wideband inductors

[John Larkin]

Suppose one needs an inductor that's a high impedance over a very wide
frequency range. Right now, I'd like something around 100 uH, good for
200 mA maybe, and that looks like a small fraction of a pF and stays
above a couple of kohms impedance up to a couple GHz maybe, and has no
nasty resonances or such.

Our current idea is to make a string of inductors, starting with an
0402 surfmount ferrite bead and working up to maybe a couple of
roughly 56 uH axials in series on the other end of the string (leaded
axials seem to have much higher SRFs than surface mount inductors.)

Seems to me there should be an optimum staging of values, maybe some
shunt resistors to kill Qs, and some theory somewhere. Sounds like an
ideal topic for an academic paper or so.

No serious luck googling. Anybody have leads?

John

 

[John Woodgate]

I read in sci.electronics.design that John Larkin <jjlarkin@highSNIPland
THIStechPLEASEnology.com> wrote (in <pbour05t3cph5jo29qom7db3npsackosrl@
4ax.com&gt about 'wideband inductors', on Tue, 14 Dec 2004:

No serious luck googling. Anybody have leads?

ARRL or RSGB Handbook. This is the sort of thing that skilled amateurs
develop.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Ken Smith]

In article <pbour05t3cph5jo29qom7db3npsackosrl@4ax.com>,
John Larkin <jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

Suppose one needs an inductor that's a high impedance over a very wide
frequency range. Right now, I'd like something around 100 uH, good for
200 mA maybe, and that looks like a small fraction of a pF and stays
above a couple of kohms impedance up to a couple GHz maybe, and has no
nasty resonances or such.

C = 1/(2 * PI * F * Xc)
= 1/(2 * PI * 1GHz * 2K) = 0.08pF


This looks like trouble if you are doing a PCB but I suggest you look at
Toko 33CS or CoilCraft 1606 inductors as the smallest real one. Depending
on what the "couple of GHz" is, you may want to make the smallest inductor
a PCB trace.
--
--
kensmith@rahul.net forging knowledge

 

[Jim Thompson]

On Tue, 14 Dec 2004 17:42:14 -0800, John Larkin
<jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:

[snip]

Well, it *is* a Topic. Would you rather see a picture of my grandson?

[snip]
John

Yes ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Winfield Hill]

Jim Thompson wrote...

John Larkin wrote:

Well ... Would you rather see a picture of my grandson?

Yes ;-)

Oops! Here we go again!


--
Thanks,
- Win

 

[Rich Grise]

On Tue, 14 Dec 2004 20:18:46 -0700, Jim Thompson wrote:

On Tue, 14 Dec 2004 17:42:14 -0800, John Larkin
jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:

[snip]

Well, it *is* a Topic. Would you rather see a picture of my grandson?

[snip]
John


Yes ;-)

Well, why not? I can ignore it as easily as, say, Clarence. ;-)

Cheers!
Rich

 

[Winfield Hill]

Rich Grise wrote...

Jim Thompson wrote:

John Larkin wrote:

Would you rather see a picture of my grandson?

Yes ;-)

Well, why not? I can ignore it as easily as, say, Clarence. ;-)

Hey, let's not encourage this pictures-of-the-kids business.


--
Thanks,
- Win

 

[John Larkin]

On Wed, 15 Dec 2004 06:50:18 GMT, Scott Stephens <scottxs@comcast.net>
wrote:

You are simulating ferrite? Doesn't that tend to vary a bit depending on
the batch you get, especially over wide bandwidth?

No, we're just using lumped models. The ferrite beads look like L || R
(based on TDR measurements) and the axial inductors look like L || C,
and we add a little node capacitance for PCB pads. This is an
important problem, but it's still a small part of a bigger problem, so
we're looking for a quick solution based on purchased parts. In other
words, we can't be too concerned about things below the lumped-element
model. Too bad, it would be an interesting problem if one had lots of
leisure.

John

 

[John Larkin]

On Tue, 14 Dec 2004 20:18:46 -0700, Jim Thompson
<thegreatone@example.com> wrote:

On Tue, 14 Dec 2004 17:42:14 -0800, John Larkin
jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:

[snip]

Well, it *is* a Topic. Would you rather see a picture of my grandson?

[snip]
John


Yes ;-)

...Jim Thompson

He looks a little like me. I'll give you some time to reconsider.

John

 

[Winfield Hill]

John Larkin wrote...

Hey, Win,

We just TDRd a couple of PSPL bias tees. Their thru performance is
impressive - very clean path, apparently - and the side L seems to
have virtualy no capacitance, but the Ls are pretty obviously there.
One box has an effective impedance of 45 ohms, making the L leg
appear to have about 500 ohms shunt resistance. The other one has a
sort of impedance belly about 5 ns out. Given that we want less than
1% abberations over a 50 ns time span, they wouldn't be very good.

Just add to the L side on your own...

TDR pics to abse later. It's tempting to open one up.

I imagine if you win one on eBay, you'll feel more comfortable opening
it up. I've bought four on eBay so far, but they're used two each by
a research group and myself, so no "spares" are available.


--
Thanks,
- Win

 

[Uwe Bonnes]

Joerg <notthisjoergsch@removethispacbell.net> wrote:

Hello Uwe,

....

cell phone. When you take apart the tuner of an older (1970's or so) TV
you see that concept. They are housed in tin cans with several
compartments. Except for very tiny holes to pass signals each stage
cannot 'see' the others. Same in RF attenuators, cable TV amps and so
on. It used to be that way in FM stereos as well but now they even skimp

I have seen those compartments before. Doing a design with gain blocks
(Minicircuits, Agilent MGA-xxxx, RFMD RF24XX) at 3 GHz at the moment, I
wonder where these compartments walls should be:
- over the line connecting two gain stages
or
- over the gain block

In most designs I have seen, the wall are over the connecting lines. However
placing the walls over the gain blocks would better break the "line of
sight" between input and output of the gain block. Do I get something wrong?

Thanks for the help so long!

--
Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------

 

[Uwe Bonnes]

John Larkin <jjlarkin@highlandsniptechthisnologyplease.com> wrote:

The shunt capacitance is a killer. A drum-core 100 uH inductor we
tried was 160 pF. A 22 uH surfmount 1812 was about 4. The best we've
found so far is a 47 uH Vishay axial, whose SRF calculated out to 0.8
pF shunt capacitance. We'll have some tomorrow morning to verify this
amazing number.

The digikey page with the Panasonic inductors gives a good picture. The
smaller the size, the higher the self resonance frequency. There is also
some influence of the construction, but the size is the most important.

Bye
--
Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------
e mount inductors.)

Seems to me there should be an optimum staging of values, maybe some
shunt resistors to kill Qs, and some theory somewhere. Sounds like an
ideal topic for an academic paper or so.

No serious luck googling. Anybody have leads?

==========================================

I'm surprised it is necessary to ask such a question.


Well, it *is* a Topic. Would you rather see a picture of my grandson?

The obvious ideal, is a large number of series-connected coils progressively
increasing in inductance.

Sure, but there may be an optimum strategy, given, I suppose, a
fundamental relationship between L and distributed C for a given
inductor technology.

But the limits must be pre-stated - the minimum impedance required and the
frequency range to be covered.

The low frequency inductance is an elementary matter.

The shunt capacitance is a killer. A drum-core 100 uH inductor we
tried was 160 pF. A 22 uH surfmount 1812 was about 4. The best we've
found so far is a 47 uH Vishay axial, whose SRF calculated out to 0.8
pF shunt capacitance. We'll have some tomorrow morning to verify this
amazing number.

We considered a transistor cc source instead of the last big L, but
were sorta surprised that no PNP exists with low enough Co and high
enough power capability. Interestingly, the antique 2N3906 was about
as good as anything else we could find, but still terrible compared to
a real inductor.

The self resonant frequency of a coil is not nasty. It may not be useful.
But to damp it down with a shunt resistor degrades the minumum impedance
performance of the whole string at all frequencies. Resistors are OUT.

Our latest simulations indicate that q-killing resistors (across the
various Ls) are mandatory.

In most practical wide-band cases only two, or at most three
series-connected coils are necessary.

Our situation is applying power to the output pin of a distributed
amplifier. We need extreme fidelity for shaped pulses up to 50 ns
maybe, so 100 uH would be nice, and we need to keep the bandwidth in
the 4-5 GHz range with no resonances or reflections or nasty things.

Our latest simulation looks promising with two 0402 ferrite beads +
two axial inductors, various resistors, and some droop compensation
elsewhere in the signal chain to make up for the finite L. The beads
are interesting, circuit-equivalent-wise.

John

--
Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------

 

[John Larkin]

On Thu, 16 Dec 2004 21:21:01 +0000 (UTC), Uwe Bonnes
<bon@elektron.ikp.physik.tu-darmstadt.de> wrote:

John Larkin <jjlarkin@highlandsniptechthisnologyplease.com> wrote:

The shunt capacitance is a killer. A drum-core 100 uH inductor we
tried was 160 pF. A 22 uH surfmount 1812 was about 4. The best we've
found so far is a 47 uH Vishay axial, whose SRF calculated out to 0.8
pF shunt capacitance. We'll have some tomorrow morning to verify this
amazing number.

The digikey page with the Panasonic inductors gives a good picture. The
smaller the size, the higher the self resonance frequency. There is also
some influence of the construction, but the size is the most important.

The Vishay axial is 47 uH with a measured SRF of 28 MHz, which is only
0.69 pF, a fraction of that of a typical surface-mount part.
Interesting that an old thru-hole part would be so much better.

John