| Electronics > Beginners |
| High frequency capacitor selection |
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| ZeroResistance:
--- Quote from: helius on August 14, 2018, 10:02:12 am ---The permissible peak voltage typically decreases with higher frequencies, as the capacitor's impedance decreases until the resonant frequency is reached. The specific phenomenon that weakens the voltage withstood is called "dielectric loss". --- Quote from: From Kemet's white paper What Is A Capacitor? ---DF [dissipation factor] is frequency-dependent and displays a small increase with increasing frequency. --- End quote --- --- End quote --- Ok, so all capacitors will face this roadblock right? At higher frequencies eventually their Xc will be a limiting factor right? For eg. for a 1nF capacitor the Xc comes to 160 ohm for a frequency of 1Mhz. So how come the ceramic capacitors are used for RF power at frequencies in the 10's of Mhz to 10's of Ghz? |
| iMo:
--- Quote ---At higher frequencies eventually their Xc will be a limiting factor right? For eg. for a 1nF capacitor the Xc comes to 160 ohm for a frequency of 1Mhz. So how come the ceramic capacitors are used for RF power at frequencies in the 10's of Mhz to 10's of Ghz? --- End quote --- All types of capacitors have a quality Q. The quality depends on "parasitic" serial/parallel R, L, and frequency. The same with ceramics for 0.01-100GHz. The 1nF polypropylen bulk Wimo could have 500nH L, 2000ohm R (because of its mechanical construction). The ceramics for xxxxMHz are of different construction - those are leadless pieces of ceramics of special shapes. The 1nF may have 0.xxnH L and 0.xxohm R for example. |
| Alex Nikitin:
--- Quote from: ZeroResistance on August 14, 2018, 10:21:00 am --- --- Quote from: helius on August 14, 2018, 10:02:12 am ---The permissible peak voltage typically decreases with higher frequencies, as the capacitor's impedance decreases until the resonant frequency is reached. The specific phenomenon that weakens the voltage withstood is called "dielectric loss". --- Quote from: From Kemet's white paper What Is A Capacitor? ---DF [dissipation factor] is frequency-dependent and displays a small increase with increasing frequency. --- End quote --- --- End quote --- Ok, so all capacitors will face this roadblock right? At higher frequencies eventually their Xc will be a limiting factor right? For eg. for a 1nF capacitor the Xc comes to 160 ohm for a frequency of 1Mhz. So how come the ceramic capacitors are used for RF power at frequencies in the 10's of Mhz to 10's of Ghz? --- End quote --- http://www.atceramics.com/technical-notes.aspx Cheers Alex |
| iMo:
@ZeroResistance: better you define the required capacity (in nF/uF) at specific frequencies, max currents (Ip-p), voltages(Vp-p), and other params you are aware of, and the experts here may identify the proper caps for you easier. |
| dmills:
The reason the applied voltage limit drops with frequency, is that you are really hitting a current limit! Xc drops with increasing frequency, so the voltage you can apply without exceeding the maximum rated current (Yes, there always is one) drops with increasing frequency. What you probably want are 'doorknob' capacitors used in high power transmitters, these are specified for RF current handling and voltage. ATC make excellent parts but mainly for higher then the wiggly DC that is 13.56MHz. Regards, Dan. |
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