We have assembled a laptop;problem-background humming noise-noise while moving mouse

Discussion in 'General Electronics Chat' started by kthurst, Jan 26, 2009.

  1. kthurst

    Thread Starter New Member

    Jan 25, 2009
    Hello everybody...

    What we are building... :

    We are building a mini laptop by assembling a Single board computer (SBC), DC-DC converter (24V i/p 5V and 12 V o/p ), LCD, mouse, 4x4 matrix keyboard etc.. for a particular application*.
    We have one more module** (small PCB) which is connected to SBC through PC104 and line-in. Also it shares same power supply.
    We have designed a PCB (called as base board) which holds this module and SBC together. Base board is used for routing lines of pc104, audio, mouse, usb, vga, ethernet etc. It is multi-layered and one layer is ground (reference line) itself.
    We have used 2W audio amplifier ST TDA7268 which takes i/p from line-out and feeds to a 8 ohm speaker.
    computer is loaded with Windows XP.
    All the modules work fine when tested separately..
    (*Please excuse me. I can't give more details about 'application' and **'the module'. Hope, without this information we can debug..)

    When the computer is on there is a constant humming noise (loud enough) in the speaker. Also while moving the mouse (no matter usb or ps2) some other noise adds to it. Same is the case if you keep pressing a key in the keyboard.
    Since our final product is simply 'a laptop + our module' connected to it through PC104 and line in, detaching the module simply leaves a laptop. (laptop in the sense every part is separately bought and integrated)
    Case1: Completely detach 'our module' (testing the laptop built by us)
    - Constant loud humming background noise
    - PS/2 mouse adds noise while moving
    - USB mouse adds noise while moving
    - Keyboard (both external and our 4x4 keyboard) causes noise if a key i keep pressing a key.
    - In windows we have muted all the input audio except line-out (line-in,cd-in etc). It has no effect.
    our conclusion:
    - No idea why this loud background humming is appearing. Could not conclude.
    - mouse/usb/keyboard clock is causing noise. May be problem is with dc grounding. (But we have one complete layer of base board PCB grounded (reference), and all grounding connections are made to that. It will give a star like connection. So i hope ground loops cannot happen).
    ( What else can cause this kind of problem?)
    Case2: Our module connected (It shares power supply, PC104 with SBC, also connected to line-in)
    - Same humming, mouse, keyboard noises.
    - When in windows line-in is muted, no noise. Every thing works perfectly.
    - When line-in connection from the module to SBC is removed (same as muting line-in), no noise.
    Couldn't come to any conclusion because in previous case line-in had no effect, now it is completely depending on line-in.

    Other observations:

    - With proper capacitors added, no fluctuations in power supply lines.
    - When checked seperately audio amplifier is working fine.
    - Please note that we have connected dc ground to chassis of the laptop.
    - We cannot mute line-in while operating. It is very much needed. Just for checking we muted line in.
    - We are using complete audible frequency range. So filters cannot be added.
    - We are using 24V DC battery for power supply. (given to DC-DC converter to get 12V and 5V). So there should not be ground loops which can be caused from earthing line (third line in ac supplies; usually connected to the body of an electronic device. Also called as ground).

    What I want...:

    - There should not be noise from mouse/keyboard.
    - Background humming noise should be minimal.
    - I want to know whether it is grounding issue.
    - I am stuck;not being able to suspect on anything. So, what do you think? what is causing noise? Can anyone give some probable causes?
    - Can anyone suggest me how to proceed my troubleshooting process?
    - I may not be able to give schematics. Since it is a normal laptop, I request you to 'guess' the problem with your experience and help me with some suggestions.

    Thank you for reading my problem patiently...
    Waiting for your response.