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# what is the difference between harmonic and spur?

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#### fanisani

##### Newbie level 6
I want to know the relation of spur and non-linearity in ADC and

what is the difference between harmonic and spur?

in ADC because of non-linearity, input driving and s\h circuit

at output we see that harmonics of input signal appear at ADC

spectrum.

The ideal ADC has the quantization noise as a white noise (in certain condition, and

i think it appear because of non-linearity of transfer function of ADC)but if the input

is a pure single tone it has no spur in the spectrum . so i think about INL and DNL

error and how can they cause spur in the spectrum in low frequency.

but in calculation of SFDR we must think of worst spur.references say that spur can

be harmonic of input signal or may be distortion.

at level of signal near full scale worst spur be a harmonic , but at low level signal

may be not a An ideal N-bit ADC, sampling at a rate fs, produces quantization

noise having an rms value of q/(sqrt 12) measured in the Nyquist bandwidth dc to

fs/2.
An actual ADC will produce noise in excess of the theoretical

quantization noise, as well as distortion products caused by a non-linear transfer

function. but i think quantization noise is also because of non-linearity.????????????

Harmonics are exact interger multiples of the frequency of the input tone. Spurs are all other spectral lines.

### fanisani

Points: 2
Hi
we have the Spure in the ideal adc, because of quantization noise.
see Pan's paper

regards

### fanisani

Points: 2
hi, first thank you for answering me.
in ideal adc we just have harmonics that aliased back to Nyquist zone. in the first zone we just consider first 6 harmonics and other as noise.
but in typical adc we have some other thing that are not harmonics and call them...
in sfdr calculation we must consider worst spur that can be harmonic or not!!!, i want to know what cause spur that are not harmonics.

Hi
many things such as
capacitor mismatch
non linearity
in general non idealities
regards

hi , what do you mean capacitor mismatch?
and ideal ADC also has non-linearity and on certain condition it has quantization noise as white noise and on the other case it has very high harmonics.so i think spur(that are not harmonic) can not be cause of non-linearity. and what cause non-idealities , i think some error for example INL & DNL , but these are also non-linearity and can cause harmonics!!!!!
so we have harmonics of input signal, so what cause spur when we have pure sine wave at the input of ADC?!

Hi,

here is what I know:

1. ADC (an ideal one) is by nature nonlinear. because its characteristic is stair case (obviosly it is not linear). This transfer characteriic generates harmonics and spurs.

2. Quantization noise is another expression for the above phenamena. Although it is modeled by white noise, it is not.

3. Nonidealities like capacitor mismatch generate INL/DNL. They make the characeristic og the converter MORE nonlinear. In frequency domain, this means that some spurs are increased compared to an Ideal ADC.

4. Not that a harmonic need not be an exact mutiply of fin. As you said harmonics fold back. for example fin=3MHz, fs=10Mhz, then the second harmonic apears at 4MHz.

Hi,
in 2)quantization noise may appear as harmonics of input frequency if there is a relation between input signal and sampler.

in 3) what do you mean MORE non-linearity ??
i can model some non-linearity with tailor sum, so i have each harmonics ( with INL & DNL just change the amplitude of harmonics)
in 4) i call the folded back harmonics as harmonics because of sampling they are fold back in the Nyquist zone.
but in practical adc i have some frequency that they are not harmonic of input frequency,i think called them distortion.
and i want to know what cause them to creative ?

Hi
I'm not sure but
I think there is a kind of modulation and intermodulation.
maybe this is the source of spures.
regards

### fanisani

Points: 2
hi, thank you because of your help .
you said about mismatch capacitor. so i want to say that in ideal ADC we have only harmonics of input signal but in practical ADC for example pipeline ADC that construct of some flash ADC and stages , each ADC has it's own noises ,..and so that they are unwanted frequency for the other stages.(a kind of inter modulation)
best regards

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