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

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

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