Paging the guys with VNA’s - Page 1
Forum section: electronics / projects

author: Scott in Michigan
author_rank: Contributor
timestamp: April 28, 2015, 02:38:28 am
content: Paging the guys with VNA’s.Would anyone have a recommendation for a physically small 0.1 uF capacitor with S.R.F. of at least 10 Mhz?I’m searching for a 0.1 uF coupling cap to use in a miniaturized 7 Mhz H.F. QRP amplifier I’m building.Manufacturer graphs show several that should be adequate but when I sweep them with my VNA the SRF is well below the frequency I need them to behave at. I’ve swept samples of about 10 or so different disc cap’s in different packages and even an SMT package, but they all go inductive by about 5 Mhz.I was able to find decoupling cap’s in 0.01 uF size that are still nicely capacitive without much value change at the frequency of interest, but the larger 0.1 uF value is evading me.Thanks,Scott

author: ConKbot
author_rank: Super Contributor
timestamp: April 28, 2015, 06:12:07 am
content: Was your test setup constructed properly calibrated? I.e. 2 microstrips, and do a though cal, or a proper OSLT, with one microstip, and the cap in series on the other. Or just a single stripline, bare, calibrate to that, then solder the cap from the stripline to ground. 10 MHz shouldn't be that big of a deal for even generic 0.1 uF 0402 MLCC's even without getting one of the 'sideways' (0204/0306/0508) low ESL types.

author: Scott in Michigan
author_rank: Contributor
timestamp: April 28, 2015, 12:43:29 pm
content: No sir, test fixtures are not the problem. Each one has a correlating calibration file I load before using.Also, I can see SRF's in the 100 Mhz range for some silver mica capacitors, and the 0.1 uF decoupling caps all have SRF's above 10 Mhz.It's the pesky 0.1 uF coupling cap's I've encountered so far that have SRFs below what the manufacturer's spec sheets state they should.So, my question, has anyone with a VNA swept 0.1 caps and can recommend one?

author: mzzj
author_rank: Super Contributor
timestamp: April 28, 2015, 01:22:23 pm
content: 5Mhz SFR would indicate approx 13nH of series inductance. That is about a ½ inc of wire.What physical size your "disk capacitors" are and how big wiring loops you got?

author: Scott in Michigan
author_rank: Contributor
timestamp: April 28, 2015, 05:40:00 pm
content: Lead length is equivalent to the thickness of a PCB. I know where you guys are headed, but it's not the jig or excessive lead length. Remember, I can see SRF's in excess of 100 Mhz w/ a quality silver mica. What I need is someone who has a VNA who has swept some .1's to share the P/N of a low SRF device.

author: Scott in Michigan
author_rank: Contributor
timestamp: April 28, 2015, 05:42:48 pm
content: Correction to my post above regarding the decoupling caps that sweep > 10 Mhz: they are 0.01 uF.

author: ruffy91
author_rank: Regular Contributor
timestamp: April 28, 2015, 05:47:39 pm
content: Try smaller packages. A 0402 should give you a srf above 10MHz.

author: mzzj
author_rank: Super Contributor
timestamp: April 28, 2015, 05:52:12 pm
content: Quote from: Scott in Michigan on April 28, 2015, 05:40:00 pmBut I doubt that your silver micas are 0,1uF in capacitance...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 28, 2015, 06:09:04 pm
content: You're asking for < 2.5nH.What circuit could you possibly connect this to that would have a loop that small? What's more important: what circuit could you possibly be building that's so horribly designed that the sign of a 0.16 ohm reactance is critical!?The purpose of a constant voltage type supply rail is not to be capacitive. It is to reflect AC signals. As long as the impedance is much lower than the system impedance, it will do its job, regardless of the angle of its impedance.Tim

author: mzzj
author_rank: Super Contributor
timestamp: April 28, 2015, 07:11:04 pm
content: Quote from: ruffy91 on April 28, 2015, 05:47:39 pmEven a 1206 should be above 10Mhz if you mount it with minimal inductance.I did try 100nF (more like 80nF measured) 1206 cap soldered directly to ground plane with just a kapton tape under the cap. Got a 20Mhz srf so that indicates about 1nH inductance. Pretty much in line with published fiqures ie AVX appnotes.

author: Scott in Michigan
author_rank: Contributor
timestamp: April 28, 2015, 11:52:04 pm
content: Mzzj - I mention the silver mica to show my test jig behaves at higher frequencies. Yes, a silver mica of that capacity would be larger than my entire circuit board. Ordered some 1206's on your advice. Problem w/ going any smaller is - I can't see nor handle them.

author: G0HZU
author_rank: Super Contributor
timestamp: April 29, 2015, 12:25:07 am
content: I agree with Tim. I think you are worrying unecessarily about the performance of a typical 100nF disk ceramic cap at 7MHz.i.e. it seems odd that you are concerned about this when you seem to just want a typical decoupling cap for a low power 7MHz transmitter.I'd be happy to pick a typical 100nF disk cap for this and I'd not be too concerned about the SRF. However, I'd expect the SRF to be approx 10MHz if the cap is fitted snugly. However, a typical 1206 100nF cap would have an SRF up around 15MHz. But I don't think this is that relevant for your task. I would consider a 100nF ceramic disk cap to be ideal if you wanted to use a leaded part. But if you want to use SMD then use 1206 as these are fairly easy to solder.Your biggest worry with 1206 caps (if you are new to SMD) will be PCB flexing because they can develop microscopic cracks if you handle the PCB a lot and it has flex in it. If they crack then they become a long term time bomb for failure.1206 caps often crack easily because they are long and thin. You won't be able to see the cracks as they happen internally and the part will become intermittent over time.But don't be too put off by this. Just be careful not to flex the PCB. One common way to design badly with a 1206 cap is to place it (as a series DC blocking cap) very near an RF connector on a PCB that isn't rigidly held down. So after a few fittings and removals of the RF connector the 1206 cap will crack due to the PCB flexing that occurs as the connector is fitted by hand.

author: mzzj
author_rank: Super Contributor
timestamp: April 29, 2015, 06:40:30 am
content: Quote from: Scott in Michigan on April 28, 2015, 11:52:04 pmNot really a proof that your jig behaves, more like a clue that it is misbehaving.You said that 0,01uF behaves nicely with above 10Mhz SRF. well thats no supprise because for the same jig stray inductance it should have SRF somewhere 15Mhz if your 0,1 has a 5Mhz SRF.and your 100pF(just example value) silver mica would be abt 150Mhz SRF with the same jig inductance.

author: Scott in Michigan
author_rank: Contributor
timestamp: April 29, 2015, 11:01:58 pm
content: Mzzy - The OSL calib process renders the test fixture capacitance and inductance essentially transparent.G0HZU - Copy on the microscopic cracks in 1206 due to flexing. I learned that lesson the hard way but still appreciate the tip. Can you offer evidence though, that seeking a cap w/ SRF > the frequency of interest is unneccessary concern? Seems that is actually a prudent concern.

author: G0HZU
author_rank: Super Contributor
timestamp: April 29, 2015, 11:53:32 pm
content: Quote from: Scott in Michigan on April 29, 2015, 11:01:58 pmI don't think it matters for a low power 7MHz transmitter and I think Tim gave good reasons why alreadyBut if you want a real world commercial example you can look at numerous CB radio schematics to see that they often use 47nF or 100nF ceramic disk caps as decouplers on the PA power rail up at 27MHz at 12 Watt power levels. The mag Z of such a cap is still going to be in the order of 1 ohm or less provided it is fitted tightly with short legs.However, there is some technical risk if the cap has an SRF that is at the frequency of interest on a high power transmitter because the cap may get hot. But in your case there won't be any risk at all. If you are still concerned then go to a 47nF cap instead. But a decent 1206 100nF X7R cap should keep below 0.5 Ohm magZ right through to 50MHz or more even though the SRF is going to be just under 20MHz typically.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 30, 2015, 02:06:11 am
content: I'd have to see your circuit to really know for sure, but it's pretty easy to figure out based on the expected voltages, impedances, etc. in the circuit.The most revealing facet should be the most apparent: if you're connecting to this capacitor with other leaded components (or traces), surely those inductances add to the intrinsic inductance of the capacitor alone? In which case, how few components do you need before your condition (SRF > Fo) is completely blown to hell from unavoidable geometry alone? And surely the same concern must befall the commercial guys, yet they don't seem to suffer such problems..?It always bothered me that there is such a stated property as "SRF". The inductance is essentially proportional to length dimensions, so it varies directly and inseperably with circuit layout. Perhaps you can determine this figure in a fixed, standardized jig situation, but who defines that standard, how accessible is it (open vs. free; expensive spring clamps vs. carved hunk of FR-4?), and how useful/applicable is it (does it work on 1206s but not 0603s? Do you need a different one for disc types?)?So, fortunately, the answer is: no, it does not matter, and you don't have to worry about it. A 1uF 0805 hasexactly as low impedanceas a 0.01uF 0805 at high frequencies. There is no need to use smaller caps in parallel to "distribute resonances" (indeed, this can make thingsmuch worseat certain frequencies).The general design of RF circuitry is to have signals transmitted with relative freedom along the signal paths, to have them well blocked by supply and ground nodes (where applicable), and either pass or block when signal and bias cross paths (i.e., bias tees, coupling capacitors.. those sorts of things).Passage is achieved by matching the impedances of source, load, transmission line, filter, etc. (Often this impedance is something other than the small signal, maximum power point, because active devices often have input and output impedances very different from a more practical impedance, like the highest-undistorted-power point. Resistors often get involved to modify those dynamic impedances -- example, a grounded-base amplifier has an input impedance of approximately zero ohms, so that even with a matching transformer, some damping resistance is worthwhile.)Blockage is achieved by using an intentional mismatch: very high or very low impedances.It doesn't matter what the angle of that impedance is, so long as the result is very different from system impedance.Indeed, part of the power supply network need not even be low inductance at all; a 1/4 wave trap or stub can be used to make an open or short circuit at the system frequency (assuming it's an acceptable approach for the inevitable harmonics and parasitics of the system, as well). This approach is frequently used in microwave microstrip construction, where brute force bypass capacitors simply aren't possible (the body length of the capacitor is simply more transmission line length!).That said, maintaining a sufficiently low, and dissipative (lossy, resistive, attenuating) power supply network is an important goal, since you don't want that transmitter power feeding back to earlier stages (let alone to the receiver, if they're in common), or anything like that. Which usually means maintaining nice low impedances with lots of bypass caps, and also punctuating that with chokes and whatnot (hopefully with some lossy electrolytics or anything else with ESR to dampen the LF resonances thus created).Tim

author: mzzj
author_rank: Super Contributor
timestamp: April 30, 2015, 06:22:59 am
content: Quote from: Scott in Michigan on April 29, 2015, 11:01:58 pmShould. yes. Without actually seeing or knowing your text fixtures cant really comment on that. How did you calibrate the "short" in your fixture?VNA-wrestling aside I agree with Tim and G0HZU that it probably wont matter anyway.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: May 10, 2015, 01:34:04 am
content: PHOTOS, screenshots? ... or it did not happen