#### SteveM99

Joined Feb 10, 2011
10
There is currently another thread about inductors but i didn't want to hi-jack it. In it a poster mentions using an inductor to block Backward Radiation of an RF Source.

I have a pc board with a similiar circuit that combines a RF Freq. Source (1Mhz) and a DC Source (20v) (both outputs are tied together to drive a RF Coil).

The output of the RF circuit uses a Series Capacitor to block the DC (from getting into the RF Section) and the output of the DC circuit uses a Series Inductor to block the AC (RF from getting into the DC Section).

The DC uses a series 2mH inductor (on the DC output) but 200mv of the (1Mhz) signal still gets through the inductor into the DC section (200mv@1Mhz).

Would putting two back/back diodes in series with the inductor block the 200mv from getting through? Since the diodes need ~= 600mv to conduct? How can I totally block the 200mv/1Mhz?

Appreciate any info... thanks.

#### Wendy

Joined Mar 24, 2008
22,368
You could use diodes to regulate voltage, I do this all the time, but they are shunts in this case, similar to capacitors with a different operating theory. You can also use diodes as true switches, I have seen some very high quality RF switches using diodes, blocking or passing RF depending on bias.

Coils and caps are inverse of each other. In a nutshell a cap resists change in voltage, while a coil resists change in current.

One of the several directives of this concept is a cap tends to pass the effects of AC, and block DC, while a coil tends to pass DC, and block the effects of AC.

Of course, it is actually more complex than that. The fact coils and caps are mathematically inverse of each other means some very interesting things can happen with reactance and resonance.

You want to try an interesting thought experiment, work the voltage drop of a coil and cap in series being fed a resonant signal. Calculate the voltage across each. It is a really good example why reactance is not resistance.