برق_مخابرات_کامپیوتر_تاریخی
INTRODUCTION
1.1 Receiver Architectures
The traditionally used architecture, the superheterodyne or superhet in short
(Fig. 1.1), down-converts a RF signal to baseband in one or sometimes two stages
depending on the frequency plan for the chosen architecture. Mixers, used for this down
conversion, convert the RF frequency to a lower intermediate frequency (IF) for
filtering and amplification before finally converting it to baseband . The IF is defined as:
(1.1)
Where
respectively. These signals form the input to the mixer, which performs analog
multiplication on them to give rise to the signal at IF. The major drawback of this
architecture which also limits its integrability onto a single chip is the presence of image
frequencies. An image is defined as a signal other than the signal of interest that mixes
to the same IF as the desired signal. Down-conversion of two different frequencies to
the same IF occurs because of the inability of the mixer to recognize the polarity of the
frequency difference between the RF and LO. For example, if the RF signal is located
one IF higher than the LO (low-side injection), the image frequency is located at,
And if the RF signal is located one IF lower than the LO (high-side injection), image
Frequency is located at,
Therefore in the superhet architecture the mixer has to be preceded by filters for the
Purpose of image rejection and channel selection and these filters have to have steep
roll-off and very high out-of-band rejection to attenuate the unwanted signals. To
acoommodate these stringent requirements, the filters need to be designed with high
Quality (Q) factor resonators and multiple poles. The low Q factor of on-chip inductors
results in prohibitively high passband insertion loss for multiple-poled integrated
inductor and capacitor (LC) filters. Furthermore, since monolithic inductor and
capacitors require a great deal of die area, multiple pole LC filters quickly become
excessively large for on-chip integration.
Figure 1.1 Superheterodyne receiver architecture[2]
1.1.1 Direct Conversion Receivers (DCRs)
An alternative approach to heterodyning described above is to convert the RF
Frequency directly to baseband frequency thereby getting rid of multiple stages of
? نویسنده: چنگیزی