Dumb newbie has another question...

[Dave]

How can I increase the Q of a tank circuit? ANY ideas would be
appreciated...

Thanks,

Dave
db5151@hotmail.com

 

[no_one]

reduce R
"Dave" <db5151@hotmail.com> wrote in message
news:crhisc$jhf@library2.airnews.net...

How can I increase the Q of a tank circuit? ANY ideas would be
appreciated...

Thanks,

Dave
db5151@hotmail.com

 

[Andrew Holme]

Dave wrote:

How can I increase the Q of a tank circuit? ANY ideas would be
appreciated...

Thanks,

Dave
db5151@hotmail.com

Reduce the load on it:
1. increase load impedance;
2. change the tap point (inductive or capacitive tap).

Use higher quality (Q) components

Optimise the L/C ratio

 

[Bob Eldred]

"Dave" <db5151@hotmail.com> wrote in message
news:crhisc$jhf@library2.airnews.net...

How can I increase the Q of a tank circuit? ANY ideas would be
appreciated...

Thanks,

Dave
db5151@hotmail.com

You can drive the tank with negative resistance to increase the Q. You do
this with an amplifier appropriately connected to provide positive feedback
sensing the current in the tank. A bridge circuit can be used to do this.
Care must be taken as the circuit can easily burst into oscillation if too
much positive feed back is applied. Needless to say, the bandwidth of the
amplifier must be great enough to cover the frequency of interest. Key
words: positive feedback, negative resistance, negative impedance
converters, NIC, gyrator, variable damping.
Bob

 

[Andrew Holme]

Bob Eldred wrote:

"Dave" <db5151@hotmail.com> wrote in message
news:crhisc$jhf@library2.airnews.net...
How can I increase the Q of a tank circuit? ANY ideas would be
appreciated...

Thanks,

Dave
db5151@hotmail.com


You can drive the tank with negative resistance to increase the Q.
You do this with an amplifier appropriately connected to provide
positive feedback sensing the current in the tank. A bridge circuit
can be used to do this. Care must be taken as the circuit can easily
burst into oscillation if too much positive feed back is applied.
Needless to say, the bandwidth of the amplifier must be great enough
to cover the frequency of interest. Key words: positive feedback,
negative resistance, negative impedance converters, NIC, gyrator,
variable damping.
Bob

Is that a "Q-multiplier" ?

 

[Dave]

"no_one" <no_one@earthlink.net> wrote in message
news:efYCd.1202$Pm6.1042@newsread1.news.pas.earthlink.net...

reduce R

In other words, higher quality components? Or are you thinking something
else. Really *am* new to RF...

Dave
db5151

"Dave" <db5151@hotmail.com> wrote in message
news:crhisc$jhf@library2.airnews.net...
How can I increase the Q of a tank circuit? ANY ideas would be
appreciated...

Thanks,

Dave
db5151@hotmail.com

 

[Dave]

"Andrew Holme" <andrew@nospam.com> wrote in message
news:crhjv0$fup$1$8300dec7@news.demon.co.uk...

Dave wrote:
How can I increase the Q of a tank circuit? ANY ideas would be
appreciated...

Thanks,

Dave
db5151@hotmail.com

Reduce the load on it:
1. increase load impedance;

Okay, I *think* I am working on that...

2. change the tap point (inductive or capacitive tap).

Use higher quality (Q) components

Optimise the L/C ratio

How would I optimise the LC ratio? Seems like I saw something about this in

one of my books, but now can't find it. What should I look up?


Thanks,

Dave
db5151@hotmail.com

 

[Andrew Holme]

Dave wrote:

How would I optimise the LC ratio? Seems like I saw something about
this in one of my books, but now can't find it. What should I look up?

There is stuff about this in the RSGB and ARRL handbooks.

Loaded Q = X / R = wL / R = 1 / wCR

Where w = omega (2*pi*f)

You can increase Q by increasing L and reducing C - but only to a point;
you're also increasing the losses in L.

 

[Rich Grise]

On Wed, 05 Jan 2005 16:38:20 -0600, Dave wrote:

"no_one" <no_one@earthlink.net> wrote in message
news:efYCd.1202$Pm6.1042@newsread1.news.pas.earthlink.net...
reduce R

In other words, higher quality components? Or are you thinking something
else. Really *am* new to RF...

So, draw a graph. Overlay a graph of Xc = 1/(2*pi*f*c) and Xl = 2*pi*f*l,
and draw a phasor diagram of them, along with R, whether it's in series
with C, in series with L, in parallel with both, in parallel to series RC,
whatever.

Nothing to it! ;-P

Cheers!
Rich

 

[Rich Grise]

On Wed, 05 Jan 2005 16:40:55 -0600, Dave wrote:

"Andrew Holme" <andrew@nospam.com> wrote in message
Dave wrote:
How can I increase the Q of a tank circuit? ANY ideas would be
appreciated...

Reduce the load on it:
1. increase load impedance;

Okay, I *think* I am working on that...

2. change the tap point (inductive or capacitive tap).

Use higher quality (Q) components

Optimise the L/C ratio

How would I optimise the LC ratio? Seems like I saw something about this in
one of my books, but now can't find it. What should I look up?

That's the crux of the whole thing.

This one item is the thing that you were supposed to have learned in the
last three or so months in this class.

If you haven't got it after three months of classroom instruction, how do
you expect to get it from a freaking newsgroup post?

Good Luck!
Rich

 

[Paul Burridge]

On Wed, 5 Jan 2005 14:30:49 -0600, "Dave" <db5151@hotmail.com> wrote:

How can I increase the Q of a tank circuit? ANY ideas would be
appreciated...

Use a decent quality capacitor with low internal losses and wind your
own coil. Make it from silver/silver plated/gold plated wire and wind
it with nice, open, wide diameter, well-spaced turns and no core
material.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[Dave]

"Paul Burridge" <pb@notthisbit.osiris1.co.uk> wrote in message
news:t3tqt0pnm1r54k83hde9bvp57jg9j4t2gv@4ax.com...

On Wed, 5 Jan 2005 14:30:49 -0600, "Dave" <db5151@hotmail.com> wrote:

How can I increase the Q of a tank circuit? ANY ideas would be
appreciated...

Use a decent quality capacitor with low internal losses and wind your
own coil. Make it from silver/silver plated/gold plated wire and wind
it with nice, open, wide diameter, well-spaced turns and no core
material.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

Thank you. I think this answers something I just asked Tam, about the
practicallity of using RF chokes for my inductors. I *think* I have
something on winding inductors. Maybe in the ARRL handbook I bought late
last year (couldn't wait, too impatient.)

Much appreciation.

Dave
db5151@hotmail.com

 

[JeffM]

Make it from silver/silver plated/gold plated wire
Paul Burridge

Gold plating is not used in electronic assemblies
because of gold's *superior* conductivity characteristics.
Both copper and silver beat it here--at a lower cost.
http://www.google.com/search?&q=resistivity+gold+silver+copper+aluminum+platinum+ibiblio+OR+stealth316+OR+filtranmicro
Gold's resistance to oxidation is the reason it is used.

 

[Dave]

"Tam/WB2TT" <t-tammaru@c0mca$t.net> wrote in message
news:w9KdnVHN-dvOEEDcRVn-2A@comcast.com...

"Dave" <db5151@hotmail.com> wrote in message
news:crjvh0$5ho@library2.airnews.net...

"Tam/WB2TT" <t-tammaru@c0mca$t.net> wrote in message
news:z8OdnfjmWMCdz0DcRVn-og@comcast.com...

"Dave" <db5151@hotmail.com> wrote in message
news:crhqg9$e9k@library2.airnews.net...

"Andrew Holme" <andrew@nospam.com> wrote in message
news:crhjv0$fup$1$8300dec7@news.demon.co.uk...
Dave wrote:
How can I increase the Q of a tank circuit? ANY ideas would be
appreciated...

Thanks,

Dave
db5151@hotmail.com

Reduce the load on it:
1. increase load impedance;

Okay, I *think* I am working on that...


2. change the tap point (inductive or capacitive tap).

Use higher quality (Q) components

Optimise the L/C ratio

How would I optimise the LC ratio? Seems like I saw something about
this
in
one of my books, but now can't find it. What should I look up?





Thanks,

Dave
db5151@hotmail.com


Dave,
To a first approximation, if you have an R, an L, and a C in parallel,
Then
at resonance
X(L) = X(C) and Q = R/X(L).
Now, to make Q larger, make R bigger, or the inductor smaller. There is
a
limit as to how far you can go with this, because the loaded Q can not
be
bigger than the unloaded Q of the inductor.
; you will then need a better inductor. For example, when you get to
around
50 MHz, it is not unusual to see air wound inductors made with #10
wire.
On
the other hand, below about 20 MHz, you are probably best off with an
inductor wound on a toroidal core. Amidon publishes Q curves for
various
toroids for different L and frequency combinations.

If you tap down on the inductor, say connect the load to the center tap
of
the inductor, the effective load the inductor sees is 4x the R value,
but
you will get 1/2 the voltage into the load.

Lastly, use decent capacitors. Mica is best. The El Cheapo capacitors
sold
for bypassing will not work in a tuned circuit.

Tam



Uh HUH. Okay. I do appreciate the help. I am currently working with a
big, clunky variable capacitor and some RF chokes, connected by wire and
a
switch (to choose which inductor is in the circuit). Unless I am
mistaken,
my R is already quite low because of this. So I should ADD R to the
circuit?
Would it be best to have a 1:1 relationship between R and X(L)? Or
should
I
go for 2:1, 10:1, or 100:1? My old textbook doesn't seem to cover
anything
like this. One more thing, are RF chokes okay for tank circuits, or
should I try something else?

Thanks much for the help...

Dave
db5151@hotmail.com

Dave,

If you are at frequency F, X(L)= 2 x Pi x F x L, where F is in Hertz, and
L
is in Henrys. So, at 8 MHz, for example, a 10 uH inductor has a reactance
of
2 * 3.14 * 8E6 * 10E-6 = 50.24 Ohms. If you want the Q to be at least 100,
R
has to be 50.24 * 100 = 5024 OR GREATER,

RF chokes are NOT OK for tuned circuits. They are specifically made to be
fairly low Q. I don't remember what your load was, but the gate input of a
FET is about as good (high impedance) as you can get. What is your
frequency, what Q do you want, and what is the load impedance. What are
you
driving it from? that has an effect also.

Tam

Damn. Now I understand a little better why I am seeing what I am seeing.
The amazing thing is that my project works as well as it does. And now I
understand why the schematic for the end product (that I hope to end up with
someday) uses the components it does (air-wound coils/FET's etc.) Thank you
so much for the illuminating information. I went to an electronics trade
school 24 years ago and learned enough to fix computers, but RF is so
totally different as to be mind-boggling. That mind-bogglingness is
attractive to me though, and as a hobby I don't think anything else could
quite match it for obsessive interest. I am currently playing with
shortwave receiving rigs, but people keep telling me I ought to get into HAM
radio, and I may someday. Once I understand a few things.

Many thanks. More questions later, I am sure.

Dave
db5151@hotmail.com

 

[Reg Edwards]

How can I increase the Q of a tank circuit? ANY ideas would be
appreciated...

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

If R is in series with L and both are in parallel with C then, at resonance,
all is fully explained by -

Q = 1 / 2 / Pi / F / C / R

or

Q = 2 * Pi * F * L / R

and that's all there is to it.

People who missed out arithmetic at school have a hard time understanding
radio. Using arithmetic it is so precise and simple to describe.
----
Reg.

 

[Dave]

"Bob Eldred" <nsmontassoc@yahoo.com> wrote in message
news:1104959771.793038@news-1.nethere.net...

"Dave" <db5151@hotmail.com> wrote in message
news:crhisc$jhf@library2.airnews.net...
How can I increase the Q of a tank circuit? ANY ideas would be
appreciated...

Thanks,

Dave
db5151@hotmail.com


You can drive the tank with negative resistance to increase the Q. You do
this with an amplifier appropriately connected to provide positive
feedback
sensing the current in the tank. A bridge circuit can be used to do this.
Care must be taken as the circuit can easily burst into oscillation if too
much positive feed back is applied. Needless to say, the bandwidth of the
amplifier must be great enough to cover the frequency of interest. Key
words: positive feedback, negative resistance, negative impedance
converters, NIC, gyrator, variable damping.
Bob

Negative resistance, like a tunnel diode? Maybe with it's own tank circuit?
(I don't *think* that would be hard to implement with my setup.) I am
working on a simple RF amplification stage that provides positive feedback,
could that help? Was going to use a Darlington pair of 2N3906's until I saw
if/how it worked, then rebuild it with a pair of NTE 10 low-noise VHF/UHF
amplifiers. Starting with 3906's cause that's what I have handy. Does that
sound like a decent idea? If so, should I just cut to the chase and go with
the NTE 10's?

Will have to look negative impedance converters. What is a gyrator? And how
could I use variable damping?

Sorry for all the questions. Total newbie. Thanks for the help.

Dave
db5151@hotmail.com

 

[Dave]

"Rich Grise" <richgrise@example.net> wrote in message
newsan.2005.01.06.09.41.38.725950@example.net...

On Wed, 05 Jan 2005 16:40:55 -0600, Dave wrote:
"Andrew Holme" <andrew@nospam.com> wrote in message
Dave wrote:
How can I increase the Q of a tank circuit? ANY ideas would be
appreciated...

Reduce the load on it:
1. increase load impedance;

Okay, I *think* I am working on that...

2. change the tap point (inductive or capacitive tap).

Use higher quality (Q) components

Optimise the L/C ratio

How would I optimise the LC ratio? Seems like I saw something about
this in
one of my books, but now can't find it. What should I look up?

That's the crux of the whole thing.

This one item is the thing that you were supposed to have learned in the
last three or so months in this class.

If you haven't got it after three months of classroom instruction, how do
you expect to get it from a freaking newsgroup post?

Good Luck!
Rich

What class? I haven't been in a class on this topic in over 20 years. Since
then I spent 12 years swapping boards and 11 years sleeping (currently on
disability for a sleep disorder.) Consequently, I have forgotten more than
I ever knew. That's why I am asking, and am not up on some of the current
technology. Also why I appreciate the help, and patience...

Dave
db5151@hotmail.com