Hi-Q inductors

A question for the RF boys:

I've been making 50 MHz LC oscillators using some 150 nH 1008-sized
surface-mount inductors. These are ceramic-core types, not ferrite.

They work fine, but Qs are low, in the 30 range. Coilcraft has
"surface mount" parts with Q above 100, but they are either bare
springs or springs glued into a plastic case...

http://www.coilcraft.com/1515sq.cfm

and

http://www.coilcraft.com/maxi.cfm

(We have used this Coilcraft "maxi" inductor in the past, and found
that the plastic cold-flows over time and slowly changes L. So we bake
them for a day or so to relieve stresses. Coilcraft claims they don't
cold flow!)

My concerns are...

For the highest Q parts, the bare springs, does L change as the FR4
board changes dims with temperature? I suspect Coilcraft doesn't test
them soldered to a board... they apparently use some HP q-meter
fixture.

What's the effect of a PCB ground plane on L and Q? Ditto.

Are these things significantly microphonic?


Any experience or suggestions for stable, repeatable TC [1], high Q
inductors, preferable surface mount?

John

[1] so we can pad them with NTC caps and kill most of the temperature
drift. Actually getting NTC caps is a whole nother story.

On 08/20/2010 05:37 PM, John Larkin wrote:

A question for the RF boys:

I've been making 50 MHz LC oscillators using some 150 nH 1008-sized
surface-mount inductors. These are ceramic-core types, not ferrite.

They work fine, but Qs are low, in the 30 range. Coilcraft has
"surface mount" parts with Q above 100, but they are either bare
springs or springs glued into a plastic case...

http://www.coilcraft.com/1515sq.cfm

and

http://www.coilcraft.com/maxi.cfm

(We have used this Coilcraft "maxi" inductor in the past, and found
that the plastic cold-flows over time and slowly changes L. So we bake
them for a day or so to relieve stresses. Coilcraft claims they don't
cold flow!)

My concerns are...

For the highest Q parts, the bare springs, does L change as the FR4
board changes dims with temperature? I suspect Coilcraft doesn't test
them soldered to a board... they apparently use some HP q-meter
fixture.

What's the effect of a PCB ground plane on L and Q? Ditto.

Are these things significantly microphonic?


Any experience or suggestions for stable, repeatable TC [1], high Q
inductors, preferable surface mount?

John

[1] so we can pad them with NTC caps and kill most of the temperature
drift. Actually getting NTC caps is a whole nother story.

I thought that C0G caps were a good approximation to NTC?

I suspect that not many folks care much about TC, because they're
assuming that you'll put this into a phase-locked oscillator. Which
makes me wonder why you're not phase-locking your oscillator to a crystal...

On Fri, 20 Aug 2010 17:51:50 -0700, Tim Wescott<tim_at_seemywebsite.com
wrote:

On 08/20/2010 05:37 PM, John Larkin wrote:

A question for the RF boys:

I've been making 50 MHz LC oscillators using some 150 nH 1008-sized
surface-mount inductors. These are ceramic-core types, not ferrite.

They work fine, but Qs are low, in the 30 range. Coilcraft has
"surface mount" parts with Q above 100, but they are either bare
springs or springs glued into a plastic case...

http://www.coilcraft.com/1515sq.cfm

and

http://www.coilcraft.com/maxi.cfm

(We have used this Coilcraft "maxi" inductor in the past, and found
that the plastic cold-flows over time and slowly changes L. So we bake
them for a day or so to relieve stresses. Coilcraft claims they don't
cold flow!)

My concerns are...

For the highest Q parts, the bare springs, does L change as the FR4
board changes dims with temperature? I suspect Coilcraft doesn't test
them soldered to a board... they apparently use some HP q-meter
fixture.

What's the effect of a PCB ground plane on L and Q? Ditto.

Are these things significantly microphonic?


Any experience or suggestions for stable, repeatable TC [1], high Q
inductors, preferable surface mount?

John

[1] so we can pad them with NTC caps and kill most of the temperature
drift. Actually getting NTC caps is a whole nother story.

I thought that C0G caps were a good approximation to NTC?

NTC is Negative Temperature Coefficient. Inductors tend to have
positive TCs, often in the +120 PPM range.

I suspect that not many folks care much about TC, because they're
assuming that you'll put this into a phase-locked oscillator. Which
makes me wonder why you're not phase-locking your oscillator to a crystal...

It's a triggered oscillator, starts instantly when it gets triggered.
Phase locking is a nuisance under those circumstances.

Ah. No kidding. Using a DDS comes to mind, although the parts count

A question for the RF boys:

I've been making 50 MHz LC oscillators using some 150 nH 1008-sized
surface-mount inductors. These are ceramic-core types, not ferrite.

They work fine, but Qs are low, in the 30 range. Coilcraft has
"surface mount" parts with Q above 100, but they are either bare
springs or springs glued into a plastic case...

http://www.coilcraft.com/1515sq.cfm

and

http://www.coilcraft.com/maxi.cfm

(We have used this Coilcraft "maxi" inductor in the past, and found
that the plastic cold-flows over time and slowly changes L. So we bake
them for a day or so to relieve stresses. Coilcraft claims they don't
cold flow!)

My concerns are...

For the highest Q parts, the bare springs, does L change as the FR4
board changes dims with temperature? I suspect Coilcraft doesn't test
them soldered to a board... they apparently use some HP q-meter
fixture.

What's the effect of a PCB ground plane on L and Q? Ditto.

Are these things significantly microphonic?


Any experience or suggestions for stable, repeatable TC [1], high Q
inductors, preferable surface mount?

John

[1] so we can pad them with NTC caps and kill most of the temperature
drift. Actually getting NTC caps is a whole nother story.

A question for the RF boys:

I've been making 50 MHz LC oscillators using some 150 nH 1008-sized
surface-mount inductors. These are ceramic-core types, not ferrite.

They work fine, but Qs are low, in the 30 range. Coilcraft has
"surface mount" parts with Q above 100, but they are either bare
springs or springs glued into a plastic case...

http://www.coilcraft.com/1515sq.cfm

and

http://www.coilcraft.com/maxi.cfm

(We have used this Coilcraft "maxi" inductor in the past, and found
that the plastic cold-flows over time and slowly changes L. So we bake
them for a day or so to relieve stresses. Coilcraft claims they don't
cold flow!)

My concerns are...

For the highest Q parts, the bare springs, does L change as the FR4
board changes dims with temperature? I suspect Coilcraft doesn't test
them soldered to a board... they apparently use some HP q-meter
fixture.

What's the effect of a PCB ground plane on L and Q? Ditto.

A question for the RF boys:

I've been making 50 MHz LC oscillators using some 150 nH 1008-sized
surface-mount inductors. These are ceramic-core types, not ferrite.

They work fine, but Qs are low, in the 30 range. Coilcraft has
"surface mount" parts with Q above 100, but they are either bare
springs or springs glued into a plastic case...

http://www.coilcraft.com/1515sq.cfm

and

http://www.coilcraft.com/maxi.cfm

(We have used this Coilcraft "maxi" inductor in the past, and found
that the plastic cold-flows over time and slowly changes L. So we bake
them for a day or so to relieve stresses. Coilcraft claims they don't
cold flow!)

My concerns are...

For the highest Q parts, the bare springs, does L change as the FR4
board changes dims with temperature? I suspect Coilcraft doesn't test
them soldered to a board... they apparently use some HP q-meter
fixture.

What's the effect of a PCB ground plane on L and Q? Ditto.

Are these things significantly microphonic?


Any experience or suggestions for stable, repeatable TC [1], high Q
inductors, preferable surface mount?

John

[1] so we can pad them with NTC caps and kill most of the temperature
drift. Actually getting NTC caps is a whole nother story.


1) I would guess that it is "obvious" that the L would change as the FR4

John Larkin wrote:
A question for the RF boys:

I've been making 50 MHz LC oscillators using some 150 nH 1008-sized
surface-mount inductors. These are ceramic-core types, not ferrite.
They work fine, but Qs are low, in the 30 range. Coilcraft has
"surface mount" parts with Q above 100, but they are either bare
springs or springs glued into a plastic case...

http://www.coilcraft.com/1515sq.cfm

and

http://www.coilcraft.com/maxi.cfm

(We have used this Coilcraft "maxi" inductor in the past, and found
that the plastic cold-flows over time and slowly changes L. So we bake
them for a day or so to relieve stresses. Coilcraft claims they don't
cold flow!)

My concerns are...

For the highest Q parts, the bare springs, does L change as the FR4
board changes dims with temperature? I suspect Coilcraft doesn't test
them soldered to a board... they apparently use some HP q-meter
fixture.

What's the effect of a PCB ground plane on L and Q? Ditto.

Are these things significantly microphonic?


Any experience or suggestions for stable, repeatable TC [1], high Q
inductors, preferable surface mount?

John

[1] so we can pad them with NTC caps and kill most of the temperature
drift. Actually getting NTC caps is a whole nother story.



For single-layer coils, the old formula from NBS Circular C74 (1924) is
useful:

a**2 n**2
L(uH) = ----------
9a + 10b

where n is the number of turns (counting the half-turn represented by
the external circuit), a is the radius and b is the length, both in
inches. It's good to a couple of percent over the usual range of coil
dimensions.

Differentiating this formula will let you estimate the change in
inductance with stretching and heating. (The little Coilcraft springs
have wires whose radius is a significant fraction of the coil radius, so
the formula will be less accurate for those, but it's close enough for
an estimate.)

To get the stretching effect right, you have to take account of the
reduction of diameter with stretching--the wire stays the same length,
more or less.

Cheers

Phil Hobbs


I do not think the diameter changes as copper is too strong for

Phil Hobbs wrote:
John Larkin wrote:
A question for the RF boys:

I've been making 50 MHz LC oscillators using some 150 nH 1008-sized
surface-mount inductors. These are ceramic-core types, not ferrite.
They work fine, but Qs are low, in the 30 range. Coilcraft has
"surface mount" parts with Q above 100, but they are either bare
springs or springs glued into a plastic case...

http://www.coilcraft.com/1515sq.cfm

and

http://www.coilcraft.com/maxi.cfm

(We have used this Coilcraft "maxi" inductor in the past, and found
that the plastic cold-flows over time and slowly changes L. So we bake
them for a day or so to relieve stresses. Coilcraft claims they don't
cold flow!)

My concerns are...

For the highest Q parts, the bare springs, does L change as the FR4
board changes dims with temperature? I suspect Coilcraft doesn't test
them soldered to a board... they apparently use some HP q-meter
fixture.

What's the effect of a PCB ground plane on L and Q? Ditto.

Are these things significantly microphonic?


Any experience or suggestions for stable, repeatable TC [1], high Q
inductors, preferable surface mount?

John

[1] so we can pad them with NTC caps and kill most of the temperature
drift. Actually getting NTC caps is a whole nother story.



For single-layer coils, the old formula from NBS Circular C74 (1924)
is useful:

a**2 n**2
L(uH) = ----------
9a + 10b

where n is the number of turns (counting the half-turn represented by
the external circuit), a is the radius and b is the length, both in
inches. It's good to a couple of percent over the usual range of coil
dimensions.

Differentiating this formula will let you estimate the change in
inductance with stretching and heating. (The little Coilcraft springs
have wires whose radius is a significant fraction of the coil radius,
so the formula will be less accurate for those, but it's close enough
for an estimate.)

To get the stretching effect right, you have to take account of the
reduction of diameter with stretching--the wire stays the same length,
more or less.

Cheers

Phil Hobbs


I do not think the diameter changes as copper is too strong for
that..i think that the coil tries to wind/unwind and in that process the
coil instead of having a straight line center thru all loops, that the
loop-center line would curve.

John Larkin wrote:
A question for the RF boys:

I've been making 50 MHz LC oscillators using some 150 nH 1008-sized
surface-mount inductors. These are ceramic-core types, not ferrite.
They work fine, but Qs are low, in the 30 range. Coilcraft has
"surface mount" parts with Q above 100, but they are either bare
springs or springs glued into a plastic case...

http://www.coilcraft.com/1515sq.cfm

and

http://www.coilcraft.com/maxi.cfm

(We have used this Coilcraft "maxi" inductor in the past, and found
that the plastic cold-flows over time and slowly changes L. So we bake
them for a day or so to relieve stresses. Coilcraft claims they don't
cold flow!)

My concerns are...

For the highest Q parts, the bare springs, does L change as the FR4
board changes dims with temperature? I suspect Coilcraft doesn't test
them soldered to a board... they apparently use some HP q-meter
fixture.

What's the effect of a PCB ground plane on L and Q? Ditto.

Are these things significantly microphonic?


Any experience or suggestions for stable, repeatable TC [1], high Q
inductors, preferable surface mount?

John

[1] so we can pad them with NTC caps and kill most of the temperature
drift. Actually getting NTC caps is a whole nother story.


1) I would guess that it is "obvious" that the L would change as the FR4
PCB's temperature changes - but not a whole heck of a lot.
Why? The change of length is (guess without looking up CTE specs)
maybe 0.1 mil for every 10C, and the length of those coils are 100X or
more than that = = percentage change very small.
2) Ground plane would decrease effective L and Q; the Q would go down
some due to PCB losses.
3) Microphonic? What is that? I am in Egypt near this big river..

Robert Baer wrote:
Phil Hobbs wrote:
John Larkin wrote:
A question for the RF boys:

I've been making 50 MHz LC oscillators using some 150 nH 1008-sized
surface-mount inductors. These are ceramic-core types, not ferrite.
They work fine, but Qs are low, in the 30 range. Coilcraft has
"surface mount" parts with Q above 100, but they are either bare
springs or springs glued into a plastic case...

http://www.coilcraft.com/1515sq.cfm

and

http://www.coilcraft.com/maxi.cfm

(We have used this Coilcraft "maxi" inductor in the past, and found
that the plastic cold-flows over time and slowly changes L. So we bake
them for a day or so to relieve stresses. Coilcraft claims they don't
cold flow!)

My concerns are...

For the highest Q parts, the bare springs, does L change as the FR4
board changes dims with temperature? I suspect Coilcraft doesn't test
them soldered to a board... they apparently use some HP q-meter
fixture.

What's the effect of a PCB ground plane on L and Q? Ditto.

Are these things significantly microphonic?


Any experience or suggestions for stable, repeatable TC [1], high Q
inductors, preferable surface mount?

John

[1] so we can pad them with NTC caps and kill most of the temperature
drift. Actually getting NTC caps is a whole nother story.



For single-layer coils, the old formula from NBS Circular C74 (1924)
is useful:

a**2 n**2
L(uH) = ----------
9a + 10b

where n is the number of turns (counting the half-turn represented by
the external circuit), a is the radius and b is the length, both in
inches. It's good to a couple of percent over the usual range of
coil dimensions.

Differentiating this formula will let you estimate the change in
inductance with stretching and heating. (The little Coilcraft springs
have wires whose radius is a significant fraction of the coil radius,
so the formula will be less accurate for those, but it's close enough
for an estimate.)

To get the stretching effect right, you have to take account of the
reduction of diameter with stretching--the wire stays the same
length, more or less.

Cheers

Phil Hobbs


I do not think the diameter changes as copper is too strong for
that..i think that the coil tries to wind/unwind and in that process
the coil instead of having a straight line center thru all loops, that
the loop-center line would curve.

When you heat something up, all its dimensions increase by CTE*DeltaT.

If the board expands differently than the copper, one of them has to
stretch. Do you think the poor little inductor stretches a big chunk of
FR4 much? Because one has to stretch, or else the solder joint has to
break.

Cheers

Phil Hobbs

If the board shrinks more than the copper, the inductor may buckle a
little, but it has to go fairly far to take up the length change--the
height of the arch goes like sqrt(CTE*DeltaT). It's like the classical
railway rail problem--if you take a 1-mile railroad track held very
firmly at both ends, and then somebody comes along in the middle of the
night and welds in a 1-foot extra length, causing the rails to bend
upwards in a circular arc, how tall is the arch above the ground?

(Guess before looking below)

About 44.5 feet.

Robert Baer wrote:
Phil Hobbs wrote:
John Larkin wrote:
A question for the RF boys:

I've been making 50 MHz LC oscillators using some 150 nH 1008-sized
surface-mount inductors. These are ceramic-core types, not ferrite.
They work fine, but Qs are low, in the 30 range. Coilcraft has
"surface mount" parts with Q above 100, but they are either bare
springs or springs glued into a plastic case...

http://www.coilcraft.com/1515sq.cfm

and

http://www.coilcraft.com/maxi.cfm

(We have used this Coilcraft "maxi" inductor in the past, and found
that the plastic cold-flows over time and slowly changes L. So we bake
them for a day or so to relieve stresses. Coilcraft claims they don't
cold flow!)

My concerns are...

For the highest Q parts, the bare springs, does L change as the FR4
board changes dims with temperature? I suspect Coilcraft doesn't test
them soldered to a board... they apparently use some HP q-meter
fixture.

What's the effect of a PCB ground plane on L and Q? Ditto.

Are these things significantly microphonic?


Any experience or suggestions for stable, repeatable TC [1], high Q
inductors, preferable surface mount?

John

[1] so we can pad them with NTC caps and kill most of the temperature
drift. Actually getting NTC caps is a whole nother story.



For single-layer coils, the old formula from NBS Circular C74 (1924)
is useful:

a**2 n**2
L(uH) = ----------
9a + 10b

where n is the number of turns (counting the half-turn represented by
the external circuit), a is the radius and b is the length, both in
inches. It's good to a couple of percent over the usual range of coil
dimensions.

Differentiating this formula will let you estimate the change in
inductance with stretching and heating. (The little Coilcraft springs
have wires whose radius is a significant fraction of the coil radius,
so the formula will be less accurate for those, but it's close enough
for an estimate.)

To get the stretching effect right, you have to take account of the
reduction of diameter with stretching--the wire stays the same length,
more or less.

Cheers

Phil Hobbs


I do not think the diameter changes as copper is too strong for
that..i think that the coil tries to wind/unwind and in that process the
coil instead of having a straight line center thru all loops, that the
loop-center line would curve.

When you heat something up, all its dimensions increase by CTE*DeltaT.

If the board expands differently than the copper, one of them has to
stretch. Do you think the poor little inductor stretches a big chunk of
FR4 much? Because one has to stretch, or else the solder joint has to
break.

A question for the RF boys:

I've been making 50 MHz LC oscillators using some 150 nH 1008-sized
surface-mount inductors. These are ceramic-core types, not ferrite.

They work fine, but Qs are low, in the 30 range. Coilcraft has
"surface mount" parts with Q above 100, but they are either bare
springs or springs glued into a plastic case...

http://www.coilcraft.com/1515sq.cfm

and

http://www.coilcraft.com/maxi.cfm

(We have used this Coilcraft "maxi" inductor in the past, and found
that the plastic cold-flows over time and slowly changes L. So we bake
them for a day or so to relieve stresses. Coilcraft claims they don't
cold flow!)

My concerns are...

For the highest Q parts, the bare springs, does L change as the FR4
board changes dims with temperature? I suspect Coilcraft doesn't test
them soldered to a board... they apparently use some HP q-meter
fixture.

What's the effect of a PCB ground plane on L and Q? Ditto.

Are these things significantly microphonic?

Any experience or suggestions for stable, repeatable TC [1], high Q
inductors, preferable surface mount?

John

[1] so we can pad them with NTC caps and kill most of the temperature
drift. Actually getting NTC caps is a whole nother story.