CPWG trace widths and spaces

The Hittite document "Layout Guidelines For MMIC Components" is a great help but they only provide a single dielectric thickness, trace width, trace space and trace thickness combination that gives a 50ohm trace. It would be great to have a list of other combinations that work well with the various Rogers 4350 standard thicknesses. I noticed that many of the Hittite evaluation boards have trace widths and spaces that differ from the recommendations in the document but unfortunately the dielectric thickness and copper/plating thickness isn't mentioned so it is not so easy to duplicate.

Specifically I am looking for something that works well for a trace width of 15 to 20 mils (similar to what the HMC424 or HMC525 evaluation boards use).

  • 0
    •  Analog Employees 
    on Dec 16, 2014 3:21 PM

    Hi Peter,

    Thru lines on the Hittite EVB`s sometimes also takes into account the PCB edge connector launch effects; hence PCBs are designed to compensate the mismatch introduced by the connector connection.

    That "Layout Guidelines For MMIC Components" is to provide a good starting point. For further analysis you can use simulation programs or freeware calculators. If you are using Rogers substrate, then Rogers Corp also provides a freeware for those calculations. Other substrate vendors like taconic or arlon may also be providing similar software.

    Best,

    Kagan

  • 0
    •  Analog Employees 
    on Dec 16, 2014 5:38 PM

    Hi Peter,

    To elaborate on Kagan's response.  The HMC525 eval board uses Rogers 4350 on the top layer (Er ~ 3.7)  The RF trace width is 16mils and the ground spacing is 13mils. We use 1/2 oz. copper on the top layer, when plating is complete the finished metal is 2.0 oz. or approximately 2.8 mils.  If you enter these into a grounded CPW calculator you will get approximately 55Ω.  The parasitic effects of the SMA connector drop the impedance to 50Ω.

    Regards,

    Mark


  • Thanks for your answers. The Rogers calculator is very nice.

    Mark, you don't mention the Rogers dielectric thickness. Using the Rogers calculator, I concluded it must be 10mils. Correct?

    Do you have any additional information regarding the SMA compensation you use?

    Thanks again

    Peter

  • 0
    •  Analog Employees 
    on Dec 17, 2014 1:15 AM

    Hi Peter,

    Correct, the Rogers 4350 is 10mils.  The effects of the SMA connector are typically de-embedded using an RF modeling tool.  I recommend using the CPW specifications I outlined in the previous response to achieve the 50Ω CPW.

    Regards,

    Mark


  • Hi Mark,

    I found this statement very interesting:

    "If you enter these into a grounded CPW calculator you will get approximately 55Ω.  The parasitic effects of the SMA connector drop the impedance to 50Ω."

    Is this a rule of thumb, from experimentation, or was this modeled using an EM simulator with the pcb board properties and the connector geometries?