Radio Frequency (RF) Carrier Signal
As mentioned above, a carrier sinusoid carries the
information signal over the airwaves in the case of radio transmission or though
wires or optical fibres in the case of telephony or computer communication.
The carrier for radio is a sinusoid with a high frequency.
For example, the carrier frequency for FM signals will be around 100 MHz which
means that the transmitter will produce a sinewave which goes through one cycle
100,000,000 times per second, or one cycle lasts for a period of 10 nanoseconds
(ns) which is 10 billionths of a second. A DAB carrier frequency will be around
200 MHz (for its present use in band III) so would have a period of 5ns because
the frequency is the reciprocal of the period and vice versa, i.e.:
frequency = 1 / period
The carrier “carries” the information by being
modulated by an information signal such as an audio signal. Modulation just
means “is modified in some way”. The easiest way to describe modulation is
using the example of amplitude modulation (AM). Here, the audio signal will
alter the amplitude of the carrier; when the amplitude of the audio signal goes
up, the amplitude of the carrier goes up too by a proportional amount, when the
signal goes down, the carrier frequency goes down, and so on. For frequency
modulaton (FM), the amplitude of the audio signal changes the frequency of the
carrier so that when the amplitude of the audio signal goes up, the carrier
frequency will be increased to a higher frequency and when it does down the
carrier frequency will go down. Another type of modulation is phase modulation.
Here the phase angle of the carrier is changed. This is more important for
digital modulation and will be dealt with later.
The audio signal is called a baseband signal or lowpass
signal because its band of frequencies go down to zero frequency, or DC (direct
current) or to a low frequency relative
to high frequencies such as radio frequencies. For example, audio signals cover
the range of frequencies from 20Hz up to 20kHz and so as this band of
frequencies is very low in comparison with the radio frequency (RF) carrier the
audio signal is termed a baseband- or lowpass-signal.
The result of modulating the carrier with the baseband
signal is that the band of frequencies (spectrum) of the baseband signal is
translated up to the carrier frequency, hence another name for this is
upconversion. One thing that confuses people not familiar with communication
theory is that as a result of using Fourier Theory, the signal contains negative
as well as positive frequencies. Don’t
bother trying to visualize what negative frequency really means in the physical
world and just accept that a signal such as an audio signal is analyzed in the
frequency domain using both positive and negative frequencies components and an
audio signal’s negative frequency components are the mirror image of the
positive frequency components, mirrored around zero Hz, so the spectrum looks
symmetrical and the symmetric axis is 0 Hz. The effect of translating this
baseband spectrum up in frequency though is that both the positive and negative
frequency components are translated up in frequency. For example, if you just
multiplied the carrier frequency with a frequency of 100 MHz by a baseband
signal, this translates the baseband signal up in frequency so that the
upconverted signal’s spectrum now is symmetrical about the carrier frequency
at 100 MHz. So, if you had a baseband signal with a bandwidth of 20kHz and
upconverted this signal by multiplying it by the carrier, the new spectrum at
the higher frequency will cover the frequency range
f_high = 100 MHz + 20kHz = 100.02 MHz
down to
f_low = 100 MHz – 20 kHz = 99.98 MHz
so its bandwidth is:
B = f_high -
f_low = 100.02 MHz
- 99.98
MHz = 40 kHz
That is, because both the positive and negative frequency
components have been translated up in frequency the baseband bandwidth has
doubled from the original 20 kHz to 40 kHz.
This higher frequency signal is called a passband signal as
opposed to a baseband signal.
Radio signals are often converted first up to an
intermediate frequency (IF), before being converted higher to the radio
frequency (RF). This is to do with filters being easier to build with a given
frequency response when the filter has a fixed centre frequency than are filters
that have a variable centre frequency. The centre frequency is halfway between
the filter’s upper and lower cutoff frequencies and a cutoff frequency is
where the frequency outputs a frequency component with half the power that it
contained at the filter input.
As well as at the IF, there will be a filter at the higher
RF, which is the final transmitter centre frequency. This filter is used to
limit the amount of power that strays into adjacent frequency bands and thus
interfere with signals in the adjacent bands.
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