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Help. Transistor circuit that connect or disconnect 75ohm terminator for video application

Discussion in 'General Electronics' started by LRCR, Feb 2, 2006.

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  1. LRCR

    LRCR Guest

    I breadboard separate two transistor circuits to compares the
    performance of switching in and out a 75 ohm resistor terminator to a
    video source. One circuit is based on a bi-polar 2N3904 and the other
    is a FET transistor(it's a DMOS N-channel MOSFET). Using a video
    generator that is set to sweep 1 to 30MHz and an oscilloscope to
    monitor the response, I observed that both type of terminators have
    issues. On the bipolar method, the frequency response is pretty flat
    to 30MHz when the 2n3904 is truned off, but when it is turned on, the
    frequency response has gain peaks and loss valleys. This cause the
    video sync to measure 40IRE (that's good), but the color burst
    measures 48IRE (too high). I need the frequency response to be flat to
    10 MHz. The FET mehtod has the reverse effect. When the FET is on,
    the frequency response is pretty flat to 30MHz. When the FET is OFF,
    the frequency response has a number of gain peaks and loss valleys.
    Also, on both method, the video signal wobbles slowly slightly up and
    down (on or foo on both methods). I can't seem to get rid of the
    wobble. Must be due to the stray capacitance of both FET and bipolar.
    Any help will be appreciated.

    Circuit of both termination method:

    video source------------------*-------------------Oscilloscope
    0V or 5VDC-->10k-------->B 2N3904
    75 ohm

    video source------------------*-------------------Oscilloscope
    0V or 5VDC-->10k-------->G N-channel DMOS FET RON(MAX)=2ohm
    S for VGS= 5V
    75 ohm
  2. Robert Baer

    Robert Baer Guest

    Err..a signal source of 75 ohms travelling down what pretends to be a
    transmission line and that line not terminated *will* give "peaks and
    valleys" due to reflections off the end of the line.
    Also, nothing is said as to how the signal magically goes to the
    scope; virtually any probe used for that purpose will alter a resistice
    75 ohm termination, to the detriment of the signal.
    Methinks you need to go back to square one and think this over
  3. LRCR

    LRCR Guest


    You do make a good point. I am using from the source to the
    oscilloscope an RG59 cable with a T at the scope. The circuit is at
    the T. Also. I attached a large capacitor 220uF (Crazy) from the
    gate to ground(the gate source is from a lab Power Supply) and it
    appears to help. Maybe that a clue.

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