# Dumb newbie has another question...

T

#### Tam/WB2TT

Jan 1, 1970
0
Dave said:
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
[email protected]

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

D

#### Dave

Jan 1, 1970
0
Tam/WB2TT said:
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
[email protected]

R

#### Reg Edwards

Jan 1, 1970
0
How can I increase the Q of a tank circuit? ANY ideas would be============================

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.

R

#### Rich Grise

Jan 1, 1970
0
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...
Well, I apologize for assuming. What "optimize the L/C ratio" means is,
select your inductor and capacitor such that they present a suitable
impedance to the rest of the circuit. Increase Q by decreasing
dissipation, by using a coil with a lower resistance but the same
inductance, that sort of thing.

And sit at google searching on keywords until you find something helpful,
or go to the public library, around 621.384 in the Dewey system.

OK?

Good Luck!
Rich

R

#### Rich Grise

Jan 1, 1970
0
Bob Eldred wrote:
How can I increase the Q of a tank circuit? ANY ideas would be
appreciated...

Thanks,

Dave
[email protected]

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" ?

Yes, and sometimes "BFO".

Cheers!
Rich

[0] Beat Frequency Oscillator.

A BFO is different - it is an oscillator used to generate the tone you
hear from a CW Morse code transmission - nothing to do with
regeneration or Q multiplication.

Yeah, except in this one guy's Hallicrafters SX-140. It had a knob
labeled, "Selectivity/BFO". It was nothing but a Q mult that could be
cranked up until it oscillated.

Cheers!
Rich

D

#### Dave

Jan 1, 1970
0
Rich Grise said:
Well, I apologize for assuming. What "optimize the L/C ratio" means is,
select your inductor and capacitor such that they present a suitable
impedance to the rest of the circuit. Increase Q by decreasing
dissipation, by using a coil with a lower resistance but the same
inductance, that sort of thing.

And sit at google searching on keywords until you find something helpful,
or go to the public library, around 621.384 in the Dewey system.

OK?

Good Luck!
Rich

Gotcha. Thanks. I've already been googling everything I could think of,
and have a wealth of material to peruse. And thanks to all those here I
know, more or less, what direction I need to head in.

Much appreciated.

Dave
[email protected]

D
Replies
10
Views
1K
D
Replies
16
Views
6K
Replies
5
Views
1K
D
Replies
4
Views
1K
D
Replies
10
Views
289