Inductance specification
 

As I was re-reading Boswick's RF Design book it occurred to me that I
may have overlooked something very fundamental.

I often transform components from series to parallel form, and visa
versa ,depending on what I am wanting to achieve.

What I have over looked is. What form do manufacturer's state Q and
inductance ?

I took a look at a coil manufacturer's specs and they don't explicitly
say whether the test circuit was series or parallel. I would assume that
the standard would be series form ?

Would someone please advise me which it is ?

Though the Q would be same in either form, the inductance and loading
could be slightly different.(ie. If I have assumed series form then when
I transform into parallel form and calculate a specific Rp and L it
would be incorrect if the inductance and Q in the manufacturer's data
were specified in parallel form.)

Hopefully someone can make sense of what I am getting at here.

Thanks in advance

Regards

David

Inductance specification
 

The Q should be identical when transformed. Q = Xs/Rs = Rp/Xp. It's true
that Xs isn't exactly the same as Xp(*) (and therefore Ls isn't exactly
equal to Lp), but for even a poor Q, they're much closer than typical
component tolerances or even your ability to measure. Any design
requiring this kind of accuracy should be reconsidered.

Of course, a series-parallel conversion is correct only at a single
frequency, and the Q also varies with frequency. So such conversions
need to be applied with caution.

(*) Xp/Xs = (Q^2 + 1) / Q^2, so the ratio is 1.01 when Q is 10, 1.0001
when Q is 100, etc.

Roy Lewallen, W7EL

David wrote:
As I was re-reading Boswick's RF Design book it occurred to me that I
may have overlooked something very fundamental.

I often transform components from series to parallel form, and visa
versa ,depending on what I am wanting to achieve.

What I have over looked is. What form do manufacturer's state Q and
inductance ?

I took a look at a coil manufacturer's specs and they don't explicitly
say whether the test circuit was series or parallel. I would assume that
the standard would be series form ?

Would someone please advise me which it is ?

Though the Q would be same in either form, the inductance and loading
could be slightly different.(ie. If I have assumed series form then when
I transform into parallel form and calculate a specific Rp and L it
would be incorrect if the inductance and Q in the manufacturer's data
were specified in parallel form.)

Hopefully someone can make sense of what I am getting at here.

Thanks in advance

Regards

David

Inductance specification
 

Roy,

Thanks for your response. So basically it makes no real practical
difference which way they specify the parameters. The lowest Q inductors
I use have Q = 30 anyway.

Regards

David




Roy Lewallen wrote:
The Q should be identical when transformed. Q = Xs/Rs = Rp/Xp. It's true
that Xs isn't exactly the same as Xp(*) (and therefore Ls isn't exactly
equal to Lp), but for even a poor Q, they're much closer than typical
component tolerances or even your ability to measure. Any design
requiring this kind of accuracy should be reconsidered.

Of course, a series-parallel conversion is correct only at a single
frequency, and the Q also varies with frequency. So such conversions
need to be applied with caution.

(*) Xp/Xs = (Q^2 + 1) / Q^2, so the ratio is 1.01 when Q is 10, 1.0001
when Q is 100, etc.

Roy Lewallen, W7EL

David wrote:
As I was re-reading Boswick's RF Design book it occurred to me that I
may have overlooked something very fundamental.

I often transform components from series to parallel form, and visa
versa ,depending on what I am wanting to achieve.

What I have over looked is. What form do manufacturer's state Q and
inductance ?

I took a look at a coil manufacturer's specs and they don't explicitly
say whether the test circuit was series or parallel. I would assume
that the standard would be series form ?

Would someone please advise me which it is ?

Though the Q would be same in either form, the inductance and loading
could be slightly different.(ie. If I have assumed series form then
when I transform into parallel form and calculate a specific Rp and L
it would be incorrect if the inductance and Q in the manufacturer's
data were specified in parallel form.)

Hopefully someone can make sense of what I am getting at here.

Thanks in advance

Regards

David