Choosing capacitors fo RF

[grapeman]

Hi there,

I have been reading around about capacitors in RF design. I would like to find a suitable capacitor to use as an in series DC block. I understand the self resonance frequency needs to be above the upper working frequency and the reactance needs to be low for the working frequencies. I'm having trouble finding a capacitor to meet my needs though.

For example:

Working frequencies: 1.4-1.6GHz
From this I want the reactance to be less than 1ohm at 1.6GHz so the capacitor should be around 220pF. However finding a 220pF capacitor and have a self resonance frequency above 1.6GHz apparently doesn't exist. What do people recommend?

 

[mtwieg]

Depending on your application it may be fine for the capacitor to have a fsr at or even below the working frequency, so long as you take its actual impedance into account when designing the circuit. The closest to your spec I can find is the ATC600S series which shows fsr around 1.6GHz at 100pF.

 

[ferdem]

I m not aware of typical SRF values of the capacitors but I do not have problem with dc blocking capacitors(typical values 1nF,10nF,100nF) up to 6 GHz. As I said, I do not know their SRF. However, I guess that even though the SRF is exceeded, they may still show low enough impedance not causing significant attenuation.

 

[jiripolivka]

There are specific designs of ceramic capacitors that can be used to high frequencies. Check maker's specifications.
Even with such precaution I have found that some "RF" SMD ceramic caps resonated around 2...3 GHz, so the solution was to use another type or stack two SMD caps in parallel, to move the resonance higher.

 

[D.A.(Tony)Stewart]

A few deci-dB is usually adequate for all applications for S21 insertion loss. ( i.e. 0.1~0.3dB). Often this requires operating above the SRF ( or fSR) and near the PRF *( or fPR). As you may recall, the equivalent circuit has 2 caps, Cs and Cp and thus a series resonance ( Z min ) and many parallel resonances which show up as notches in the s21.

Normally I would choose the smallest case size , or 402 in 47 pF and consider 2 in parallel.(stacked)

But ATC who specializes in microwave Caps suggests using their 1nF ceramic cap.

Read why., It has to do with X7R lossy high dielectric constant properties and same criteria above for 50 MHz to 3.,6 GHz

I also stated in previous questions that vertical mounting of microwave caps eliminates all the odd PRF notches including the 1st. ( easier said than done)

For exceptionally broad spectrum AC coupling or DC decoupling noise, the old method of using 3 caps 2 decades apart still holds. such as case 10pF, // 100pF //0,1uF.

More good choices come from Murata, if you working high volume. GQM and GJM for different applications.
You may need to import the s parameters and work with those.

 

[volker@muehlhaus]

Often this requires operating above the SRF ( or fSR) and near the PRF *( or fPR).

Why would we want to go above SRF to a frequency "near PRF"?