Problem with logic level and using a logic analyzer

Discussion in 'General Electronics Chat' started by 2amTech, Jan 5, 2013.

  1. 2amTech

    Thread Starter New Member

    Jan 4, 2013
    Using a Zeroplus Logic Cube I'm tapping into an 8bit one direction bus that has a 20Mhz clock. Since the Logic Cube can not be driven synchronously with the clock, I wanted to capture it's pulses for reference.

    Problem is I see missing or erroneous samples no matter what trigger, sample rate, or voltage level I set the Logic Cube to.

    The inputs should all be running between 0v and 5v (Vcc) according to the specifications of the chip being fed.

    I've used two scopes (a DSO and Analog scope) and I see a stable clock pulse but it is 2v total with the upper 1v being a triangle wave (see attachment). It isn't square or near square as I had expected.

    The circuit is still operating normally with the probes connected.

    I'm tempted to put a pulldown resistor on this but afraid of damaging something or causing the circuit to not work correctly.

    Anyone familiar with this scenario and how to get this signal in a useful way to look at with a logic analyzer?

    I have a few 4050B hex buffers but looks like without the proper logic levels they won't be much help either.

    Note: Yellow Marker on the left in this screen shot is at GND
    Last edited: Jan 5, 2013
  2. ScottWang


    Aug 23, 2012
    1. How is the frequency response of scope?
    2. The waveform looks like the frequency response of scope is not enough, so when a high signal can't up to high, when a low signal can't drop down to low.
    3. The propagation delay time of CD4050 maybe too long for 20Mhz signal, you may use 74HC4050.
  3. 2amTech

    Thread Starter New Member

    Jan 4, 2013
    50Mhz DSO, 100Mhz Analog

    I've tried different probes for grins (rated @100Mhz, and 300Mhz) and see the same thing.

    I don't have anything to generate a 20Mhz square wave off hand and seeing what I have handy to make something. Theoretically the scopes are within range.

    Thanks for the heads up, I see that now in the 4050B data sheet. I'm digging through parts to see what else I have.
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    The 100Mhz analog scope will only be able to accurately portray a signal to about 10Mhz.

    Can you slow the clock down to 5Mhz and verify your scope shows the waveform?

    It does look like a scope bandwidth issue.
  5. ScottWang


    Aug 23, 2012
  6. Brownout

    Well-Known Member

    Jan 10, 2012
    Your scope has pleanty of bandwidth. Either you have a probe grouding issue, or your scope is trash.