Oops - L/R ISTR!

"K9SQG" wrote in message
...
High L/C ratio increases Q.

Circuit Q = omega*L/R. Reducing L has little or no effect on Q because,
after winding L, you will find R has decreased in about the same proportion.
The fewer the number of turns, the shorter the length of wire, and the lower
the resistance.

The ratio of L to C also has little effect on circuit Q because the
intrinsic Q of capacitors is usually an order of magnitude or more greater
than Q of L. L and C values of a tuned circuit are usually selected by the
reactances required of them at resonance for reasons independent of circuit
Q. Eg., the reactance of L and C may be required to be 300 ohms at
resonance because other components will have to be connected to them.

Usually it is the value of C which controls the value of L. C may have to
be trimmer. If it is a fixed value it will have to conform to a preferred
series of values and tolerances. If it is too small it will get lost in
stray and other circuit capacitances.

IMPORTANT - Intrinsic Q of a solenoid is directly proportional only to its
physical size. Double all dimensions, including wire diameter, and Q is
doubled. Its the the amount of space you have which decides the value of Q.
And there's a similar relationship even for magnetic cored components. If
you havn't got the room then you will have to put up with a low coil Q. And
it's always lower than what you think it is. Its impossible to measure in
situ. Spice is of no help.

Don't forget that a tuned circuit is never used in isolation. If it is used
as a filter in transistor collector circuit then it forms only part of the
transistor load. And whatever else is connected will reduce the effective
circuit Q. It could be that it doesn't matter what the intrinsic Q of the
coil may be provided it is not ridiculously low. Which I suspect to be true
in your case. You may be doing your nut about nothing.

To put it crudely, it is seldom that coil Q matters. Nearly always whatever
you've got is good enough. If in a particular application you might think it
does then you are barking up the wrong tree.
----
Reg, G4FGQ

Circuit Q = omega*L/R. Reducing L has little or no effect on Q because,
after winding L, you will find R has decreased in about the same proportion.
The fewer the number of turns, the shorter the length of wire, and the lower
the resistance.

The ratio of L to C also has little effect on circuit Q because the
intrinsic Q of capacitors is usually an order of magnitude or more greater
than Q of L. L and C values of a tuned circuit are usually selected by the
reactances required of them at resonance for reasons independent of circuit
Q. Eg., the reactance of L and C may be required to be 300 ohms at
resonance because other components will have to be connected to them.

Usually it is the value of C which controls the value of L. C may have to
be trimmer. If it is a fixed value it will have to conform to a preferred
series of values and tolerances. If it is too small it will get lost in
stray and other circuit capacitances.

IMPORTANT - Intrinsic Q of a solenoid is directly proportional only to its
physical size. Double all dimensions, including wire diameter, and Q is
doubled. Its the the amount of space you have which decides the value of Q.
And there's a similar relationship even for magnetic cored components. If
you havn't got the room then you will have to put up with a low coil Q. And
it's always lower than what you think it is. Its impossible to measure in
situ. Spice is of no help.

Don't forget that a tuned circuit is never used in isolation. If it is used
as a filter in transistor collector circuit then it forms only part of the
transistor load. And whatever else is connected will reduce the effective
circuit Q. It could be that it doesn't matter what the intrinsic Q of the
coil may be provided it is not ridiculously low. Which I suspect to be true
in your case. You may be doing your nut about nothing.

To put it crudely, it is seldom that coil Q matters. Nearly always whatever
you've got is good enough. If in a particular application you might think it
does then you are barking up the wrong tree.
----
Reg, G4FGQ

Absolutely not!!!!!!!!!!!!!!!!!!
He needs best possible loaded Q for maximum selectivity! Good unloaded Q
will help minimuze circuit losses, but won't help with selectivity.

Joe
W3JDR

"Airy R. Bean" wrote in message
...
Then it is unloaded Q that you are interested in.

"Paul Burridge" wrote in message
...
On Sun, 21 Mar 2004 16:02:18 GMT, "W3JDR" wrote:
Airy,
What you said would be relevant only if you were trying to determine
circuit
losses due to "unloaded" Q of the components. I believe Paul is trying
to
determine the 'loaded" Q in order to obtain best selectivity (narrowest
bandwidth). Is this true Paul?
Some clarification is necessary!
The application is the tank in a frequency multiplier.
I am seeking to select for the 5th harmonic. Therefore, the tank needs
to have as little loss as possible given the fact that the 5th will be
way down dB-wise on the fundamental. I can't afford to attenuate it
too much as it's already weak to begin with. Ergo, I need the lowest
loss components and the best selectivity for the desired 5th harmonic.

Absolutely not!!!!!!!!!!!!!!!!!!
He needs best possible loaded Q for maximum selectivity! Good unloaded Q
will help minimuze circuit losses, but won't help with selectivity.

Joe
W3JDR

"Airy R. Bean" wrote in message
...
Then it is unloaded Q that you are interested in.

"Paul Burridge" wrote in message
...
On Sun, 21 Mar 2004 16:02:18 GMT, "W3JDR" wrote:
Airy,
What you said would be relevant only if you were trying to determine
circuit
losses due to "unloaded" Q of the components. I believe Paul is trying
to
determine the 'loaded" Q in order to obtain best selectivity (narrowest
bandwidth). Is this true Paul?
Some clarification is necessary!
The application is the tank in a frequency multiplier.
I am seeking to select for the 5th harmonic. Therefore, the tank needs
to have as little loss as possible given the fact that the 5th will be
way down dB-wise on the fundamental. I can't afford to attenuate it
too much as it's already weak to begin with. Ergo, I need the lowest
loss components and the best selectivity for the desired 5th harmonic.

I don't agree. The selection of a harmonic is a
signal processing function and not one of power transfer.

"W3JDR" wrote in message
...
Absolutely not!!!!!!!!!!!!!!!!!!
He needs best possible loaded Q for maximum selectivity! Good unloaded Q
will help minimuze circuit losses, but won't help with selectivity.

"Airy R. Bean" wrote in message
...
Then it is unloaded Q that you are interested in.

"Paul Burridge" wrote in message
...
I am seeking to select for the 5th harmonic.

I don't agree. The selection of a harmonic is a
signal processing function and not one of power transfer.

"W3JDR" wrote in message
...
Absolutely not!!!!!!!!!!!!!!!!!!
He needs best possible loaded Q for maximum selectivity! Good unloaded Q
will help minimuze circuit losses, but won't help with selectivity.

"Airy R. Bean" wrote in message
...
Then it is unloaded Q that you are interested in.

"Paul Burridge" wrote in message
...
I am seeking to select for the 5th harmonic.

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.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

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.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Loaded Q or unloaded Q? If the load is in parallel with the tank and
a fixed value, decrease the reactance of the tank elements. If the
fixed resistive load is in series with the L and C, increase the
reactance of the tank elements. Generally you should design the Q to
fit the task. (You could expand on that: generally you should design
the circuit to fit the task...)

Unloaded Q is increased by things like using the right core material
and right winding techniques. There's not a simple answer. For
air-core coils, the larger the coil the higher the Q possible, up to
the point where radiation becomes significant. You might hear that
helical resonators are very high Q, but actually the same coil in
freespace will be higher Q, so long as it's not so large it radiates a
lot. Reg has a program that estimates unloaded Q of air-core RF
coils.

It's a fairly complex subject...don't expect one answer to fit all
situations.

Cheers,
Tom

Paul Burridge wrote in message . ..
Hi guys,

ISTR that one can improve Q in resonant tanks by having a low L-C
ratio. Or was it high L-C ratio. I can't remember but need to know.
Can any kind soul help me out? Thanks.

p.