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Transmission line analysis help

  1. Sep 9, 2010 #1
    I am at a loss on how to apply transmission line theory when a probe is connected to a high speed bus or for a matched RF line.
    For example, a signal going from a PCI clock to a bufffer which has a very short trace(so no termination is required). Now, if I connect a logic analyzer probe to this trace, how do I analyze this circuit.
    The probe is in parallel with the buffer. Can I ignore the buffer (since the buffer input impedance is high) and just think of the circuit as a clock source connected to the logic analyzer?
    This will reduce to a clock source with some internal impedance(maybe 10-20 ohms) connected to 20K probe tip and terminated by 1M resistor. So, the clock source will see an open circuit? Wouldn't there be any reflections because of the mismatch. (Fig 1)

    In case of a 50 ohm matched source, tx line and load. How do I analyze, when a probe is connected at the source or load? (fig 2)
     

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  2. jcsd
  3. Sep 9, 2010 #2

    vk6kro

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    If you have a very short line (ie short compared with the wavelength of the signal), then you can live with the reflection due to a mismatch because it will be very nearly in phase with the incoming wave.

    So, if you put a 20 K probe in parallel with the input impedance of a buffer, the SWR will actually get slightly better if the line has an impedance of about 50 ohms or so. However, it still won't be a match and there will be some reflection.

    This reflection occurs at the tip of the probe, so the length of the probe cable won't matter. Again, you will get a reflection and how important it is depends on the distances compared with a wavelength at the signal frequency.

    If the line is matched and you put 20 K across it, you will upset the match very slightly. Instead of a 50 ohm load, you would then have a load of 50 ohms in parallel with 20 K, or 49.87 ohms. So, it isn't going to make any difference.
     
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