[SOLVED] How decap helps power steady in high frequency?

[coxstreet]

One block needs to work at 960M Hz (such as USB2 Rx) but in which the internal voltage regulator only has 1M Hz bandwidth and 6M Hz UGF. Apparently, above 6M Hz, large decap helps to make power stabe without regulator. Please help to explain this? Thanks.

 

[D.A.(Tony)Stewart]

Any negative feedback regulator or unity gain Op Amp has low output impedance at DC which rises with f as feedback decreases from lack of gain-bandwidth.

Capacitors can have lower impedance Zc=1/ωC up to the series resonant frequency, (SRF)

 

[coxstreet]

Any negative feedback regulator or unity gain Op Amp has low output impedance at DC which rises with f as feedback decreases from lack of gain-bandwidth.

Capacitors can have lower impedance Zc=1/ωC up to the series resonant frequency, (SRF)

Yes, decap does have lower impedance at high frequency. But how does it stabilize the power? Thanks.

 

[ahsan_i_h]

But how does it stabilize the power?

The function of decap is similar to the function of a surge tank in mechanical system.

 

[csolis_GT]

Yes, decap does have lower impedance at high frequency. But how does it stabilize the power? Thanks.

If you don't have a decap, the impedance will keep increasing, meaning that when you have a current spike demanded from the load (or any current change with high frequency content) the voltage drop can be huge. It is similar to keeping IR drops low at DC but the decap does it at high frequencies. In that way the supply is stable because it retain its voltage for a wider frequency range.

 

[dick_freebird]

HF decoupling capacitors are meant to create an AC
short circuit from pin to pin, keeping impulse and tonal
perturbations close in, short loop, not impacting the
upstream regulator nor demanding anything from it.

Your DC / bulk decoupling plays into regulator stability.
The HF stuff takes care of the regions where the supply
regulator is basically helpless (limited BW, far end of an
inductive power distribution network).