# Self-Resonant Frequency and Resonant Frequency

Discussion in 'Electronic Basics' started by [email protected], Aug 22, 2006.

2. ### w2aewGuest

Resonance (in electrical parlance) refers to the frequency where the
capacitive reactance is equal to the inductive reactance. At this
point, the phases cancel and you get a sharp change in the overall
impedance. For example, a parallel LC circuit (capacitor in parallel
with an inductor) will be in resonance at a frequency where 2*pi*f*L =
1/(2*pi*f*C). At this frequency, the impedance of this parallel LC
circuit will be extremely high due to the phase combination.

Normally, the term "self-resonant" frequency refers to the resonant
frequency of a component due to the parasitic characteristics of the
component. For example, the physical construction of a capacitor will
determine the parasitic inductance of the leads/connections to the cap.
The frequency where the capacitive reactance of the cap and the
inductive reactance of the lead's parasitic inductance, is the
self-resonant frequency. It is a property of the physical
characteristics of the device - and to some extent, defines where the
cap no longer behaves like a cap, but begins to behave like an
inductor.

When designing resonant circuits, you pick the capacitance and
inductance. Ideally, you will operate at frequencies that are far away
from the self-resonant frequency of the individual components. Of
course, there are special circumstances where you might do a design
that involves utilizing components near their self-resonant frequency,
but it isn't that common.

3. ### Jim LandGuest

It would help if you tell us what kind of transducer you're talking about.

4. ### defaultGuest

An inductor has a "self resonant" frequency bacause the turns of wire
have small "parasitic" capacitors to each other and its surroundings.
Two wires laying next to each other form a capacitor . . . the self
resonance in a tesla coil produces higher voltages than the simple
turns ratio of the transformer might account for. Electrical
resonance.

Self resonance in a transducer may be electrical and mechanical. For
instance, a piezo transducer when struck will produce voltage, but it
will also vibrate for a period (like striking a steel bar) after it is
struck. The mechanical vibration also produces voltage till it dies
out.

If you drive a transducer at its resonant frequency it is much more
efficient at producing output. The analogy is to push a swing just as
it is about to start its return motion.

5. ### Guest

I would like to clarify the components I talked about.

For Inductor:
http://www.mouser.com/catalog/627/782.pdf

For Transducer:
http://web.pui.com/AudioCommerce/images/catalog/audiopdfs/AI-4228-P-SP250.pdf

They use "Self-Resonant Frequency" for Inductor and "Resonant
Frequency" for Transducer. I wonder if that is the same term. Is the
word "Self" implied something?

Thanks...

6. ### Michael A. TerrellGuest

No, they are not the same. Self resonant frequency happens because
no inductor is pure inductance. It also has resistance and capacitance.
As the frequency goes up, you reach a point where the inductor no longer
has the characteristics you need. When you choose an inductor, you want
to make sure that the SRF is above the frequency you need. I prefer at
least a 50% margin.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

7. ### Guest

No, they are not the same. Self resonant frequency happens because
Thanks. I understand Self-Resonant Frequency now. However, I may miss
this but let me make sure I got everything correctly. So Resonant
Frequency of a Transducer is to get to the max output (the closer, the
better), while Self-Resonant Frequency of an Inductor is to "stay away"
(the higher, the better). Am I wrong?

8. ### Michael A. TerrellGuest

No, that's it in it's simplest form.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida