Blog posts of '2013' 'August'

Iron Mike

Q. In my job, I work with microphones a lot. How does a microphone work?

A. There are three types: dynamic, crystal and electret. In the dynamic type, there is a thin diaphragm which vibrates along with the sound waves in the air. The diaphragm is attached to a coil suspended in the field of a permanent magnet. When the coil vibrates, a voltage is induced in it that varies along with the sound vibrations.

This voltage is the output signal of the microphone. Dynamic mikes are designed to operate into a low impedance, such as 600 ohms. Typical signal levels are about 50mV.

Crystal microphones use a piezoelectric crystal in place of the coil and magnet. In these types of crystals, mechanical pressure produces a voltage. Crystal mikes are of poorer quality than dynamic mikes, and are used in applications where an inexpensive microphone is acceptable.

They operate into a high impedance, usually 100 K to 1 Megohm, and have a high output voltage level, typically 1 volt.

Electret microphones use a capacitor with a special dielectric which holds a charge indefinitely. The plates of the capacitor vibrate along with the sound vibrations, which changes the value of the capacitor. This makes the voltage across it vary, and it is this voltage that becomes the microphone's output signal.

These microphones have excellent frequency response, and are generally used where small size and a high-quality signal are important. Electret mikes usually require a small battery. They are designed to operate into a high impedance.

An older type of microphone used a container full of carbon particles. The diaphragm was attached to the container so that the vibrations would change the pressure on the carbon particles. Thus their resistance varied along with the sound vibrations. The resistance element was connected in series with a fixed resistor and driven by a source of DC. Thus, the voltage across the resistance element served as the microphone's output signal.

Carbon mikes tended to be unreliable. The carbon particles would become compacted, and the mike had to be tapped a few times to loosen them. They have become obsolete, and are rarely used nowadays.

Dopple Duty

Q. I have seen doppler radar in weather reports. What is the difference between regular radar and doppler radar?

A. Basically, radar emits a burst of radio waves. The radio waves reflects off an object and the radar antenna receives the reflection. Since radio energy travels at the speed of light, if in the time between the transmission and reception of the burst is measured, the distance to the object can be determined. The greater the distance, the longer it takes for the burst to reach the object and be reflected back to the radar unit.

If the object, such as a cloud, is not moving, the frequency of the reflected burst will be the same as the transmitted one. But if the object is moving toward the radar unit, the reflected frequency will be higher. If it is moving away from the radar unit, the frequency will be lower.

This is known as the "Doppler shift" after the name of its discoverer. The faster the object is moving toward or away from the radar unit, the greater will be the doppler shift. A doppler radar can thus display the position of a cloud, and it can indicate how the cloud is moving.

How do neon signs work?

A. The glass tube is filled with a gas at a low pressure. Electrodes at the ends of the tube are connected to a source of high voltage AC, typically 30 kV for a small sign. The high voltage ionizes the gas. Electrons are pulled out of the outer orbits of the gas atoms, resulting if free electrons and positively charged gas atoms.

Both of these serve as charge carriers, and support the flow of current through the tube. When electrons fall back into orbit around the ionized gas atoms, photons of light are emitted.

The wavelength of the photon, and hence the color of the light, is determined by the amount of energy the electrons give up when they fall into orbit around the gas atoms. This amount is different for different kinds of gas atoms. Thus, different gases produce different colors of light. Neon give a reddish-orange color, while Krypton gives off a green color, and Argon white. Different blends of gases are also used to produce different shades.

Small neon indicator lamps for panels were popular before the advent of LEDs. These were usually red, although yellow and green were available. About 70 to 80 volts was required to make them conduct, and the voltage across them stayed about the same for a wide range of currents. Usually, they were operated off the AC power line. A current limiting resistor was placed in series with them. They are still used on some types of industrial equipment and vending machines.

Weird Science

Q. What is a tunnel diode?

A. A tunnel diode has an extremely thin insulating layer between the P and N material. We normally can think of electrons as little points of mass with a negative electric field surrounding them. The real picture is more complicated. Electrons not only have charge and mass, they have a wavelength.

In a tunnel diode, the insulating layer is thin compared to the wavelength of an electron. Normally, an electron can't get through an insulator. But in the tunnel diode electrons can sort of pop through the insulating layer due to their wavelike property. This gives the tunnel diode a property called "negative resistance." In ordinary resistance, as we increase the voltage across the resistor, the current increases along with it. In a negative resistance, the opposite happens. As we increase the voltage, the current decreases. This makes the tunnel diode useful as an oscillator.

The tunnel diode can also be used as a very fast switch in digital circuits. The reason it is called a "tunnel" diode comes from a graph of the energy of the barrier voltage at the diode's junction. This graph rises to a peak at the junction. The graph for an electron that pops through the insulating layers looks like a little tunnel cut through the peak of the curve.