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THS6012: My question is related to an increase in voltage at the end of a “transmission line”.

Part Number: THS6012

My question is related to an increase in voltage at the end of a “transmission line”.

First, I don’t want to confuse you. There are 2 signals that are referred to in the attached schematics.

 

A “W” beam signal and a “N” beam signal.

The circuits for each of them are identical.

The same problem appears on both signal lines.

I will refer to only one of them.

 

The driver is a TI THS6012IDWP (lower right).

The voltage measured between the WBEAM_WF High and Low is 10 Vpp/Sine wave at about 160 kHz.

The receiver is a TI OPA37GU (upper left).

The voltage measured between the TS_IN_WB High and Low is 16 Vpp.

 

Between the driver and receiver there is about 360 feet of twisted shielded cable.

 

Here are my questions:

1. What is causing this “phenomena”?

2. I am not an electronics engineer. So, is there some sort of an analysis that can be performed that reflects the measurements noted above?

3. What can be done in the electronic design to ensure that a 10 Vpp signal at the driver output will provide a 10 Vpp signal at the receiver input?

 

 

  • Hi Justin,

    Would it be possible for you to please re-attach the schematic diagram? It looks like it did not get through.

    Best Regards,
    Rohit
  •  

    My question is related to an increase in voltage at the end of a “transmission line”.

    First, I don’t want to confuse you.  There are 2 signals that are referred to in the attached schematics.

     

    A “W” beam signal and a “N” beam signal.

    The circuits for each of them are identical.

    The same problem appears on both signal lines.

    I will refer to only one of them.

     

    The driver is a TI THS6012IDWP (lower right).

    The voltage measured between the WBEAM_WF High and Low is 10 Vpp/Sine wave at about 160 kHz.

    The receiver is a TI OPA37GU (upper left).

    The voltage measured between the TS_IN_WB High and Low is 16 Vpp.

     

    Between the driver and receiver there is about 360 feet of twisted shielded cable.

     

    Here are my questions:

    1. What is causing this “phenomena”?

    2. I am not an electronics engineer.  So, is there some sort of an analysis that can be performed that reflects the measurements noted above?

    3. What can be done in the electronic design to ensure that a 10 Vpp signal at the driver output will provide a 10 Vpp signal at the receiver input?

     

     

    Thanks,

    Justin

    Receiver schematic.pdfDriver schematic.pdf

  • Hi Justin,

    For a 360 feet cable (~110 meters) and 160kHz sine wave, the wavelength of the 160kHz sine wave is too long (~1240 m) so as to not see significant transmission lines effects. This is assuming a dielectric constant of 2.29 for a standard coaxial cable.

    Are you measuring the voltage at the WBEAM_WF High and Low  on the THS6012 side in a single-ended fashion by probing each of the outputs with respect to GND? Or are you measuring them differentially with respect to each other? Would it be possible for you to provide oscilloscope screen-shots of the same?

    You can also try to increase the value of R105 and R106 resistors to 500 or 1k ohms in the driver schematic, to see if the signal swing at the receiver reduces.

    Best Regards,

    Rohit

     

  • The cable is not a coaxial cable.  It is a military type of cable with the type being:   2XSAOW-19.    That translates to 19 twisted shielded pairs of size 22 AWG wire with an additional shield over all of the 19 pairs..  Two of those pairs are the WBEAM and NBEAM where we are having the problem.  The remaining 17 pairs consist of spares and other types of control (TTI) signals.

    The measurement of voltage was made with a battery powered Fluke O'scope across the high and low leads at the end of the cable at the receiver input at 16 Vpp.  Measurements were also made at the driver output at 10 Vpp.  Nothing was grounded.

    Pictures of the waveform are not available but I can say that it was a clean sine wave pulse about 300 usecs long at both the driver output and the receiver input. 

    At this point in time, we cannot physically change any components.

  • Hi Justin,

    Ok. Can you confirm whether the O'scope probes are set to 1Mohm impedance and not 50-ohms, and is DC-coupled? Also, the other thing you can try out is measure each of the single-ended receiver input with respect to ground, so as to confirm the differential probe measurement is correct. I would recommend to do the same on the driver output. You can also try changing the signal frequency from 160kHz to 80kHz/320kHz keeping the same swing at the driver output 10Vpp, and see if there is any difference in the receiver input swing.

    I cannot suggest you more than this as it looks like you are physically constrained on changing the components. Also, measurement plots of the voltage would be helpful for any further debugging process.

    Best Regards,

    Rohit