What is the use of the serial coupling capacitor ?

[Matiw]

In WD drives with built-in USB connector, there are four serially connected coupling capacitors in between the USB bridge and the MCU. What is the purposes of those components ? Were they connected in parallel to ground, one could have claimed that they are there to dump high frequency AC interference; but, they are in series ! Are they there to block DC voltages/currents ? Why ? If one wants to block the DC, why can not one just leave the connections open ?

Appreciate any help.

fzabkar input is highly appreciated.

[fzabkar]

The purpose of the coupling capacitors is to block DC. A 10nF capacitor looks like an open circuit to DC voltages and a short circuit to high frequency (GHz) AC voltages. The reason that DC needs to be blocked is that the DC bias levels on either side of the SATA interface would be different. The DC bias is the quiescent voltage level of the Tx/Rx pins in the absence of a signal. Without the capacitors, the DC bias of the MCU would disturb the DC bias of the bridge IC. BTW, after removing the capacitors, there will still be coupling capacitors on the computer's motherboard to provide DC isolation.

Perhaps the following example will better illustrate what is happening.

Code:

        +12V           +50V
         |              |
         |              |
       |\| 6V      0V |\|
    ___| \_____||_____| \___
       | /     ||     | /
       |/|     C      |/|
         |              |
         |              |
         0V            -50V

    preamplifier    power amplifier

The preamplifier has a 12V supply. Its output is biased at the midpoint of the supply rail, namely 6V. The AC signal then swings symmetrically above and below this bias point. For example, a 1Vpp (peak-to-peak) AC signal would swing between 5.5V and 6.5V.

The power amp has a +/-50V supply. Its input has a DC bias of 0V. The coupling capacitor, C, blocks the DC component of the input signal, resulting in a swing of -0.5V to +0.5V.

[HaQue]

fzabkar, thanks for taking the time to explain that, I have wondered about in series caps before a well. could you possibly explain in plain english what "biased" means? One of the things I think I understand but never really totally sure about.

[fzabkar]

The DC bias voltage is the voltage at the input or output when there is no signal.

In the case of the preamplifier, the output voltage is given by ...

Vout = DC bias voltage + AC signal voltage

In the above example where the AC signal is 1Vpp ...

Vout = 6V + 0.5 x sin(2.pi.f.t)

... where f = frequency, t = time.

Similarly for the power amp ...

Vin = 0V + 0.5 x sin(2.pi.f.t)

The reason that you would select a bias voltage at the centre of the range is that it allows for the maximum voltage swing. In the case of the preamp, the maximum possible signal output would be 12Vpp, ie 6 x sin(2.pi.f.t).

Another example of DC biasing is in the VCM servo. A small DC current needs to flow in the voice coil even when the headstack is stationery because the headstack is subject to certain forces including latch force and gravitational forces (in vertical orientations).

[Matiw]

Thank you fzabkar. You have explained it better than how my college time digital electronics teacher did.

[Alexii]

My wife always wanted to visit Australia. And if we ever do - ill bring Frank a bottle of something nice and strong =)

[digitalferret]

just thought "let franc explain it"

After all who better to explain AC DC than an Oz )))

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[Doomer]

Where is the "Like" button ?

[HaQue]

Where is the "Like" button ?

I think it is on that other website people go to to get their PII stolen, get malware and whine about their friends behind their back in language they "think" no one will know who they are talking about...

[Doomer]

Where is the "Like" button ?

I think it is on that other website people go to to get their PII stolen, get malware and whine about their friends behind their back in language they "think" no one will know who they are talking about...

Now I'm wondering what that site would be?
Seems like a lot of conspiracy going on there.

[HaQue]

Where is the "Like" button ?

I think it is on that other website people go to to get their PII stolen, get malware and whine about their friends behind their back in language they "think" no one will know who they are talking about...

Now I'm wondering what that site would be?
Seems like a lot of conspiracy going on there.

does it show I hate facebook? I mean I am pretty subtle

[digitalferret]

Facebook?
The NSA couldn't have done a better job of collating information without appearing as BigBrother.
All they have to do now is hack Facebook and ... oh hang on ...

btw, in case there was any confusion about the intent of my cartoon;
just to say i hold Fzabkar's contributions in the highest regard.
I find his explanations so complete and so succinct it's hard to follow any of his replies with much by way of valid further contribution.

The cartoon was my attempt at humour using his avatar as a nod of recognition.

Right ... "Mr Embarrassed" here is gonna walk off and trough breakfast.

Kern