Digital Signal Processing Reference
In-Depth Information
17.6 Digital Upconversion
Alternately, this process can be done digitally. Let us assume
that the information content, whether it is voice, music, or data, is
in a sampled digital form. In fact, as we covered in the earlier
chapter on modulation, this digital signal is often in a complex
constellation form, such as QPSK or QAM, for example.
In order to transmit this information signal, at some point it
must be converted to the analog domain. In the past, the
conversion from digital to analog occurred when the signal was
in baseband form, because the data converters could not
handle higher frequencies. As the speeds and capabilities of
analog-to-digital converters (ADC) and digital-to-analog
converters (DAC) have improved, it has become possible to
perform the up- and downconversions digitally, using a digital
carrier frequency. The upconverted signal, which has much
higher frequency content, can then be converted to analog form
using a high-speed DAC.
Upconversion is accomplished by multiplying the complex
baseband signal (with I and Q quadrature signals) with a
complex exponential of frequency equal to the desired carrier
frequency.
The complex carrier sinusoid can be generated using a lookup
table, or implemented using any circuit capable of generating two
sampled sinusoids offset by 90 degrees. In order to do this digi-
tally, the sample rates of the baseband and carrier sinusoid signal
must be equal. Since the carrier signal will usually be of much
higher frequency than the baseband signal, the baseband signal
will have to be interpolated, or upsampled, to match the sample
frequency of the carrier signal. Then the mixing or upconversion
process will result in the frequency spectrum shift depicted
in
Figure 17.16
Carrier Signal
Baseband on
Carrier Signal
Baseband
Fc = Carrier
Signal
Frequency, F
Baseband
Carrier
Signal
Frequency, F
Figure 17.16. Frequency spectrum of upconversion.
