Direct Current Theory

If we take a light bulb and connect it to a battery, the bulb will light up. The lamp lights up because current flows through it. The current leaves the battery at the negative terminal, flows through the bulb, and returns to the positive terminal of the battery. The electrons flow in one direction.

This is known in electronics as DIRECT CURRENT flow because the electrons flow only in one direction. The arrows in the figure show the direction that the current would flow in this circuit. As long as we can follow the current from the negative terminal of the battery throughout the entire circuit, and back to the positive terminal, we have a COMPLETE CIRCUIT PATH.

It is very important to remember that current will ONLY flow if the circuit path is complete. If we were to remove the light bulb from the circuit, the circuit path would not be complete, and while voltage would still exist on the battery, no current would flow through the circuit. In order to have any complete circuit, you are required to have at least 3 parts:

(1) The SOURCE or SUPPLY of Voltage.
(2) The LOAD which uses the source Voltage.
(3) A complete path of connecting wires.

Schematic Symbols

Sometime over the years, some bright soul determined that it would be difficult to draw a picture of every component that you decided to put into a circuit. However, they needed a way to tell their colleagues about discoveries and accomplishments. So a system was developed that was a sort of “electrical shorthand”.

They call it a SCHEMATIC DIAGRAM and the individual component representations are called SCHEMATIC SYMBOLS. Throughout the course, I will be introducing you to the various SCHEMATIC SYMBOLS one by one. This lesson will take you through the first two symbols, and describe how they are used in a circuit. The first three SCHEMATIC SYMBOLS you will be introduced to are the lamp, battery and resistor.

Remember that a resistor is any device which causes electrical friction. In electronics, the resistor can be substituted for any current load. The schematic symbol for a battery can likewise be substituted for any direct current supply voltage. So, in essence, we could theoretically use our battery and resistor to represent our light bulb circuit.

Modes and Modulation

Modulation is the process by which voice, music, and other “intelligence” is added to the radio waves produced by a transmitter. The different methods of modulating a radio signal are called modes. An unmodulated radio signal is known as a carrier. When you hear “dead air” between songs or announcements on a radio station, you’re “hearing” the carrier. While a carrier contains no intelligence, you can tell it is being transmitted because of the way it quiets the background noise on your radio.The different modes of modulation have their advantages and disadvantages. Here is a summary:

Continuous Wave (CW)

CW is the simplest form of modulation. The output of the transmitter is switched on and off, typically to form the characters of the Morse code.

CW transmitters are simple and inexpensive, and the transmitted CW signal doesn’t occupy much frequency space (usually less than 500 Hz). However, the CW signals will be difficult to hear on a normal receiver; you’ll just hear the faint quieting of the background noise as the CW signals are transmitted. To overcome this problem, shortwave and ham radio receivers include a beat frequency oscillator (BFO) circuit. The BFO circuit produces an internally-generated second carrier that “beats” against the received CW signal, producing a tone that turns on and off in step with the received CW signal. This is how Morse code signals are received on shortwave.

Amplitude Modulation (AM)

In amplitude modulation, the strength (amplitude) of the carrier from a transmitter is varied according to how a modulating signal varies.

When you speak into the microphone of an AM transmitter, the microphone converts your voice into a varying voltage. This voltage is amplified and then used to vary the strength of the transmitter’s output. Amplitude modulation adds power to the carrier, with the amount added depending on the strength of the modulating voltage. Amplitude modulation results in three separate frequencies being transmitted: the original carrier frequency, a lower sideband (LSB) below the carrier frequency, and an upper sideband (USB) above the carrier frequency. The sidebands are “mirror images” of each other and contain the same intelligence. When an AM signal is received, these frequencies are combined to produce the sounds you hear.

Each sideband occupies as much frequency space as the highest audio frequency being transmitted. If the highest audio frequency being transmitted is 5 kHz, then the total frequency space occupied by an AM signal will be 10 kHz (the carrier occupies negligible frequency space).

AM has the advantages of being easy to produce in a transmitter and AM receivers are simple in design. Its main disadvantage is its inefficiency. About two-thirds of an AM signal’s power is concentrated in the carrier, which contains no intelligence. One-third of the power is in the sidebands, which contain the signal’s intelligence. Since the sidebands contain the same intelligence, however, one is essentially “wasted.” Of the total power output of an AM transmitter, only about one-sixth is actually productive, useful output!

Other disadvantages of AM include the relatively wide amount of frequency space an AM signal occupies and its susceptibility to static and other forms of electrical noise. Despite this, AM is simple to tune on ordinary receivers, and that is why it is used for almost all shortwave broadcasting.

UTC & Standard Time

Since radio signals can cross multiple time zones and the international date line, some worldwide standard for time and date is needed. This standard is Coordinated Universal Time, abbreviated UTC. This was formerly known as Greenwich mean time (GMT). Other terms used to refer to it include “Zulu time” (after the “Z” often used after UTC times), “universal time,” and “world time.”UTC is used by international short-wave broadcasters in their broadcast and program schedules. Ham radio operators, short-wave listeners, the military, and utility radio services are also big users of UTC.

Greenwich mean time was based upon the time at the zero degree meridian that crossed through Greenwich, England. GMT became a world time and date standard because it was used by Britain’s Royal Navy and merchant fleet during the nineteenth century. Today, UTC uses precise atomic clocks, short-wave time signals, and satellites to ensure that UTC remains a reliable, accurate standard for scientific and navigational purposes. Despite the improvements in accuracy, however, the same principles used in GMT have been carried over into UTC.

UTC uses a 24-hour system of time notation. “1:00 a.m.” in UTC is expressed as 0100, pronounced “zero one hundred.” Fifteen minutes after 0100 is expressed as 0115; thirty-eight minutes after 0100 is 0138 (usually pronounced “zero one thirty-eight”). The time one minute after 0159 is 0200. The time one minute after 1259 is 1300 (pronounced “thirteen hundred”). This continues until 2359. One minute later is 0000 (”zero hundred”), and the start of a new UTC day.

To convert UTC to local time, you have to add or subtract hours from it. For persons west of the zero meridian to the international date line, hours are subtracted from UTC to convert to local time.