal.

Re: VCO Duty cycle

dhasmana said:
Hi,

Is it true that the duty cycle of every VCO is 50% (assuming no variation because of PVT)? If we add inverters in the VCO for getting rail to rail output then they will add jitter due to threshold variations and thus duty cycle will drift away from 50%.Please comment in this regard.

Regards,
Jitendra.
the invertor definitely add jitter, however, it's very small

Re: VCO Duty cycle

how to simuliate jitter?

To simulate a deterministic jitter, a simple way is to use frequency modulation around the main frequency. Not very realistic, but easy to make.

--------
Electrocoop

Re: VCO Duty cycle

RDRyan said:
Some times, a VCO does not have a 50% duty cycle precisely.
Adding inverters after the VCO, duty cycle will drift, I agree.
but I don't think the jitter is added by inverters, jitter is produce by VCO itself.

Ryan

no, you are wrong, the inverter will add jitter to the clock, because of the inverter has worse psrr

Status
Not open for further replies.