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LPF IC PCB design mini project

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arr_baobao

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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.
 

You mention coupling capacitor and decoupling capacitor.
Can you understand things correctly ？
 
Last edited by a moderator:

pancho_hideboo said:
Show us terminated impedance value.
Click to expand...

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
 

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

SMD Filter top.jpg
 
pancho_hideboo said:
You still use coupling and decoupling capacitor.

Again can you understand things correctly ?
Click to expand...

hi adding a decoupling cap at input and output of the LPF IC is not right? Please educate me ....
 

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?
 
FvM said:
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?
Click to expand...

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.
LPF.JPG
 

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.
 
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.layer stack manager.JPG
 

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.
 

FvM said:
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.
Click to expand...

Hi , i have no experience in this, so which layer i shud change to make it an overall 0.8mm ?
thanks
 

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

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

FvM said:
I apologize for using the synonymous substrate for dielectric...

Seriously, isn't the answer obvious when looking at the Altium stackup display?
Click to expand...

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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