Choosing Capacitors for Rf Applications - Page 1
Forum section: electronics / rf-microwave

author: Sparky49
author_rank: Regular Contributor
timestamp: March 04, 2021, 06:08:24 pm
content: I'm wanting to replicate a circuit from this (https://www.minicircuits.com/app/AN60-064.pdf) appnote for a LNA for use up to 1GHz. A couple of components for DC blocking are rated at 10nF (C1 and C2). However, when I search suppliers for a 10nF capacitor, I find the 'RF' rated ones are quite expensive - more than a dollar in price each. When I look at the cheaper smt capacitors, I find that the few that have plots of effective capacitance and self resonant frequency show that the cheaper caps resonate at around 100 MHz or less.Is this a simple factor I need to suck up, or am I missing how to choose/use the cheaper components in sub-GHz circuits? It seems to me that the more expensive 'RF' rated devices are suitable up to at least 3 or 4GHz, which is a bit overkill for my application, but I can't seem to find anything with a self resonant frequency higher than a 100MHz or so from cheaper devices - it seems like there's no middle ground.Thanks in advance for any suggestions/explanations.

author: TimFox
author_rank: Super Contributor
timestamp: March 04, 2021, 06:16:29 pm
content: There are expensive capacitors designed for your application that are rated as “mode free” up to the GHz range. Normal units around 10 nF, as you have found, have first self-resonant (series resonance with self-inductance) frequencies well below 1 GHz. I would be curious to know construction details, but they may be proprietary.

author: mawyatt
author_rank: Super Contributor
timestamp: March 04, 2021, 06:47:48 pm
content: You can still use capacitors past the self-resonate frequency (SRF) for decoupling and coupling applications where the capacitance is just an "impedance" and the lower the better. How far above the SRF depends on the application. A quick look at the impedance vs. frequency plot should show what to expect from your cap, and remember in the actual use the mounting and PCB traces will add additional inductance and lower the resonance below the cap SRF.For tuned circuits and such then the SRF becomes a barrier, then more expensive higher quality caps are required.Hope this helps,Edit: Should mention long ago we often used and even selected caps near SRF for decoupling, because these would tend to be smaller than capacitors with better ceramic materials. Small size was a limiting requirement in many applications and we would measure certain vendors caps and at the used supply voltage to verify the SRF and proper cap parameters. Not recommending this procedure for production use, unless one has a compelling requirement, because these selected caps required special spec control drawings, and verification tests for each lot. So the actual cap cost was much higher than getting a better quality cap in the first place...but in our case those better caps were just too big!!Best,

author: Sparky49
author_rank: Regular Contributor
timestamp: March 04, 2021, 07:04:06 pm
content: I see, thanks Tim and mawyatt - I suppose it makes that the SRF isn't necessarily too important in a flitering/DC block application - as you say you want the impedance to be low. Though the other issue is capacitance vs frequency , correct? I may try something like thishttps://product.tdk.com/info/en/documents/chara_sheet/C1608C0G1E103J080AA.pdf- any potential problems?

author: TimFox
author_rank: Super Contributor
timestamp: March 04, 2021, 07:18:28 pm
content: Also, note that there is a second resonant frequency above the series-mode resonance that is a parallel-resonant mode, where the high impedance would be a problem in a coupling application. I used to use porcelain capacitors from ATC, who published full characteristics on their (expensive) components, including both the series and parallel resonant frequencies. Unfortunately, I don’t recall the manufacturer of the mode-free capacitors I bought for a high-speed data application, but I remember that they were otherwise mediocre, i.e. lowish Q, but had very impressive high frequency limits compared with SMD capacitors of the same physical size. Ideal for coupling, but low performance in a tuned circuit.The capacitance vs. frequency graph in the TDK data sheet is just another representation of the series-resonant mode: on a test fixture calibrated for capacitance, as the total reactance falls approaching resonance, the capacitance value increases, then changes sign (inductive reactance) above resonance (not shown on that graph).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 04, 2021, 09:28:51 pm
content: Well, there are two things. First of all, the SRF is largely due to the capacitor's body length as measured in a test jig. If you connect it in circuit with a different layout (particularly with length of connecting traces/vias), you'll get a lower SRF.The SRF doesn't mean anything magical, or even very intrinsic about the part itself. It simply is where, given a part of that length and in the test jig used, it has its lowest impedance.If that lowest impedance is adequate, and the impedance at nearby frequencies is also adequate, and it's not resonating with other things in your circuit -- that's it, you're done, nothing more to be concerned about!The other thing is if the capacitor has any higher resonant modes. Consider an MLCC is a stack of interleaved layers; it is effectively a massive parallel stack of very short transmission line stubs. At low frequencies, the equivalent impedance of all those in parallel, is very low, i.e., capacitive -- hence it's a useful capacitor. Near resonant frequencies, you may get peaks or troughs due to resonances between these stubs. Again, these frequencies depend on the dimension of the part; it may be in the 10s of GHz.Or maybe you don't really much weirdness at all, because the Q of those modes is going to be low, due to the low [transmission line] impedance of a given layer, its relatively high DCR (the metallization is very thin), and the high losses of type 2 dielectric if applicable.You choose a more specialized type, when these are a concern. Very precise frequency response may demand this of bypass caps. Filters and matching networks, with high Q or at high power, are demanding of the resonant caps used in them, which may rule out the relatively high DCR of jellybean C0G (type 1 dielectric) or similar types, necessitating an RF type.For coupling of low frequency (<GHz) signals, yep, doesn't matter one lick. The SRF completely disappears when the capacitor is part of a transmission line; indeed, if the component body width and height above ground plane are judiciously chosen, what manifests at low frequencies/impedances as its stray inductance, is simply matched transmission line of its body length. That is to say, if the body/pad widths are the same as a 50 ohm trace, and the capacitor is placed inline with such a trace: it is nothing more than another length of that trace, and maintains the same impedance all the way through. The only reason it shows up as inductance in another context, is because at low frequencies and low impedances, that length of 50 ohm (or whatever) transmission line manifests as equivalent inductance.At really low frequencies (~audio), you will probably find a different problem: piezoelectricity, coupling mechanical vibrations (strain and especially flex) of the PCB, into electrical signals. Typically on the order of ~mV. It is recommended to use film capacitors for audio coupling and other sensitive / low noise applications.Tim

author: TimFox
author_rank: Super Contributor
timestamp: March 04, 2021, 09:47:09 pm
content: The book “RF Capacitor Handbook”, which can be ordered through the “Technical Notes” page on atceramics.com, is a good reference for capacitors in your frequency range.

author: rfclown
author_rank: Frequent Contributor
timestamp: March 05, 2021, 05:35:31 am
content: Quote from: Sparky49 on March 04, 2021, 07:04:06 pmI see others have already answered your question well. Yes, the cap in this datasheet is fine, and is the type of thing the reference design is expecting that you'll use. You can figure that SMT caps from 0402 to 0805 have roughly 0.5 nH of inductance. That would match the resonance the datasheet is showing. If you calculate the reactance, at 50 MHz it would be -0.2 ohms, and at 1GHz it would be +3.1 ohms. You want this to be significanly less than 50 ohms, which it is. If you didn't need to go down to 50 MHz, a smaller cap (say 100pF) could be better for something like 500-1000 MHz (which would give -1.6 to +1.6 ohms reactance).You want C0G/NPO (which this datasheet cap is). Small value caps usually are, but something as big as 10nF might not be.Like T3sl4co1l said, it's good to match the width of the cap to the microstrip trace width.For low power applications, the RF caps from KEMET and Johanson aren't expensive. Using KEMET as an example, I just looked at Digikey for a 27pF 0402 50v 5% NPO. Their regular part is $0.10 for one, $0.034 per 100. Their RF part is $0.18 for one, $0.064 / 100. These aren't used for broadband DC blocks though since they don't come in large values. I've only used ATC caps for high power amplifiers.So you're covered for the DC blocking cap... However, the RF choke is a different matter. That broadband Mini Circuits part is $3.65 with a minimum quantity of 20 (ouch). You'll sometimes see datasheets of wideband gain block amplifiers showing different recommended RF chokes for different frequency ranges.

author: salbayeng
author_rank: Frequent Contributor
timestamp: March 05, 2021, 09:47:06 am
content: I would try using NPO capacitors for your bypassing, they tend to be physically larger than X7R caps, so should have a lower inductance. your PCB layout will probably have a larger effect on resonances. if you use a pair of capacitors bridging the trace to ground on the left and the right side, the self inductances cancel to a large extent.And as RFclown says above, finding good inductors is harder than good capacitors.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: March 05, 2021, 11:07:39 am
content: Quote from: salbayeng on March 05, 2021, 09:47:06 amA physically larger capacitance will have more inductance. you want the largest capacitor in the package you have to use.An important point can be linearity - class 2 ceramics (X5R, X7R) etc have lower linearity. This can be important when DC-blocking to power amplifiers or LNAs with blockers present, where your linearity requirement might be high enough such that you have to take that into account.

author: Sparky49
author_rank: Regular Contributor
timestamp: March 05, 2021, 03:25:26 pm
content: Thanks again everyone - all really useful answers!I'll stick to the 0603 device, as my microstrip is supposed to be 38mil (for a 0.53mm high, 4.5 Er PCB), so that should be roughly the same width. thanks also for the nod to the capacitor book - will request copy. And also thanks for the other device suggestions and explanations - very helpful.I'll come back and post some measurements when everything arrives.

author: oliviasmithh900
author_rank: Contributor
timestamp: March 07, 2021, 01:17:29 am
content: 0306 device, I just did a search, wow its so small

author: rfclown
author_rank: Frequent Contributor
timestamp: March 07, 2021, 02:46:46 am
content: Quote from: oliviasmithh900 on March 07, 2021, 01:17:29 amFor small SMT parts, you need a microscope and fine tweezers; it's just part of the game. But once you have that, 0603 really isn't small. 0201 is small and difficult to work with by hand (at least for me). 0402 is my goto size unless I have a reason not to use it (power for resistors, voltage for caps, etc).