PREPARED BY: PRASHASTI (15MECC19)

SUDHIR (15MECC24)

RF RECIEVER ARCHITECTURE

OUTLINE INTRODUCTIONFRONT-ENDRECIEVER ARCHITECTUREFRONT-END NON-LINEARITIES

INTRODUCTIONThe 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.

BLOCK DIAGRAM OF RF SYSTEM

The front end is the main focus of this architecture

FRONT END BLOCKThe 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).

WHY DO WE NEED A FRONT END?

ECHANNEL FFRONT END

ADD NOISE,DISTORTION AND INTERFERENCE TO THE TRANSMITTED SIGNAL

REDUCE NOISE,DISTORTION AND INTERFERENCE AND THUS INCREASE THE SNR OF THE SIGNAL

IS USED TO

FUNCTIONS OF FRONT END Filtering-out the interference and

noise Amplification of the signal Frequency down conversion using

a mixer and a local oscillator.

RECEIVER ARCHITECTURE

HETRODYNE RECEIVER

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.

SELECTION OF IF (LOW IF OR HIGH IF)

SELECTION OF IF (CONT..)

IMAGE REJECTION V/S CHANNEL SELECTION

DIRECT CONVERSION RECEVIER

WIDEBAND IF WITH DOUBLE CONVERSION RECEIVER

ARCHITECTURE (WBIFDC)

WIDEBAND IF WITH DOUBLE CONVERSION RECEIVER ARCHITECTURE (WBIFDC)

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 suppress nearby interference and select desired channel.

LOW IF RECEIVER

LOW IF RECEIVERIn 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.

COMPARSION

FRONT END NON IDEALITIESThe 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.

CIRCUIT NON-LINEARITY This is of great concern in RF system design,

since non-linearity gives rise to many difficult problems, they are:

1)Harmonics distortion2)Inter-modulation3)Third order Intercept point4)Gain compressor5)Noise figure6)Blocking 

HARMONIC DISTORTIONHarmonic 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.

MATHEMATICAL ANALYSIS

MATHEMATICAL ANALYSIS (CONT..)

MATHEMATICAL ANALYSIS (CONT..)

MATHEMATICAL ANALYSIS (CONT..)

INTERMODULATION DISTORTION(IMD)

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.

INTERMODULATION DISTORTION(IMD)

THIRD ORDER INTER MODULATION PRODUCTS• 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.

THIRD ORDER INTER MODULATION PRODUCTS

As a general rule, the 3rd order products will increase (grow) 3-times faster than the fundamental signal (the signal of interest).

INTERMODULATION IS MORE HARMFUL THAN HARMONICS DISTORTIONThe 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.

THIRD ORDER INTERCEPT POINT (IP3)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

GAIN COMPRESSIONThe 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.

GAIN COMPRESSION

The idea of the 1-dB compression point is shown graphically in Figure below

SENSITIVITY AND BLOCKING• 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.

CIRCUIT INTRODUCE NOISE“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 signalsSources:

1.Random noise in resistors and transistors2.Mixer noise3.Undesired cross-coupling noise4.Power supply noise

Dynamic range – capability of detecting weak signals in presence of large-amplitude signals.

CIRCUIT INTRODUCE NOISE

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-

CIRCUIT INTRODUCE NOISE

CASCADED NOISY STAGES

CASCADED NOISY STAGES (cont.)