Choice of DC blocking Capacitors in Microstrip Design
 

Hi,

I was wondering as to how to choose the correct value of a DC blocking
capacitor with least possible reactance with atleast 5V and 20mA.
Center Frequency 910MHz. The value of capacitor that i need is above
200pF..

I know the thumb rule that SRF(self resonant frequency) 2 Operating
frequency. but i enquired both Farnell and Rscomponents distributors
in UK as am here .., Only low value capacitors satify the above
criteria of SRF. How do i proceed with the same...

Thanks a lot
Alec

Choice of DC blocking Capacitors in Microstrip Design
 

I was wondering as to how to choose the correct value of a DC blocking
capacitor with least possible reactance with atleast 5V and 20mA.
Center Frequency 910MHz. The value of capacitor that i need is above
200pF..

I know the thumb rule that SRF(self resonant frequency) 2 Operating
frequency. but i enquired both Farnell and Rscomponents distributors
in UK as am here .., Only low value capacitors satify the above
criteria of SRF. How do i proceed with the same...

Thanks a lot
Alec

Well, you might try paralleling a bunch of little ones.

W4ZCB

Choice of DC blocking Capacitors in Microstrip Design
 

Alec wrote:
Hi,

I was wondering as to how to choose the correct value of a DC blocking
capacitor with least possible reactance with atleast 5V and 20mA.
Center Frequency 910MHz. The value of capacitor that i need is above
200pF..

I know the thumb rule that SRF(self resonant frequency) 2 Operating
frequency. but i enquired both Farnell and Rscomponents distributors
in UK as am here .., Only low value capacitors satify the above
criteria of SRF. How do i proceed with the same...

Thanks a lot
Alec

I'm confused. Why do you care what the capacitance is? If you choose
a capacitor that is series resonant at 910 MHz (i.e., the reactance is
zero), isn't that is the best bypass capacitor or coupling capacitor
that you can get.

If you are doing high volume work, you may want to standardize on a high
quality (porcelain, etc) capacitor from single manufacturer so that the
SRF is tightly controlled.

If you need the 200pf for DC (or low frequency) reasons, consider two
different capacitors - one for low frequencies and one for high
frequencies. You'll need to isolate them so they don't interact
unexpectedly.

I doubt whether you'll find a 200pf cap with that SRF.

Bob.