On Sun, 21 Mar 2004 17:44:09 +0000 (UTC), "Reg Edwards"
wrote:

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.

Be that as it may, Reg, how satisfied are you that your program
accurately calculates Q in the inductors in which it predicts
characteristics? For example, it is accepted by all (I hope) that
Q(unloaded) is at a maxium when the length of a coil equals its
diameter. Your program doesn't seem to reflect this fact. How come?

p.
--

The BBC: Licensed at public expense to spread lies.

On Sun, 21 Mar 2004 17:44:09 +0000 (UTC), "Reg Edwards"
wrote:

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.

Be that as it may, Reg, how satisfied are you that your program
accurately calculates Q in the inductors in which it predicts
characteristics? For example, it is accepted by all (I hope) that
Q(unloaded) is at a maxium when the length of a coil equals its
diameter. Your program doesn't seem to reflect this fact. How come?

p.
--

The BBC: Licensed at public expense to spread lies.

Reduce the L to reduce the resistive loss - the essence of L
is the energy stored in its current carrying, and it is the current that
causes I^2 R losses. The energy stored in the C is static. (Yes, there
are some losses in polarising the dielectrics but these are small enough
to be ignored)

"Paul Burridge" wrote in message
...
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.

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.

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

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.

Of course, maybe you don't need such a high Q, Paul. Qu of 30 is
quite reasonable for small SMT RF inductors, at least the type I use.
In the following list, "series" means in series from the gate output
to the next gate input, in order, and "shunt" means shunt to ground at
that point. Anyway, try this (build it or SPICE it or RFSim99
it...add resistors to any simulation to account for the Qu. I'd
suggest 3 ohms series and 12k ohms shunt for each 1.8uH.)

47pF series
1.8uH series
470pF shunt
45pF series
1.8uH shunt
3.3pF series
40pF shunt
1.8uH shunt
DC blocking cap series
high-value DC bias resistors, and the gate input (I assumed to be
about 4k net resistance to ground at the gate input, including the
bias resistors).

It should give you enough voltage gain at 18MHz to drive the second
gate at the fifth harmonic, and should attenuate the third at least
50dB if you build it properly, even with low-ish Qu inductors. This
is rather a "hack" circuit, but works. The premise is that it's
easier to get three inductors all the same value than muck about
tuning the inductors. Make the 47pF, 45pF and 40pF caps variable and
you can peak up the response at your desired frequency. Your
simulation should show a reasonably flat bandpass characteristic,
centered at about 18MHz.

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.

On 22 Mar 2004 22:01:47 -0800, (Tom Bruhns) wrote:

[filter spec snipped]
It should give you enough voltage gain at 18MHz to drive the second
gate at the fifth harmonic, and should attenuate the third at least
50dB if you build it properly, even with low-ish Qu inductors. This
is rather a "hack" circuit, but works. The premise is that it's
easier to get three inductors all the same value than muck about
tuning the inductors. Make the 47pF, 45pF and 40pF caps variable and
you can peak up the response at your desired frequency. Your
simulation should show a reasonably flat bandpass characteristic,
centered at about 18MHz.

Thanks, Tom! You're even more indefatigable than I am, by the look of
it. I shall look into it...


--

The BBC: Licensed at public expense to spread lies.

On 22 Mar 2004 22:01:47 -0800, (Tom Bruhns) wrote:

Of course, maybe you don't need such a high Q, Paul. Qu of 30 is
quite reasonable for small SMT RF inductors, at least the type I use.
In the following list, "series" means in series from the gate output
to the next gate input, in order, and "shunt" means shunt to ground at
that point. Anyway, try this (build it or SPICE it or RFSim99
it...add resistors to any simulation to account for the Qu. I'd
suggest 3 ohms series and 12k ohms shunt for each 1.8uH.)

Thanks, Tom. I've simulated the filter and posted the plot result to
abse. Looks promising. See if it meets your expectations....

p.
--

The BBC: Licensed at public expense to spread lies.