Re: Q of parallel tuned circuit
- From: "John - kd5yi" <kd5yikes@xxxxxxxxxxxxxx>
- Date: Sat, 2 Apr 2005 17:22:28 -0600
<Albert> wrote in message
news:m15u41difakgoruu25672aok9mc7uvs6lp@xxxxxxxxxx
>
> Hi John, thanks for the help.
Hi, Albert -
> I understand about the Q of the coil now. And, I understand that a
> smaller value cap will have less esr (and, therefore a higher Q).
>
> I'm not sure how the Q or the cap and the Q of the coil combine though
> (to make an overall Q for the tuned circuit.
The Qs combine just like parallel resistors. Q(total)=1/(1/Ql + 1/Qc)
whether in parallel or series.
> I'm working on a 17 Khz receiver that will be magnetically coupled so
> the antenna will be a coil (not a wire).
Yes, I understand that.
> The manufacturer of one of the chips I might use cautions about using
> a high Q coil because the bandwidth will be much to narrow and
> temperature change becomes an issue then.
I think you will have some trouble getting a Q high enough for this to be a
problem, but I might be wrong. You have already said you wanted a Q of 100,
as I recall.
> Since the receiver has to operate in 0 degree weather as well as 90
> degree weather, I had hoped to use an air wound coil in the finished
> product.
With a low Q, this is not a problem. And, I doubt you will get a high Q in
a small size without some heroic effort. But, I've been wrong before.
> The receiver has to be small and use very low power, so the physical
> size of the antenna matters alot...which is why I started with a 10 uh
> coil and a higher value cap. It also needs to be somewhat omni
> directional so I planned on having 2 coils mounted at 90 degree
> angles.
>
> If I use ferrite cores, the antenna coil can be much smaller tho-most
> likely I can't get away from ferrite coils. Since the coil is a
> receiving antenna, I can probably use high mu ferrite, which will make
> the coil MUCH smaller.
I'm not sure you understand what I proposed. Imagine a loop of (say) two
feet in diameter composed of one turn of wire. On the circumference of the
loop is a small toroid of (say) .5 inches inside diameter. Close wound on
the toroid is (say) 20 turns (or 50 or 100) of #30 wire.
Now you get a 20:1 increase in voltage or current. A 400 to 1 change in
impedance. Your very low impedance loop is now 400 times better at matching
a realistic input impedance.
Of course, this is hypothetical and I have very little to back it up except
for the example I gave in another post.
In any case, I wish you luck and I will follow your posts. They will be
educational, I'm sure.
John
.
- References:
- Re: Q of parallel tuned circuit
- From: Bob Liesenfeld
- Re: Q of parallel tuned circuit
- From: John - kd5yi
- Re: Q of parallel tuned circuit
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