3. The main purpose of the RF receiver is to detect
RF signals within a specific band and to extract
the baseband information carried in the detected
RF signal.
The design of RF receivers for wireless
applications entails many challenges at both
architecture and circuit levels.
Two important specifications are fundamental to
all receivers:
1. Sensitivity- signal strength required to achieve a
given signal-to-noise ratio
2.Selectivity- the ability to reject unwanted signals.
5. The fundamental operation of an RF front
end is fairly straight forward – it detects
and processes the radio waves that have
been transmitted.
It consists of seven active components in the
receiver that process the signal at the
original radio frequency (RF),before it is
converted to lower intermediate frequency
(IF).
8. FUNCTIONS OF FRONT END
Filtering-out the interference and
noise
Amplification of the signal
Frequency down conversion using
a mixer and a local oscillator.
9.
10.
11. The PRE SELECTION FILTER (BPF1) of the RF front
end, removes the out-of-band signal and selects the
band of interest of the received signal. Thus referred to
as BAND SELECTION FILTER.
LNA amplifies the signal.
The BPF2 also known as an ANTI-IMAGE
FILTER/IMAGE REJECTION FILTER attenuates the
undesired signals .
Frequency down conversion(RF to IF) using a MIXER
and a LOCAL OSCILLATOR.
BPF3 also known as the CHANNEL FILTER. This
narrow band channel filter is used for better selectivity.
An IF AMPLIFIER to amplify the signal and improve
further SNRdemod_in of the signal.
DEMODULATOR converts into baseband signals.
12.
13.
14.
15.
16.
17.
18. Principle is similar to heterodyne receiver but it
uses two frequency conversion block.
The first mixer utilizes an LO that is at a fixed
frequency, and all channels in the RF band are
translated to IF, retaining their positions relative to
one another.
The second mixer utilizes a tunable LO, thus
selecting the desired channel to be translated to
baseband.
IF in the wideband IF architecture is typically
high.
Employs low pass filtering in baseband to
19.
20. In the low-IF receiver architecture several
advantages of both heterodyne and the homodyne
architecture are combined.
In Low-IF receiver architecture all the RF signals
are translated to low-IF frequency which is then
down-converted to BB signal in digital domain.
In the low IF receiver there is no need of any HF
suppression of the mirror signal. The HF filter can
be broadband, unturned and cheap.
21.
22. The front end of RF receiver contains
two non idealities which is undesirable
as they degrade SNR of the signals.
1)CIRCUIT NON-LINEARITY.
2)CIRCUIT INTRODUCE NOISE.
23. This is of great concern in RF system design,
since non-linearity gives rise to many difficult
problems, they are:
1)Harmonics distortion
2)Inter-modulation
3)Third order Intercept point
4)Gain compressor
5)Noise figure
6)Blocking
24. Harmonic distortion is said to occur when a signal
is applied at the input of any non-linear circuit, the
output signal will contain several
harmonics(higher order multiples ) of that input
signal.
It adds frequencies components in output which
are not present in input.
25.
26.
27.
28.
29. When signals with different frequencies are
applied to the input of the non-linear component,
the non-linearity gives rise to spurious distortion
signals on other frequencies.
30.
31. • It is the ratio of the amplitude of the third order
inter modulation product (ID3) to the ratio of the
amplitude of the fundamental output component
(ID1)
• The 3rd order products will be the largest (loudest)
of the inter-modulation products
• Mathematically, IM3 = (ID3 / ID1)
• IM3 expressed in decibels.
32. As a general rule, the 3rd order products will increase
(grow) 3-times faster than the fundamental signal (the
signal of interest).
33. The primary difference between I.M.D versus harmonic
distortion is that two or more different frequencies must be
actively present to produce Inter-modulation distortion.
This is different than the nature of harmonic distortion,
which needs but one frequency to be present in order to
form.
Adding to this Inter-modulation distortion products may not
be harmonically related to original frequency, due to this
they may be easily detected by human ears as noise.
Great care is to taken to minimize the IM frequencies.
34. The fundamental principle of TOI is that for every
1-dB increase in the power of the input tones, the
third-order products will increase by 3 dB.
TOI is always calculated as a function of IMD:
TOI = (IMD/2) + power
35. The input to an amplifier is large, the amplifier
saturates, hence clipping the signal.
When the strength of the input is further
increased, the output signal is no longer
amplified.
At this point, the output is said to be
compressed.
We can use the concept of the 1-dB
compression point, defined as the input level
that causes the linear small-signal gain to
drop by 1 dB.
36. The idea of the 1-dB compression point is shown
graphically in Figure below
37. • Blocking happens when a large off channel signal
causes the front-end RF amplifier to be driven to
its compression point.
• As a result all other signals are lost (blocked).
• This condition is frequently called de-sensing—
the sensitivity of the receiver is reduced.
• Blocking is generally specified as the level of the
unwanted signal at a given offset.
38. “Noise is always one of the characteristics of the
design that must be balanced and weighed in. In the
process of reducing the magnitude of noise, it is
possible to adversely affect another performance
aspect of a design.”
“Any unwanted input”
Limits systems ability to process weak signals
Sources:
1.Random noise in resistors and transistors
2.Mixer noise
3.Undesired cross-coupling noise
4.Power supply noise
Dynamic range – capability of detecting weak signals
in presence of large-amplitude signals.
39. NOISE FIGURE
It is defined as the ratio of the signal to noise power supplied to
the input terminals of a receiver to the signal to noise power
supplied to the output.
Mathematically can be expressed as-
