selection of substrate in microstrip design

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abhijitrc

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what is the selection rule of substrate height ( say alumina) Ku/K band design ? whether 10 mil is preferred over 25 mil in high frequency design!! Plz guide me
 

My application is the implementation of low noise amplifier circuits , driver amplifier and filter design for ku-band Front end design .
 

abhijitrc said:
what is the selection rule of substrate height
Click to expand...

Thin substrate advantages:
- shorter via through the substrate = less inductance for vias to bottom side ground
- less radiation

Thick substrate advantages:
- wider lines for the same line impedance, so it is easier to implement high impedance lines
- in coupled lines, for the same coupling you have wider gaps = less problems with manufacturing tolerances
 
In order not to add stray capacitance, the components pads should be incorporated in the line; this means the line should be wide enough to accomodate the pad, from this point of view a thick substrate, if epsilon cannot be changed, helps.
 

I used to use 10mils substrate for 8-40G design.
10mils can be used to 40G, and it's better to design with only one substrate.
 

you can simulate ustrip line (in HFSS, ...) and find the cut off frequency for higher order modes. i think 10mils height is preferable.
 

What are the main microstrip based design recommendations:

suppose if i moved to ka band whether 10mil alumina substrate is recommended.

Is there any thumb rule which decide microstrip height and width with frequency?/
 

abhijitrc said:
What are the main microstrip based design recommendations:

suppose if i moved to ka band whether 10mil alumina substrate is recommended.

Is there any thumb rule which decide microstrip height and width with frequency?/
Click to expand...

My rule is to pick up a substrate height which is 10 times less than lambda, for example, if you use alumina H=10 mil so you operate until 40 GHz

By the way, I have never seen any hybrid RF circuit working above 20 GHz...you need a very very accurate process. If someone is willing to show us few realization that would be cool !
 

jayce3390 said:
By the way, I have never seen any hybrid RF circuit working above 20 GHz...you need a very very accurate process.
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Yeah, the high permittivity of alumina will lead to really small dimensions. I would prefer a substrate with lower permittivity, so that dimensions are larger and etching precision requirements are more relaxed. I've used 20mil RO4003 for a 24GHz radar circuit for the reasons posted in #4 above.
 

AN5408 from Avago may also provide another example on material used for similar frequency
 

I used RO4350B 10mil for 26.5GHz microwave point to point circuits.
 

tony_lth said:
I used RO4350B 10mil for 26.5GHz microwave point to point circuits.
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tony_lth,

When you design a PA at high frequency, do you use SMD passives on the RF path (capacitors, resistors) with your RO4350B ?
 

Yes, I used SMD passive components on the RF path, but they must be high enough SRF.
 

you must notice some points like dielectric constant , its best value for your application, and your frequencies
 

As per the basic microstrip/patch design, dielectric constant, resonant frequency and height of the substrate are given parameters and using this parameter we calculate W and L this length and width of microstrip/patch. It must be noted that formulas for microstrip and patch are little bit different.

And we cannot use any height for substrate because in market only specific heights are available and it is really expensive to get our required height of substrate. So for cheap/inexpensive design we must stick to basics.



- - - Updated - - -

As per the basic microstrip/patch design, dielectric constant, resonant frequency and height of the substrate are given parameters and using this parameter we calculate W and L this length and width of microstrip/patch. It must be noted that formulas for microstrip and patch are little bit different.

And we cannot use any height for substrate because in market only specific heights are available and it is really expensive to get our required height of substrate. So for cheap/inexpensive design we must stick to basics.

 

akc185 said:
And we cannot use any height for substrate because in market only specific heights are available and it is really expensive to get our required height of substrate.
Click to expand...

This is true for non-RF materials (standard FR4). For RF substrates from Rogers, Taconic etc. you can choose from a wide range of dielectric materials (different thickness, different permittivity, different metalization options).
 

what is the advantage of using alumina substrate over other soft substrate? My concern is basically for high frequency application eg ku/ka band respectively.plz give me a general guideline about this.Apart from this generally i have seen people are using kovar over which substrate has been pasted, what is the reason? Is it the coefficient of thermal expansion of kovar and the alumina are same!! kindly guide me.
 

yes, thermal expansion is a big issue. But the real fact is that different expansion can cause grounding issue. That is fatal.
 

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