T
Tom Derham
- Jan 1, 1970
- 0
I have seen it written many times that for dc blocking (e.g. series cap on
the input to a microwave amplifier), the cap is chosen as large as possible
such that the impedance is a fraction of an ohm at the operating frequency,
but the series resonant frequency must still be well above the top operating
frequency.
However, in practice I can't find a value to meet both these requirements.
e.g. at 2.4GHz, an "ideal" cap at 100pF would give fraction-of-an-ohm
impedance, but all real caps of this size have their resonant frequency well
below this frequency and so are inductive in the range of operation...
although the (negative) impedance is still fairly small at this range (maybe
a few ohms).
So how is a blocking cap chosen? The ESR of a good microwave cap (e.g. AVX
AccuP) might be only 0.2 Ohms, but this is swamped by the reactive impedance
everywhere but at the resonant frequency!
Lastly, if the application is only for a fixed frequency, is it fair game to
spec a capacitor for use *at* its resonant frequency? I can see this is not
wanted in filters because it could lead to instability, but is it ok for dc
blocking purposes?
Thanks
Tom
the input to a microwave amplifier), the cap is chosen as large as possible
such that the impedance is a fraction of an ohm at the operating frequency,
but the series resonant frequency must still be well above the top operating
frequency.
However, in practice I can't find a value to meet both these requirements.
e.g. at 2.4GHz, an "ideal" cap at 100pF would give fraction-of-an-ohm
impedance, but all real caps of this size have their resonant frequency well
below this frequency and so are inductive in the range of operation...
although the (negative) impedance is still fairly small at this range (maybe
a few ohms).
So how is a blocking cap chosen? The ESR of a good microwave cap (e.g. AVX
AccuP) might be only 0.2 Ohms, but this is swamped by the reactive impedance
everywhere but at the resonant frequency!
Lastly, if the application is only for a fixed frequency, is it fair game to
spec a capacitor for use *at* its resonant frequency? I can see this is not
wanted in filters because it could lead to instability, but is it ok for dc
blocking purposes?
Thanks
Tom