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Microstrip advice

Discussion in 'Radio and Communications' started by Mosaic, Feb 9, 2016.

  1. Mosaic

    Mosaic Well-Known Member

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    Hi All:

    I purchased one of these items..note attached PDF wit all details. Proceeded to break the PCB due to a yanked cable and was in the process of recalculating the 'impedance' of the microstrip for 1.6mm FR4.

    I realized that it adds up to several hundred ohms.
    My question is this...why would a 50 ohm RF signal require a several hundred ohm microstrip (to couple some signal to a detector) and then connect to another 50 ohm output? See Note 4 on page 11.

    PS: i asked the author...he does not know.
     

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    Last edited: Feb 9, 2016
  2. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Those microstrip impedance values shown in Note 4 are transmission line impedances.
    They don't add together.
    Ideally they should be all the same but there is some mismatch between them (apparently not enough to cause a problem).

    Do you understand what the characteristic impedance of a transmission line is?
     
  3. Mosaic

    Mosaic Well-Known Member

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    I was treating them as lumped element, but that's a wrong approach.
    I have to do some more learning in this regard.
    I think the stepped impedance.is deliberate to create some kind of Low pass arrangement, otherwise just make a single 50 Ohm microstrip for lowest VSWR and be done.
    I am working on figuring it out. I am going to do some empirical VNA tests as well, I have a new PCB from the PDF to work with . Sometimes learning works better by doing.
     
  4. dave

    Dave New Member

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  5. JimB

    JimB Super Moderator Most Helpful Member

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    I think that this may be a case of "over thinking" the problem.

    The highest frequency involved here is about 1000MHz, wavelength is 30cm.
    What are the line lengths involved here, 1cm maximum?

    I think that this is going to have little effect on the signal.
    Did Tektronix even design these as microstrip or are they just traces on a piece of exotic circuit material?

    JimB
     
  6. Mosaic

    Mosaic Well-Known Member

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    I have been advised that it is a meander line inductor.....
     
  7. JimB

    JimB Super Moderator Most Helpful Member

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    Yes, the squiggle on the FR4 board is an inductor, and is there to replace a straight trace on the ceramic(?) original.

    I am just questioning whether there was any impedance matching or filtering designed in to the Tektronix original.

    JimB
     
  8. Mosaic

    Mosaic Well-Known Member

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    It is matched to replace a squiggle on the original , both stepped impedance and phase delay. It's length is adjusted for the different propagation/permittivity of FR4 vs Alumina. The designer of the substitute said he just duplicated the functionality without finding out why it's there. As I am only 3 months into RF (IoT motivated) I have a lot to do in the fundamentals and so here I am.

    I gather the following:
    Since the peak Freq is 1 Ghz the microstrip is well below 1/4 wavelength resonance and can be considered an inductor and not a tx line. VSWR then is not as significant.
    A series inductor is a choke/LP pass filter, presumably to knock down harmonic spurs.

    What makes this complicated for me is figuring out what is the inductance of this design.
    Can you help with that?

    I found this interesting:
    http://www.qsl.net/va3iul/Microstrip_Stripline_CPW_Design/Microstrip_Stripline_and_CPW_Design.pdf

    Edit:
    using coil32 freeware I calc'd the individual inductor steps and came up with 7nH total for the original TEk strip and 9nH for the substitute. Thus it seems from a lumped element perspective the substitute is somewhat incorrect. From a tx line perspective (which appears invalid at the Frq range) the impedances and phase are correct.
     
    Last edited: Feb 11, 2016
  9. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Last edited: Feb 13, 2016
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