Variable inductor

[Grumps]

I'm looking for a variable inductor (approx 10mm size) that has a range of
at least 2:1.
This is for use in a 50-100MHz tuned circuit, and I've calculated that
Imin=185nH, and Imax=366nH.
Does anyone know if such a range is possible?
Thanks.

 

[Phil Allison]

"Grumps"

I'm looking for a variable inductor (approx 10mm size) that has a range of
at least 2:1.
This is for use in a 50-100MHz tuned circuit, and I've calculated that
Imin=185nH, and Imax=366nH.

** Lmax will need to be 4 times Lmin for a 2:1 frequency range.

Does anyone know if such a range is possible?

** Possible, no doubt.

But do you want to set and forget or have L user adjustable ?



........ Phil

 

[Grumps]

"Grumps"


** Lmax will need to be 4 times Lmin for a 2:1 frequency range.




** Possible, no doubt.

But do you want to set and forget or have L user adjustable ?

It needs to be 'tweakable' by the user. This won't happen very often.
The cap in the tuned circuit is also a variable with a 2:1 range (which are
readily available).

 

[Winfield Hill]

Grumps wrote...

I'm looking for a variable inductor (approx 10mm size) that
has a range of at least 2:1. This is for use in a 50-100MHz
tuned circuit, and I've calculated that Imin=185nH, and
Imax=366nH. Does anyone know if such a range is possible?

For a 2:1 range in resonant tuned frequency you'd need
a 4:1 range in variable inductance. That's more than
most adjustable pot-core inductors, etc., provide. A
quick review of various parts shows about 2:1 maximum.

There are several ways to create variable inductors.

One is to make an air coil and adjust a tuning slug in
and out of the coil. This may be your best approach.
Coilcraft has a nice selection of coils with tuning
slugs, but most, like the Unicoil 10mm series, can't do
a 4:1 range. http://www.coilcraft.com/uni10.cfm

A second way is the classic variometer. Here the coil
is split into two parts, wired in series, where one of
the coils is capable of rotating within the other. If
they are aligned the inductance is nearly doubled over
a single coil. But, if the orientation of one coil is
rotated with respect to the other the inductance will
be reduced, and if they're reversed it can be very low.
http://www.qsl.net/in3otd/variodes.html
http://www.g0mrf.freeserve.co.uk/variometer.htm
http://www.crystalradio.net/crystalsets/hassell
http://www.schmarder.com/radios/misc-stuff/variometer.htm
http://www.qsl.net/in3otd/vario1w.jpg

If the coils are wound in spherical shapes, it's easier
to nest one inside the other and orient it as desired.
http://www.rubylane.com/ni/shops/wavesradios/iteml/tel002#pic1
http://www2.propichosting.com/Images/450002496/17.jpg

It's easy to get > 10:1 ratios by this technique.

You can also slide one coil in and out of the other.
http://www.radioblvd.com/DoddPix/lscplr.jpg
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=9726250166

A third method for obtaining wide variable inductance is
with a rolling slider bar making a variable winding tap.
http://www.fairradio.com/t301.jpg

The links and pictures I have provided are for working
at low frequencies with large inductors, but I imagine
most of the concepts can be miniaturized.

An interesting pot-core tuning technique that may be
useful for you is to run a DC current through the coil
to saturate the core, dropping its permeability. This
scheme could probably be used to exceed a 4:1 ratio.

An issue to evaluate is your required tuning stability.

 

[Phil Allison]

"Grumps"

It needs to be 'tweakable' by the user. This won't happen very often.
The cap in the tuned circuit is also a variable with a 2:1 range (which
are readily available).

** So are variable caps with a 4:1 range.

What sort ot of dumb wank is this ?



......... Phil

 

[Reg Edwards]

If the tuned circuit is to cover a range 50-100MHz, then the coil,
with its ferrite slug, has to cover a FOUR-to-ONE inductance range.

Such coils are not available, not manufactured, no such thing.

Back to the drawing board! Try varying the capacitor.

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

 

[Grumps]

"Grumps"



** So are variable caps with a 4:1 range.

What sort ot of dumb wank is this ?

Are you this group's spunk bucket or what?

It's quite simple. Listen. An LC tuned circuit. It needs a 2:1 (or greater)
variable I. All I'd like to know is if these are available in small can
sizes.

 

[Grumps]

If the tuned circuit is to cover a range 50-100MHz, then the coil,
with its ferrite slug, has to cover a FOUR-to-ONE inductance range.

Such coils are not available, not manufactured, no such thing.

Back to the drawing board! Try varying the capacitor.

The cap is variable too.
Can you get a 2:1 variable I ?

 

[Phil Allison]

"Grumps"

Can you get a 2:1 variable I ?

** The symbol for inductance is " L ".

" I " is reserved for current.





....... Phil

 

[Grumps]

"Grumps"



** The symbol for inductance is " L ".

" I " is reserved for current.

** Yawn.
Ok.

So apart from telling me that, do you actually know the limiting factors
that contribute to the max ratio?

 

[Phil Allison]

"Grumpy Fuckwit "

** Yawn.
Ok.

So apart from telling me that,

** Go **** yourself.

do you actually know the limiting factors that contribute to the max
ratio?

** Yawn.

Go do your own Google searching - fuckhead.






......... Phil

 

[Grumps]

"Grumpy Fuckwit "



** Go **** yourself.




** Yawn.

Go do your own Google searching - fuckhead.

I just did. You seem to be quite a hero according to Google.
I'd rather you didn't try to answer questions that are beyond (way beyond)
your abilities. It's just wasting everyone's time reading your childish
spew.

 

[Phil Allison]

"Grumpy Asshole Fuckwit "


( snip abuse)


** Go **** yourself - MORON .






........ Phil

 

[Winfield Hill]

Grumps wrote...

... An LC tuned circuit. It needs a 2:1 (or greater) variable I.
All I'd like to know is if these are available in small can
sizes.

2:1 variable L is readily available in 10mm gapped pot
cores, although 4:1 is not, SFAICT.

There are several methods to get up to 10:1 inductance
ratios in a bit more space, e.g., moving large cores
into air coils, rotating concentric series-connected
coil sections (variometer), and rolling inductors.

One interesting idea to consider is using DC currents in
the winding to magnetically saturate the core, reducing
its permeability. This method can easily exceed 4:1
ratios, but it's best done inside a servo loop where
stability doesn't matter.

What kind of stability do you need?

 

[Grumps]

Grumps wrote...

For a 2:1 range in resonant tuned frequency you'd need
a 4:1 range in variable inductance. That's more than
most adjustable pot-core inductors, etc., provide. A
quick review of various parts shows about 2:1 maximum.

There are several ways to create variable inductors.

One is to make an air coil and adjust a tuning slug in
and out of the coil. This may be your best approach.
Coilcraft has a nice selection of coils with tuning
slugs, but most, like the Unicoil 10mm series, can't do
a 4:1 range. http://www.coilcraft.com/uni10.cfm

A second way is the classic variometer. Here the coil
is split into two parts, wired in series, where one of
the coils is capable of rotating within the other. If
they are aligned the inductance is nearly doubled over
a single coil. But, if the orientation of one coil is
rotated with respect to the other the inductance will
be reduced, and if they're reversed it can be very low.
http://www.qsl.net/in3otd/variodes.html
http://www.g0mrf.freeserve.co.uk/variometer.htm
http://www.crystalradio.net/crystalsets/hassell
http://www.schmarder.com/radios/misc-stuff/variometer.htm
http://www.qsl.net/in3otd/vario1w.jpg

If the coils are wound in spherical shapes, it's easier
to nest one inside the other and orient it as desired.
http://www.rubylane.com/ni/shops/wavesradios/iteml/tel002#pic1
http://www2.propichosting.com/Images/450002496/17.jpg

It's easy to get > 10:1 ratios by this technique.

You can also slide one coil in and out of the other.
http://www.radioblvd.com/DoddPix/lscplr.jpg
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=9726250166

A third method for obtaining wide variable inductance is
with a rolling slider bar making a variable winding tap.
http://www.fairradio.com/t301.jpg

The links and pictures I have provided are for working
at low frequencies with large inductors, but I imagine
most of the concepts can be miniaturized.

An interesting pot-core tuning technique that may be
useful for you is to run a DC current through the coil
to saturate the core, dropping its permeability. This
scheme could probably be used to exceed a 4:1 ratio.

An issue to evaluate is your required tuning stability.

Thanks very much for those links. I'll search for miniature variometer, but
my gut feeling says that there won't be much available.

You're right that a 4:1 range is required for a 2:1 frequency coverage.
However, there is a trimmer cap in this circuit too.
I'm not sure the size of those variometers is quite right for this project,
but I might just get one to show the boss

The stability issue had crossed my mind. Another area to investigate.

Thanks again.

 

[Tim Wescott]

Grumps wrote...



For a 2:1 range in resonant tuned frequency you'd need
a 4:1 range in variable inductance. That's more than
most adjustable pot-core inductors, etc., provide. A
quick review of various parts shows about 2:1 maximum.

There are several ways to create variable inductors.

One is to make an air coil and adjust a tuning slug in
and out of the coil. This may be your best approach.
Coilcraft has a nice selection of coils with tuning
slugs, but most, like the Unicoil 10mm series, can't do
a 4:1 range. http://www.coilcraft.com/uni10.cfm

A second way is the classic variometer. Here the coil
is split into two parts, wired in series, where one of
the coils is capable of rotating within the other. If
they are aligned the inductance is nearly doubled over
a single coil. But, if the orientation of one coil is
rotated with respect to the other the inductance will
be reduced, and if they're reversed it can be very low.
http://www.qsl.net/in3otd/variodes.html
http://www.g0mrf.freeserve.co.uk/variometer.htm
http://www.crystalradio.net/crystalsets/hassell
http://www.schmarder.com/radios/misc-stuff/variometer.htm
http://www.qsl.net/in3otd/vario1w.jpg

If the coils are wound in spherical shapes, it's easier
to nest one inside the other and orient it as desired.
http://www.rubylane.com/ni/shops/wavesradios/iteml/tel002#pic1
http://www2.propichosting.com/Images/450002496/17.jpg

It's easy to get > 10:1 ratios by this technique.

You can also slide one coil in and out of the other.
http://www.radioblvd.com/DoddPix/lscplr.jpg
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=9726250166

A third method for obtaining wide variable inductance is
with a rolling slider bar making a variable winding tap.
http://www.fairradio.com/t301.jpg

The links and pictures I have provided are for working
at low frequencies with large inductors, but I imagine
most of the concepts can be miniaturized.

An interesting pot-core tuning technique that may be
useful for you is to run a DC current through the coil
to saturate the core, dropping its permeability. This
scheme could probably be used to exceed a 4:1 ratio.

An issue to evaluate is your required tuning stability.

Another way to vary inductance is to put a shorted turn (or a copper
disk) in your coil, and vary it's orientation from right angle to
parallel with the coil. At your frequencies this may work well, but
check the effect on Q.

Another thing that may work is to use an air-core inductor in insert a
brass or copper slug to reduce inductance.

I've seen both of these techniques (well, _pictures_ of both of these
techniques) used in amateur radio installations.

If you want something you can gang to your cap I think you're going to
have to plan on doing some experimenting.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Grumps]

Grumps wrote...

2:1 variable L is readily available in 10mm gapped pot
cores, although 4:1 is not, SFAICT.

Yes, I've found a couple of manufacturers that have 2:1 (and slightly more),
but nothing in the ideal range for me (about 200-400nH).

There are several methods to get up to 10:1 inductance
ratios in a bit more space, e.g., moving large cores
into air coils, rotating concentric series-connected
coil sections (variometer), and rolling inductors.

One interesting idea to consider is using DC currents in
the winding to magnetically saturate the core, reducing
its permeability. This method can easily exceed 4:1
ratios, but it's best done inside a servo loop where
stability doesn't matter.

What kind of stability do you need?

That's a good question. Are these devices mechanically stable, or should you
'glue' the core?

 

[Jim Thompson]

I'm looking for a variable inductor (approx 10mm size) that has a range of
at least 2:1.
This is for use in a 50-100MHz tuned circuit, and I've calculated that
Imin=185nH, and Imax=366nH.
Does anyone know if such a range is possible?
Thanks.

I'm trying to remember how much variability coils with *copper* slugs
had (or were they brass?)

I've haven't seen them since the early days of TV sets (mid '50's).

...Jim Thompson

 

[Don Lancaster]

"Grumps"





** So are variable caps with a 4:1 range.

What sort ot of dumb wank is this ?



........ Phil

You can build a 4:1 variable cap by taking a 30:1 variable cap and
padding it with an external capacitor.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Winfield Hill]

Grumps wrote...

Yes, I've found a couple of manufacturers that have 2:1 (and
slightly more), but nothing in the ideal range for me (about
200-400nH).

Bare gapped cores; you do have to wind them yourself.

For example, Ferroxcube's P11/7 with 3D3 ferrite and E16
gap, with yellow adjuster, gives A_L from 16 to 35nH/t^2,
which means four turns will give you 256 to 560nH, etc.