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    LPF IC PCB design mini project

    Hi all,

    I have a mini project which i need to design a LPF with an minicircuit 700MHz LPF ic(SCLF-700+).
    the passband of my signal are between 400 to 700MHz.

    I need to do a simple a PCB design layout with the IC above and with 2 sma connector on the PCB board.
    I wanted to put a coupling cap each at input and output of my LFP IC. How do i design the decoupling cap? what value i should put?
    And also, how to decide the thickness of my pcb? Issit by the thickness of SMA connector that i need to put on the board?

    thanks.

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    Re: LPF IC PCB design mini project

    You mention coupling capacitor and decoupling capacitor.
    Can you understand things correctly ?
    Last edited by BradtheRad; 15th June 2017 at 23:35. Reason: Portion edited



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    Re: LPF IC PCB design mini project

    Quote Originally Posted by pancho_hideboo View Post
    You mention coupling capacitor and decoupling capacitor.
    Can you understand things correctly ?
    sorry typo there. coupling cap. at the input and output of the LPF.



    •   Alt16th June 2017, 01:46

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    Re: LPF IC PCB design mini project

    Show us terminated impedance value.



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    Re: LPF IC PCB design mini project

    Quote Originally Posted by pancho_hideboo View Post
    Show us terminated impedance value.
    50 Ohm input and output termination.
    i wanna redesign the project, instead of coupling cap. now i wanted to change it to decoupling cap to filter out high freq component. So what shud the cap value i shud put there.

    thanks



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    Re: LPF IC PCB design mini project

    You still use coupling and decoupling capacitor.

    Again can you understand things correctly ?


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    •   Alt16th June 2017, 13:15

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    Re: LPF IC PCB design mini project

    I agree that the question sounds confused. The low-pass filter doesn't need additional components to achieve the specified characteristic, and you shouldn't connect additional components not to worsen it. If you want the low-pass with SMA interface, I suggest SLP-750+.

    If you want to make a PCB though, consider that the width of 50 ohm traces is affected by the substrate height. And yes, the SMA connector should fit the substrate height if you are using edge mounted connectors.

    You can reduce the trace width with thick (e.g. 1.6 mm) PCB if you design it as coplanar strip with ground. Via fences required.

    - - - Updated - - -

    Or make your own low pass filter, like the lower PCB in the photo

    Click image for larger version. 

Name:	SMD Filter top.jpg 
Views:	7 
Size:	439.0 KB 
ID:	139436


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    Re: LPF IC PCB design mini project

    Quote Originally Posted by pancho_hideboo View Post
    You still use coupling and decoupling capacitor.

    Again can you understand things correctly ?
    hi adding a decoupling cap at input and output of the LPF IC is not right? Please educate me ....



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    Re: LPF IC PCB design mini project

    Not exactly clear what you mean with "decoupling cap". You have it described as low-pass in post #5. But it's conflicting with expected impedance matching for SCLF-700.

    Why do you think it should be used?


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    Re: LPF IC PCB design mini project

    Quote Originally Posted by FvM View Post
    Not exactly clear what you mean with "decoupling cap". You have it described as low-pass in post #5. But it's conflicting with expected impedance matching for SCLF-700.

    Why do you think it should be used?
    Can i add the capacitors as shown in the schmatic below? ignore the wrong lpf ic. just simple draw out for so that you all understand my question.
    Click image for larger version. 

Name:	LPF.JPG 
Views:	3 
Size:	32.9 KB 
ID:	139440



    •   Alt16th June 2017, 18:28

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    Re: LPF IC PCB design mini project

    As previously stated, you shouldn't do this. The filter is calculated to match 50 ohm impedance at both ports. Adding capacitors destroys the matching.

    If you have designed the filter yourself and know the circuit, you can calculate the capacitor effect. But if you want to modify the filter characteristic, you'll redesign the filter instead of adding arbitrary components.


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    Re: LPF IC PCB design mini project

    i have done the project pcb layout with the ic and 2 edge mount sma connector. for the pcb thickness i was ask to keep it to 0.8mm.
    so from the setting in the altium layer stack manager which one i shud setting to keep it to 0.8mm. totally hav no idea in this.Click image for larger version. 

Name:	layer stack manager.JPG 
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ID:	139441



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    Re: LPF IC PCB design mini project

    Settings in Layer Stack Manager are only relevant for documentation purposes and possibly impedance calculations. It's an optional add-on to the basic PCB design.

    You see the default settings of Altium designer, need to change the substrate thickness respectively for overall 0.8mm thickness, e.g. 29mil.



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    Re: LPF IC PCB design mini project

    Quote Originally Posted by FvM View Post
    Settings in Layer Stack Manager are only relevant for documentation purposes and possibly impedance calculations. It's an optional add-on to the basic PCB design.

    You see the default settings of Altium designer, need to change the substrate thickness respectively for overall 0.8mm thickness, e.g. 29mil.
    Hi , i have no experience in this, so which layer i shud change to make it an overall 0.8mm ?
    thanks



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    Re: LPF IC PCB design mini project

    Quote Originally Posted by arr_baobao View Post
    hi adding a decoupling cap at input and output of the LPF IC is not right?
    Your capacitors in figure are not "Decoupling Capacitor".
    http://www.edaboard.com/attachment.p...0&d=1497634105

    They are "Shunt Capacitor".


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    Re: LPF IC PCB design mini project

    I apologize for using the synonymous substrate for dielectric...

    Seriously, isn't the answer obvious when looking at the Altium stackup display?



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    Re: LPF IC PCB design mini project

    Quote Originally Posted by FvM View Post
    I apologize for using the synonymous substrate for dielectric...

    Seriously, isn't the answer obvious when looking at the Altium stackup display?
    thanks. so for my case i will need to change my dielectric FR4 layer to make it overall thickness of 0.8mm.

    thanks so much for the lesson



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    Re: LPF IC PCB design mini project

    Quote Originally Posted by pancho_hideboo View Post
    Your capacitors in figure are not "Decoupling Capacitor".
    http://www.edaboard.com/attachment.p...0&d=1497634105

    They are "Shunt Capacitor".
    thanks for pointing out.
    what are the different between shunt capacitor and decoupling capacitor.



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    Re: LPF IC PCB design mini project

    "Shunt" or "Series" specify inserting topology of capacitor.
    "Decoupling" mean a purpose or role of capacitor.

    Your shunt capacitor's role is not "decoupling".

    http://www.edaboard.com/showthread.php?t=368768


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    Re: LPF IC PCB design mini project

    Quote Originally Posted by pancho_hideboo View Post
    "Shunt" or "Series" specify inserting topology of capacitor.
    "Decoupling" mean a purpose or role of capacitor.

    Your shunt capacitor's role is not "decoupling".

    http://www.edaboard.com/showthread.php?t=368768
    thanks. If the cap act as a decoupling role, mean that it always connected shunt? any decoupling cap that connected in series?



    •   Alt20th June 2017, 03:01

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