Tej, each signal can be represented by a sum of harmonics with different magnitudes and frequencies (Fourier series). If we plot magnitude vs. frequency, we get the signal spectrum.

Many blocks of control systems have some inertia, mechanical or electrical by its nature. It means that they cannot perform high-frequency oscillations at their output even if they have a high-frequency input. In terms of input spectrum, they cut off its high-frequency part and pass only low-frequency harmonics. The bandwidth of a control system block is the range of oscillation frequencies that it can pass, i.e. reproduce at its output without significant attenuation.

If we want a block to have little time lags in its operation, we must reduce its inertia and therefore allow it to perform high-frequency oscillations if the appropriate input is given. Hence, its bandwidth must be wide. On the other hand, the noise can also be treated as a sum of high-frequency oscillations. If we wish to suppress it, we must increase the block inertia and therefore narrow its bandwidth. Thus, the bandwidth adjustment is a trade-off between the characteristic response time and the output noise level.

Vasiliy Tereshkov

That was an amazing answer. Thanks for the explanation.

Now i see some relation between these terms. Replication of input oscillation  frequency to output  gives proportionality to Bandwidth (i will just increase bandwidth if i wanted to see intergral multiple of fundamental harmonic) and Noise suppression gives inverse proportionality to Bandwidth.

It gives me some insight.  I have one more basic question, how will i know what can be the highest frequency oscillation that can appear at step input change which will give desired bandwidth ?

Thanks in advance 

Have a great day

Most systems will have electro-mechanical resonances which can interfere with ideal linear feedback loops when you extend the bandwidth.

To reduce control errors the current feedback allows direct error feedback with force current control and acceleration profiles, tach feedback direct error feedback when velocity  profiles are used. This is the Proportional wide bandwidth control.

Due to inertia and transient loads these disturbances introduce lag and increased error. So improvements in design amplify higher frequency errors a.k.a. derivative (D) feedback. This also lowers the drive impedance to disturbances which resists sudden changes, reduces overshoot and shortens settling time.

Overall lag-lead phase compensation ensures each loop has at least 30 deg phase margin depending on system requirements for stability of speed control, pref.,45 deg margin.

The demand for higher stability with step loads demands more bandwidth than ramp loads or surge loads that decay with rising speed. 

Since current is proportional to acceleration which is the derivative of velocity, it has increased gain with higher frequency, but also more noise because load torque is also proportional to current and if unexpected load occurs, then velocity feedback must be differentiated to compare with the acceleration profile. 

IN other cases shaft torque strain gage feedback and temperature feedback to compensate for changes in transfer function to optimise the motor control loop dynamically.

The system gain and bandwidth parameters may need to be conditoned (filtered) depending conditions like ; no load , full load or fault conditions , depending on the need for performance and system protection.

... step response ringing usually occurs at the unity loop gain frequency unless mechanical resonance shifts that response.