Here's a post that's mostly for my own studies, but I hope others will find it useful. Today's topic is waves.
AC signals are described using a
sine wave (from high school geometry). Anything above 0 on the typical graph is positive and below is negative. One full cycle of a sine pattern is called the
cycle. The number of cycles per second is the
frequency, which is measured in
Hertz (Hz). The inverse of the frequency is the
period, which is measured in seconds. For example, a wave with a frequency of 3 Hz cycles 3 times per second; it also has a period of 1/3 seconds.
The
wavelength of a wave is the distance that is has traveled in one cycle. Wavelength is usually measured in meters and is represented by the Greek
lambda (λ). Wavelength is the speed (usually the
speed of light, c) divided by frequency. Remember that c is a constant, so we don't need to calculate for it.
λ = c / f
f = c / λ
We can use the old D=R*T equation from middle school on this one. The rate is c, which is the speed of light. The time is the period, which is the inverse of frequency, so, instead of multiplying by period, we can divide by frequency.
Since the
speed of light is constant, it is common to have a wave talked about in terms of frequency or of wavelength. A 3MHz wave can be referred to as a 100m wave.
If capactive and inductive reactance cancel each other, you have
resonance.
Radio frequencies: > 20kHz
Audio frequencies: <> 1GHz
A
band is a range of frequencies used for a specific purpose. For example, the frequencies that ham operators use is called the amateur or ham bands.
The
phase is the position with a cycle. If you are 180 degrees out of phase, you are out of phase.
Modulation is modifying a wave to by adding information to it.
In
amplitude modulation (AM), the carrier wave is modified by the amplitude of the information it to create small peaks and valleys above the carrier wave. The carrier is then subtracted from the resulting wave to get the information out. This process is called
demodulation. Any signal above the carrier wave is called the
upper sideband (USB); any lower is the
lower sideband (LSB). The sidebands contain the information!
If you had a 1MHz carrier and wanted to carry voice, what would the sidebands be? Voice goes up to about 3kHz, so, if you added and subtracted that from the 1MHz carrier, you would have a LSB of 997kHZ and a USB of 1003kHz (1.003MHz).
AM is very inefficient. Most of the power is dedicated to the carrier and two sidebands (which are mirrors of each other). A single sideband (SSB) is a signal with the carrier and one of the sidebands removed. This is much more efficient since the power is dedicated to the data and not the carrier or other sideband. UHF and VHF use the USB of the signal.
Modulation modes that change the frequency to add the information is
frequency modulation (FM). When information is put on the carrier wave, the frequency
deviates to include the information. Amplitude is not changed in FM transmission, making it a
constant power signal. FM has many sidebands that range from 5 to 15 kHz. FM is popular because interference actually generates changes in amplitude, which can be filtered out with a limiter.
Modulation modes that change the phase of a wave when information is added is called
phase modulation (PM).
Data can be transferred over the air by using
data modes. A
modem changes data to audible signals, and, on the other end, another modem demodulates.
Protocols swch as RTTY, AX.25, B2F, PSK31, MFSK, AMTOR, and PACTOR all combine with a modulation to take care of errors and loss. A
terminal node controller (TNC) is a microprocessor that processes the protocols along with a modem. A
multiple protocol controller (MPC) is a TNC that does many protocols.