Choice of DC blocking capacitors in Microstrip Design
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Posted by Alec on August 11, 2008, 11:48 am
Please log in for more thread options 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 | |||||||||||||||||||||||||
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Posted by John Larkin on August 11, 2008, 11:56 am
Please log in for more thread options On Mon, 11 Aug 2008 08:48:46 -0700 (PDT), Alec >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... > There's no reason for that rule. Most of the time a reasonable, like 0.33 uF, 0603 maybe, surface-mount cap has a very low RF impedance, essentially the same ESL as most any other 0603 cap. SRF doesn't matter, impedance does. A 0.33 uF 0603 cap is a very low z well into the GHz. Ideally, match the cap body width and the trace width. John | |||||||||||||||||||||||||
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Posted by Alec on August 11, 2008, 1:04 pm
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On 11 Aug, 16:56, John Larkin > On Mon, 11 Aug 2008 08:48:46 -0700 (PDT), Alec
> > >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... >
> There's no reason for that rule. Most of the time a reasonable, like > 0.33 uF, 0603 maybe, surface-mount cap has a very low RF impedance, > essentially the same ESL as most any other 0603 cap. SRF doesn't > matter, impedance does. > > A 0.33 uF 0603 cap is a very low z well into the GHz. Ideally, match > the cap body width and the trace width. > > John Hi, Is it the same for 0805 case or is there a difference. Thanks | |||||||||||||||||||||||||
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Posted by BobW on August 11, 2008, 1:05 pm
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> On Mon, 11 Aug 2008 08:48:46 -0700 (PDT), Alec
> >>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. [snip] >
> There's no reason for that rule. Most of the time a reasonable, like > 0.33 uF, 0603 maybe, surface-mount cap has a very low RF impedance, > essentially the same ESL as most any other 0603 cap. SRF doesn't > matter, impedance does. > > A 0.33 uF 0603 cap is a very low z well into the GHz. Ideally, match > the cap body width and the trace width. > > John > Whose 0.33uF do you use that is not well above its resonance at 1GHz? Bob -- == All google group posts are automatically deleted due to spam == | |||||||||||||||||||||||||
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Posted by Alec on August 11, 2008, 1:19 pm
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>
> > > On Mon, 11 Aug 2008 08:48:46 -0700 (PDT), Alec
>
> >>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. >
> [snip] > > > > > There's no reason for that rule. Most of the time a reasonable, like
> > 0.33 uF, 0603 maybe, surface-mount cap has a very low RF impedance, > > essentially the same ESL as most any other 0603 cap. SRF doesn't > > matter, impedance does. >
> > A 0.33 uF 0603 cap is a very low z well into the GHz. Ideally, match
> > the cap body width and the trace width. >
> > John
>
> Whose 0.33uF do you use that is not well above its resonance at 1GHz? > > Bob > -- > == All google group posts are automatically deleted due to spam == AVX have capacitors whose SRF =950MHz at 47pF for 0805. 33pF --> srf =
880MHz for 1210
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