coupling cap's PLEASE HELP!

[telluric42]

Hi all,
ive been looking at the Tate ambient power circuit (and others) and trying very hard to get my head around series cap's(coupling??) and how they function. i have red that they block dc and allow ac to pass and the picture i get in my head is a cap as a membrane (like a drum) and the ac beats transfering the "signal" to the other side while the dc is constant pressure and therefore not transfered to rest of circuit.

my questions are:

does my above thinking sound right?
how does the value of the cap effect the behaviour (fequencey??)....filter IN or OUT
if the value filters frequency is there a calculator/formular

PLEASE dumb down reply alot i'm not a wiz at electronics 

thanks heaps

[Talath]

Here's a couple of nifty online calculators

Parallel caps.
http://www.csgnetwork.com/parallelcapacicalc.html

Series caps.
http://www.csgnetwork.com/seriescapacicalc.html

Cap resonant frequency.
http://www.csgnetwork.com/capresonancecalc.html

Hope these help you out.

-Tal

[telluric42]

thanks Talath that helps with the "standard" calculations i just also need to get my head around the mechanics of how the ac is allowed thru and how the cap rating may effect the frequencies. if at all

for example if i use a 100pf cap or a 1uf cap how wud that effect the circuit 

the general arrangement im thinking of is antenna - cap - ground

please anyone 



   

[gyulasun]

How AC is allowed through?  You may consider it as a continous sequences of chargings and dischargings at the rate of the AC frequency involved.

The capacitive reactance is measured in Ohms and you can calculate it as:

X_C=1/(2*pi*f*C)

pi=3.1415
f= AC frequency in Hz
C= capacitance value in Farad

Here is an online calculator for it: http://www.66pacific.com/calculators/xc_calc.aspx

In antenna coupling circuits you may consider variable capacitors as if they were (frequency dependent) variable resistors but the smaller the capacitor value, the higher their capacitive reactance, hence the smaller the AC current they can pass, at a given frequency of course.

In resonant circuits the variable caps can change the resonant frequency of the circuit: http://www.midnightscience.com/formulas-calculators.html
Normally the cap ratings voltage, loss, tolerance etc) does not affect its behaviour to AC, within the limits of course.

rgds, Gyula

[telluric42]

thanks heaps gyulasun