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?

  • \$\begingroup\$ how about any resonance with the solder pads? \$\endgroup\$ – analogsystemsrf Jan 12 '19 at 3:09
  • \$\begingroup\$ hum ... 6 Ohms? . that is 6 dBm @ 50 Ω \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 12 '19 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 '19 at 3:33
  • \$\begingroup\$ I hadn't considered that, @analogsystemsrf. What would I do to calc the effect of that? \$\endgroup\$ – MattHusz Jan 12 '19 at 3:36

You want an NPO Cap with a SRF >> 8GHz and ultralow ESR and ESL at 6GHz.

Not any 100 pF 402 cap will do.

|improve this answer|||||
  • \$\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\$ – Tony Stewart Sunnyskyguy EE75 Jan 12 '19 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 '19 at 5:24
  • 1
    \$\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 '19 at 15:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.