Digital Signal Processing Reference
In-Depth Information
and receiver noise level, and gives the maximum data rate that
can be sent over a channel, or link, with a given noise level and
bandwidth.
17.5 Signal Upconversion
The process of mixing a baseband signal onto a carrier
frequency is called upconversion, and is performed in the radio
transmitter. In the radio receiver, the signal is brought back down
to baseband in a process called downconversion. Traditionally,
this up- and downconversion process was done using analog
signals and analog circuits. Analog upconversion with real (not
complex) signals is depicted in
Figure 17.15
.
To see why a mixer works, consider a simple baseband signal
of 1 kHz tone and a carrier frequency of 600 kHz. Each can be
represented as a sinusoid. A real signal, such as a baseband
cosine, has both a positive and negative frequency component. At
baseband, these overlay each other, so this is not obvious. But
once upconverted, the two components can be readily seen both
above and below the carrier frequency.
The equation for the upconversion mixer is:
cos
ðu
carrier
t
Þ
cos
ðu
signal
t
Þ
1
2
½
¼
cos
ððu
carrier
þ u
signal
Þ
t
Þþ
cos
ððu
carrier
u
signal
Þ
t
Þ
The result is two tones, of half amplitude, located above and
below the carrier frequency.
ANTa
RF Oscillator
RF Signal
Band Pass
Filter
Low Pass
Filter
Baseband
Source
Mixer
Figure 17.15. Analog upconversion.
