Decoupling cap SRF

[Andrew Holme]

I just tested some ceramic caps on an AIM 4170 impedance analyser:

0603 10nF 50V X7R
SRF=47.6 MHz
Z @ 22.5 MHz: Xs=-0.557, Rs=0.083

0603 10nF 25V COG
SRF=37.7 MHz
Z @ 22.5 MHz: Xs=-0.401, Rs=0.035

0603 100nF 16V X7R
SRF=8.7 MHz
Z @ 22.5 MHz: Xs=+0.304, Rs=0.062

I'm building a 22.5 MHz IF amplifier and can't decide what decouplers to
use. The 100nF has the lowest impedance, so I suppose it would make the
best decoupler; but its SRF is less than half the operating frequency!
Which would you use?

TIA

 

[John - KD5YI]

I just tested some ceramic caps on an AIM 4170 impedance analyser:

0603 10nF 50V X7R
SRF=47.6 MHz
Z @ 22.5 MHz: Xs=-0.557, Rs=0.083

0603 10nF 25V COG
SRF=37.7 MHz
Z @ 22.5 MHz: Xs=-0.401, Rs=0.035

0603 100nF 16V X7R
SRF=8.7 MHz
Z @ 22.5 MHz: Xs=+0.304, Rs=0.062

I'm building a 22.5 MHz IF amplifier and can't decide what decouplers to
use. The 100nF has the lowest impedance, so I suppose it would make the
best decoupler; but its SRF is less than half the operating frequency!
Which would you use?

TIA

Find a capacitor that is series resonant at 22.5 MHz. From your data, it
looks like a 0603 between 10nF and 100nF might get there.

The UHF/microwave guys call them J0 bypass or J0 DC block capacitors.
The J0 DC block capacitors they use for input/output DC blocking.

Cheers,
John

 

[John - KD5YI]

The resonant frequency for the 10nF X7R jibes with the AVX datasheet for a
similar part.

What seems odd is that all the reactances are so high at resonance. Wouldn't
the net reactive component be very close to zero at resonance, that is where
the series X(l) = -X(c)?

Bob

Well, there is no reactance at resonance. And, none of his reactance
tests are at resonance.

John

 

[[email protected]]

I just tested some ceramic caps on an AIM 4170 impedance analyser:

0603 10nF 50V X7R
SRF=47.6 MHz
Z @ 22.5 MHz: Xs=-0.557, Rs=0.083

0603 10nF 25V COG
SRF=37.7 MHz
Z @ 22.5 MHz: Xs=-0.401, Rs=0.035

0603 100nF 16V X7R
SRF=8.7 MHz
Z @ 22.5 MHz: Xs=+0.304, Rs=0.062

I'm building a 22.5 MHz IF amplifier and can't decide what decouplers to
use. The 100nF has the lowest impedance, so I suppose it would make the
best decoupler; but its SRF is less than half the operating frequency!
Which would you use?

FYI, I've found the AVX SpiCap3 application great for finding such
information.

http://www.avx.com/SpiApps/default.asp#spicap

 

[Jeroen Belleman]

I just tested some ceramic caps on an AIM 4170 impedance analyser:

0603 10nF 50V X7R
SRF=47.6 MHz
Z @ 22.5 MHz: Xs=-0.557, Rs=0.083

0603 10nF 25V COG
SRF=37.7 MHz
Z @ 22.5 MHz: Xs=-0.401, Rs=0.035

0603 100nF 16V X7R
SRF=8.7 MHz
Z @ 22.5 MHz: Xs=+0.304, Rs=0.062

I'm building a 22.5 MHz IF amplifier and can't decide what decouplers to
use. The 100nF has the lowest impedance, so I suppose it would make the
best decoupler; but its SRF is less than half the operating frequency!
Which would you use?

It probably wouldn't matter, but I'd use the 100nF, by habit.
What puzzles me is the very different SRF for the two 10nF
capacitors. The COG would have an ESL in the 1.8nH ballpark,
much more than I'd expect of a 0603. How would that come about?

Tell us who makes it, so that I can avoid using it. ;-)

Jeroen Belleman

 

[Jeroen Belleman]

It probably wouldn't matter, but I'd use the 100nF, by habit.
What puzzles me is the very different SRF for the two 10nF
capacitors. The COG would have an ESL in the 1.8nH ballpark,
much more than I'd expect of a 0603. How would that come about?

Tell us who makes it, so that I can avoid using it. ;-)

Jeroen Belleman

Oh! the 100nF unit's ESL is even worse, with over 3nH, if the
SRF figure is to be believed!

Jeroen Belleman

 

[nospam]

It probably wouldn't matter, but I'd use the 100nF, by habit.
What puzzles me is the very different SRF for the two 10nF
capacitors. The COG would have an ESL in the 1.8nH ballpark,
much more than I'd expect of a 0603. How would that come about?

I think the moral of the story is you have to be very careful with test
fixtures and calibration to measure stuff like this.

A perfect 10n is 0.71 ohms at 22.5MHz.

My cheapy network analyzer measured an 0603 10n X7R 50v at 106MHz SRF and
0.82 ohms at 22.5MHz.

An 0805 100n X7R 50v in the same fixture measured 35MHz SRF and 0.05 ohms
at 22.5MHz.

 

[Andrew Holme]

SRF doesn't matter. A 10 uF 0603 would have an even lower SRF, but
that doesn't make it any worse a bypass at 25 MHz. ESL is what
matters, and the way the cap is connected matters most.

The best bypassing is when you have a solid ground plane, a thin
dielectric, a nice power pour, and several bypass caps.

But usually it doesn't matter much. Most any bypassing scheme usually
works, especially at low frequencies like 22 MHz.

How did you fixture the 0603 cap to the connector on the AIM?

I usually use TDR, to do time-domain measurements. I solder the parts
along, or at the end of, a 50 ohm microstrip.

ftp://jjlarkin.lmi.net/TDR_0.1uF.JPG

ftp://jjlarkin.lmi.net/Mini_Melf_Fixture.JPG

John

I have a BNC to binding-post adapter fitted on the front of the AIM. I
soldered wires to the ends of the 0603 caps and strung them between the
binding posts. OK, so maybe I should've rounded Xs/Rs to one decimal place.

 

[Fred Bartoli]

John Larkin a écrit :

I'd guess that your fixturing has way more inductance, 10x at least,
than the part does.

I cut fixtures out of copperclad, with an x-acto knife, and use 50 ohm
microstrips (or CPW) and SMA edge-launch connectors (or the flange
ones, which work better, if anything.) Like my mini-melf pic above. A
loop area of a fraction of a square inch will have much more L than
the cap does.

The advantage of TDR is that I can time resolve, and separate out, the
impedances of cables and connectors. A network analyzer just sees it
all in a couple of numbers.

???
That's all the purpose of calibration. Just have your reference plane at
the right place and your VNA only sees the cap.

 

[Andrew Holme]

I'd guess that your fixturing has way more inductance, 10x at least,
than the part does.

I cut fixtures out of copperclad, with an x-acto knife, and use 50 ohm
microstrips (or CPW) and SMA edge-launch connectors (or the flange
ones, which work better, if anything.) Like my mini-melf pic above. A
loop area of a fraction of a square inch will have much more L than
the cap does.

The advantage of TDR is that I can time resolve, and separate out, the
impedances of cables and connectors. A network analyzer just sees it
all in a couple of numbers.

John

Point taken about the superiority of TDR versus VNA. Thanks for the
insight.

The AIM is calibrated by connecting standard resistances to the test
fixture, which can be some distance away along a random length of
transmission line. It's intended for measuring the impedance presented by
an antenna at its feed point. I did the calibration with the binding post
adapter fitted, so it should be at least partially compensated for. I know
its only a rough measurement; but its good enough for most of the stuff I
do; and I think it's superior to say the AADE LC meter because it sweeps 1 -
200 MHz.

 

[John - KD5YI]

Tektronix Concepts book on TDR measurements:

http://slack.com/TE/TekConcepts/TekTDRMeas.pdf

Cheers

Phil Hobbs

Wow! I agree with Dave.

Thanks, Dr. Phil.

Cheers,
John

 

[John - KD5YI]

Oh, that's beautiful. On first glance it looks as if it's exactly the
sort of thing I was hoping to find.

Thank you *very* much indeed!

Hey, Dave -

Beware! Page 73 is out of place. It is stuck between pages 75 and 76.

Cheers,
John

 

[Fred Bartoli]

Joel Koltner a écrit :

Note that most decent VNAs from the past couple of decades can perform
an FFT for you and display a time-domain trace as well. Most will then
let you position a marker at a certain point in time and then re-compute
the frequency domain response as if everything prior hadn't occurred,
i.e., it's just another method of defining where you reference plane is
(although probably not as accurate as building a proper calibration jig
in the first place).

Too bad the 11802 can't do an FFT and display a frequency-domain response!

You still can post process it...