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I'm using a power amplifier that requires an external blocking capacitor for the input and output ports and I'm trying to decide the best value to use. My understanding of this is that I choose a capacitance that is sufficiently large such that the 3dB frequency is a good deal lower than my lowest frequency of operation (answered here). I also imagine that it's important to choose a capacitor with the smallest possible ESL. Are there any other considerations that are important here? What about RF currents heating such small caps?

So, I'm thinking of using a 0402 100pF cap which, given my lowest frequency of 5.3GHz and min Rload of 6ohm, should put the corner frequency around 250MHz, well below the 5.3GHz. Does that seem sensible?

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  • \$\begingroup\$ how about any resonance with the solder pads? \$\endgroup\$ – analogsystemsrf Jan 12 at 3:09
  • \$\begingroup\$ hum ... 6 Ohms? . that is 6 dBm @ 50 Ω \$\endgroup\$ – Sunnyskyguy EE75 Jan 12 at 3:18
  • \$\begingroup\$ whoops.. thanks for catching that @SunnyskyguyEE75. So 50ohm puts corner frequency at 30mhz, so i guess no need to use a cap that high. maybe 10pF makes more sense \$\endgroup\$ – MattHusz Jan 12 at 3:33
  • \$\begingroup\$ I hadn't considered that, @analogsystemsrf. What would I do to calc the effect of that? \$\endgroup\$ – MattHusz Jan 12 at 3:36
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You want an NPO Cap with a SRF >> 8GHz and ultralow ESR and ESL at 6GHz.

Not any 100 pF 402 cap will do.

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  • \$\begingroup\$ I'd like to know what is incorrect about this statement. Does he want me to do his searching? Although it appears this is his 1st rodeo in 6Ghz and microwave design. \$\endgroup\$ – Sunnyskyguy EE75 Jan 12 at 4:46
  • \$\begingroup\$ It was useful to me and I definitely do not expect you to do the searching for me. However, if you have any general recommendations for determining ESR/ESL at specific frequencies that would be great. I'm having trouble finding those numbers in the datasheets. And yes, this is my first RF design.. \$\endgroup\$ – MattHusz Jan 12 at 5:24
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    \$\begingroup\$ @MattHusz it might not be on the datasheet, sometimes you can get more detail by searching the part number on the manufacturer's website. For 8 GHz you might need to look to specialty vendors like ATC or Johansson. Or a broad line vendor like Murata might have a specialty part. \$\endgroup\$ – The Photon Jan 12 at 15:30

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