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Bypass capacitor value selection for RF application

arr_baobao

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how do i design the bypass capacitor on the dc biasing line for an amplifier.
how to i decide the value i should used to attenuate high frequency noise from dc line going in the the frequency of interest, for example gps 1575, cell low band of 800Mhz, cell high band of 2100Mhz.

should i choose the capacitor with low or close to 0 reactance value so that i will provide a low impedance path for the noise to go back to ground?
 

dick_freebird

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To begin with you want a capacitor with a SRF >>
frequency band of operation. If the amp is a
switching type then SRF needs to be even higher,
to supply the edge harmonics that supply inductance
would attenuate.
 

BradtheRad

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should i choose the capacitor with low or close to 0 reactance value so that i will provide a low impedance path for the noise to go back to ground?
There is some amount of neighboring resistance (whether unseen or unknown), which creates an RC time constant with the capacitor. Therefore as a rough calculation, find the C value which has much less impedance than the R value at the noise frequency. Such that the capacitor channels away, say 80 or 90 percent of it.
 

arr_baobao

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Useful freq range for some typical SMD capacitors.JPG

so i can use the table above as reference to design my bypass cap for different frequency of interest?
 

dick_freebird

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There is a lot of variation in the R and L of the
capacitor and the table is only anecdotes
arranged in a grid.

I'd bet that some of the big capacitor vendors
and maybe distributors, have "chooser" tools.
But you have to know what you're looking for.

It would be good if your amplifier circuit design
simulations included the power network in the
detail that would let you assess different cap
parasitic values. And good luck.

Now if you're already in module design then
I guess you go source some development kits
of assorted values that the cap vendors put
out. You can find some at distributors and I
have found that vendors often have direct
order under their applications pages or the
product family pages. And you build some and
see whose caps work for you and whose don't.
And then stake that down hard in the drawings.
 

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