On 23 Mar 2004 23:16:23 -0800, (Tom Bruhns) wrote:

Paul,

It's a series resonant circuit. The inductor is obvious. The
capacitor is actually split between the one to ground at the gate
input and the one on the other side of the inductor. Since it's a
series circuit, the inductor and first cap can be swapped, of course;
that might make it more obvious. The gate's RF input impedance
(including a resistive part) and the bias resistors are in parallel
(AC-wise) with the cap to ground, and provide a certain amount of
damping -- lowering of the Q -- or the reason loaded Q is lower than
Qu of the inductor. Since the tuning cap is in series with that cap,
only part of the resonance voltage appears across it. Remember, in a
series-resonant circuit, the voltage across the inductor or capacitor
is much higher than the exciting voltage--that's where the voltage
step-up comes from that's needed in this case. Splitting the net
capacitance up this way lets you get a reasonably high loaded Q (to
reject the other harmonics better) and control the output voltage, and
provide the proper load at to the driving gate...as I recall, my
design goal was about 100 ohms at the fifth harmonic, and a much
higher impedance at the fundamental and the other harmonics. That
way, the driving gate doesn't dissipate much power trying to drive
those other harmonics into a heavy load, and only has to deliver
significant power at the fifth harmonic. -- You could just use a cap
from the gate input to ground, and an inductor to the driving gate's
output (and then do away with the bias resistors too...), but then you
can't so easily control the loaded Q.

Thanks for the clarification, Tom. A useful piece of enlightenment.
Most helpful.

p.
--

The BBC: Licensed at public expense to spread lies.

Paul,

I updated program SOLNOID2 to 3 because somebody had difficulty with
defining the length of a coil as related to the number of turns when there
are only a very few turns on it. Its a more complicated little problem than
you might think. But the length of a coil, 1, the number of turns, 1, 2, 3
etc., and the winding pitch are all insisted on by the program.

So to clarify the point I rewrote the operating instructions. The program
itself remained unchanged.
---
====================================

You have been led up the garden path by the old wives. The L/C ratio has
very little to do with the Q of a resonant circuit. Q is controlled
independently by the losses in L and C. Since the loss in L predominates Q
= omega*L/R.

The ratio L/C, everything else remaining unchanged, affects only the
parallel impedance of the circuit. A high L/C ratio gives a high parallel
impedance. You get a higher voltage gain in an amplifier with a high L/C
ratio when the load is a parallel tuned circuit.

The parallel impedance is given by L/C/R = Q*Omega*L = Q/Omega/C.

Very often L and C are chosen from what you have in the bottom of the junk
box.
----
Reg, G4FGQ


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

"Paul Burridge" wrote in message
...
On Sun, 21 Mar 2004 18:54:19 +0000 (UTC), "Reg Edwards"
wrote:

Paul, in approaching the problem from your viewpoint havn't you set
yourself
the task of winding an inductor to have a particular value of Q ?

If you intend to use a solenoid then Q can be increased only by
increasing
its physical size without changing its proportions too much.

Utimately you will need to know what is the Q of a particular size coil,
number of turns, wire gauge, etc. It will be reduced by its proximity to
other components and circuit board by some indeterminate amount. I think
you should stop and check whether you have room for the coil in the
equipment space available. ;o)

Program SOLNOID2 may be of assistance in this onerous task.

Download in a few seconds from website below and run immediately.

Reg, SOLNOID2 has been withdrawn from your site IIRC. I *had* been
using it to great effect, but you presumably made some improvements,
implemented them, and renamed it SOLNOID3 which is what I now use.
Great program!
I'm still none the wiser as to whether it's better to have big-L ||
small C or vice versa, though. :-/
BTW, Reg - can you write a program to work out how I'm going to afford
my Council Tax this year? Thanks! :-)
--

The BBC: Licensed at public expense to spread lies.

Paul,

I updated program SOLNOID2 to 3 because somebody had difficulty with
defining the length of a coil as related to the number of turns when there
are only a very few turns on it. Its a more complicated little problem than
you might think. But the length of a coil, 1, the number of turns, 1, 2, 3
etc., and the winding pitch are all insisted on by the program.

So to clarify the point I rewrote the operating instructions. The program
itself remained unchanged.
---
====================================

You have been led up the garden path by the old wives. The L/C ratio has
very little to do with the Q of a resonant circuit. Q is controlled
independently by the losses in L and C. Since the loss in L predominates Q
= omega*L/R.

The ratio L/C, everything else remaining unchanged, affects only the
parallel impedance of the circuit. A high L/C ratio gives a high parallel
impedance. You get a higher voltage gain in an amplifier with a high L/C
ratio when the load is a parallel tuned circuit.

The parallel impedance is given by L/C/R = Q*Omega*L = Q/Omega/C.

Very often L and C are chosen from what you have in the bottom of the junk
box.
----
Reg, G4FGQ


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

"Paul Burridge" wrote in message
...
On Sun, 21 Mar 2004 18:54:19 +0000 (UTC), "Reg Edwards"
wrote:

Paul, in approaching the problem from your viewpoint havn't you set
yourself
the task of winding an inductor to have a particular value of Q ?

If you intend to use a solenoid then Q can be increased only by
increasing
its physical size without changing its proportions too much.

Utimately you will need to know what is the Q of a particular size coil,
number of turns, wire gauge, etc. It will be reduced by its proximity to
other components and circuit board by some indeterminate amount. I think
you should stop and check whether you have room for the coil in the
equipment space available. ;o)

Program SOLNOID2 may be of assistance in this onerous task.

Download in a few seconds from website below and run immediately.

Reg, SOLNOID2 has been withdrawn from your site IIRC. I *had* been
using it to great effect, but you presumably made some improvements,
implemented them, and renamed it SOLNOID3 which is what I now use.
Great program!
I'm still none the wiser as to whether it's better to have big-L ||
small C or vice versa, though. :-/
BTW, Reg - can you write a program to work out how I'm going to afford
my Council Tax this year? Thanks! :-)
--

The BBC: Licensed at public expense to spread lies.