04 Radio System Design 2

Bhaskar Banerjee, EERF 6330, Sp‘2013, UTD

Radio System Design II

Transceiver Architectures

Prof. Bhaskar Banerjee

EERF 6330- RF IC Design

Bhaskar Banerjee, EERF 6330, Sp‘2013, UTD

Outline

• Receiver Architecture

• Transmitter Architectures

• Reading:

✓Chapter 4: Fundamental of Microelectronics, B. Razavi

2

Bhaskar Banerjee, EERF 6330, Sp‘2013, UTD 3

Frequency Planning

• depends directly on – receiver topology– number of down-conversions– mode of operation


Frequency Planning

• Blockers– Understanding the wireless applications that co-exist in the frequency spectrum

surrounding the band of interest is needed– Operating close to the frequency bands of other applications places great

demands on front-end filter selectivity


Frequency Planning

• Blockers– Solution by frequency planning

• IF frequency selection–to avoid blockers that can interfere with the IF chain–Must check the availability of IF surface-acoustic wave

(SAW) filters at the chosen frequency

– Another solution• Filtering


Frequency Planning

• Spurs and De-Sensing– Spurs

• unwanted spurious frequencies that are generated by interaction between various components of our own transceiver

– De-sensing• Spurs with a higher power level lands either directly in the

band or adjacent to the band and saturates the transceiver


• Transmitter leakage– a major concern for any RF subsystems, especially in duplex systems

• de-sense the receiver by saturating the receiver front-end or causing oscillations

• makes it difficult to integrate both transmit and receive functions of a duplex system on a single chip

– Solution• stringent filtering/isolation to maintain confinement to the transmit band

Frequency Planning


Heterodyne Receivers

• Also called Super-Heterodyne• have been in use for a long time and are quite popular from

the early days of radio communication systems • two (or more) stages of down conversion


LO Leakage

• LO leakage– LO signals and their harmonics are major sources of spurious interference– Leakage paths

• IC substrate, package, or board


LO Leakage

• LO leakage– Solution by frequency planning

• Decide LO frequencies where all LO frequencies and their harmonics, and frequencies resulting from higher-order mixing of these signals do not fall in the RF or IF bands


Problem of Image Frequency

• Image frequency– One of the most problematic issues with designing traditional

heterodyne receivers – It is located on the opposite side of the LO frequency and folds

on top of the IF band as the signal in down converted in a mixer– It needs to be addressed by either filtering or image-reject mixing

topologies


Why is “Image” Frequency Critical?

• Down conversion thru a mixer generates f1-f2 component.

• ‘Image frequency(f3)’ has equal spectral distance from the LO(f2) to the RFin(f1) on the opposite side.

• Mixer generates an unwanted signal placed exactly at the same frequency as the down converted IF signal, where f1-f2 = f2-f3.

• Unwanted signal located on the ‘Image frequency’ CANNOT be filtered out once down converted to the IF.

Pass Mixer

Pass Mixer

Pass Mixer


Band-pass filter used for ‘image-rejection’.

Image-reject Filter + Mixer

• Regardless of the existence of an Image Frequency, Image Filter is always placed BEFORE a Mixer to protect the RFin signal from possibly existing spurs at ‘image’ frequency.

• Image filter prevents the unwanted spurs at image frequency from being down converted to the IF frequency.


What IF frequency to choose?

• With higher IF frequency, subsequent channel-select BPF requires “higher Q” filter due to higher frequency, resulting in lower selectivity for given Q-factor of the channel-select BPF.

• On the contrary, lower IF frequency provides better channel-selectivity on the subsequent channel-select BPF block.

• Hence, the choice of IF frequency has a trade-off between channel-selectivity and image-rejection.

• Dual IF stage can be used to achieve both benefits at higher costs.

With lower IF, BPF cannot effectively filter out the image frequency from the

RF signal.

With higher IF, BPF can effectively filter out the Image resulting in clean IF

signal.


Typical IF frequency choice

• In reality, IF frequency choice is limited by commercially available SAW filters.

• Specific application tend to use specific IF frequency value:

Typical IF frequency usedTypical IF frequency used

455kHz General equipments

10.7MHz General purpose receivers

21.4MHz Hi-performance receivers

45MHz TV’s, Cellular phones

70MHz Satellite TV’s, Military

160MHz Satellite equipments


Problem of Half IF

• Half IF frequency– It is located directly between the LO and the RF – It can double in the LNA or RF amplifiers in the front-end and get

down-converted into the IF band by mixing with the second harmonic of the LO signal

Spurious signal

12 · (!RF � !LO)


What LO frequency to choose?

• For a down mixer, two local oscillator frequency can be chosen. A Low Side LO (LSLO) and a High Side LO (HSLO). Both will down mix the RF signal to the desired IF frequency.


Heterodyne Receivers

• Selectivity: Lower Q Required

• Sensitivity: Reverse Isolation on IF stage

• Stability: Lower Gain per stage required

• Repeatability: Reuse IF stage and below, multiple carrier support.– e.g. Dual-IF Topology

• Not fully integrable


Image Reject Receivers

• To maintain an acceptable receiver signal quality, most modern wireless standards require 60 to 90 dB of image rejection

• The traditional method of image rejection is use of band stop filters at the image frequency

• Due to the stringent requirements, image rejection is typically performed through a combination of filtering and image rejection mixing techniques

• The two most popular image rejection techniques are – Hartley architecture – Weaver architecture– These techniques yield about 30 to 35 dB of image rejection


Image Reject Receivers• Hartley Architecture

– RF signal is down-converted by quadrature LO signals – Problem

• sensitivity to phase and amplitude imbalance between the quadrature mixers or the two IF paths


Hartley Architecture

• Drawbacks• Sensitivity to Gain and Phase Matching• Linearity requirement on the Adder circuit


Image Reject Receivers

• Weaver Architecture– Utilizes an additional pair of mixers to perform the phase shifting prior to IF

combination – Problem

• a second image → zero IF for the second mixing• sensitivity to phase and amplitude imbalance

sinω1t

cosω1t

sinω2t

cosω2t


• Also called Homodyne Receivers and Zero-IF Receivers• The desired signal is directly down converted to baseband

Direct Conversion Receivers (DCR)


Direct Conversion Receivers

• Main Features– Integration possible

• No Image reject filter, LNA can drive the Mixer directly• IF SAW filter - replaced by LPF and Baseband Amps

– Channel selection harder

– Flicker Noise is a big issue

– have been proposed in the early radio days, but the system performance has been usually poor

– With the advances in the digital communication area, a lower performance can be accepted with the advantage of highest possible on chip integration

– DCR is a demodulator with much higher dynamic range and higher frequency of operation than a typical IF demodulator



• The order of the baseband building blocks arrangement(Low Pass Filter, Variable Gain Amplifier, Analog Digital

Converter)– Depending on the chosen configuration, the linearity and noise

performance of the front-end and baseband blocks can be defined → need to find optimum order


Direct Conversion Receivers• The order of the baseband building blocks arrangement

– By placing the low-pass filter (LPF) in the front we can filter much of the extraneous signals that result from the mixing and relax the linearity requirements of the variable-gain amplifier (VGA) and the analog-to-digital converter (A/D)

– However, the loss through the LPF reduces the gain before the VGA and A/D and consequently increases the overall noise figure (NF) of the receiver



• The order of the baseband building blocks arrangement – As we move the LPF further down the baseband chain the NF

improves while the linearity requirements of the VGA and A/D increase


Direct Conversion Receivers• Frequency schemes of DCR and heterodyne receivers


Issues in Direct Conversion Receivers

• DC Offset– Extraneous DC voltages in the demodulated spectrum of a DCR not only

corrupt the output, but also propagate through the baseband circuitry and saturate the subsequent stages

– DC offsets are mostly generated through • self-mixing the LO signal → time-varying DC offset• mismatch in the mixers → constant DC offset



• Phase and Amplitude Imbalance – Most modern wireless modulation scheme requires I and Q signal

separation and demodulation to fully recover the information– implementing accurate phase-shifters at higher frequency becomes a

challenging task



• LO Leakage and Radiation – Since a typical DCR requires a LO signal frequency identical to the RF

input carrier frequency, the LO signal is considered an in-band interference – This LO can couple into the antenna and not only radiate out in the receive

band of other users but also penetrate and saturate the RF front-end – Solution:

• High reverse isolation in the front-end and good shielding of the receiver

• sub-harmonic mixing by moving the LO signal out of the RF frequency band of interest



• Solution for DC Offset– AC coupling of the mixer output.

• This will not only remove the unwanted DC offsets, but at the same time will corrupt the down-converted signal by attenuating the components near DC → not acceptable for demodulating most modulation schemes that exhibit a DC peak in their signal spectrum

– Therefore, DC offset cancellation techniques using DSP are necessary to accommodate the use of direct conversion topology in today’s wireless applications



• Intermodulations– susceptible to both odd- and even-order intermodulations– even-order intermodulations

• by the non-linearities of mixer and LNA• Solution:

– Balanced topologies – even-harmonic mixing

Fundamental

Frequency890, 900

Harmonic

1780 (2*890), 1800 (2*900),2670 (3*890), 2700 (3*900)

2nd IM1790 (890 + 900),

10 (900 - 890)

3rd IM

2680 (2*890 + 900),2690 (890 + 2*900),880 (2*890 - 900), 910 (2*900 - 890 )


Other Receiver Architectures

• Digital-IF Receivers– Heterodyne - but 2nd IF is digitized– Digital processing alleviates I/Q mismatch problem– A/D Converter performance needs to be very good

• Low thermal, quantization noise• Low non-linearity, high dynamic range

• Sub-sampling Receivers– Sampling at 2Δf - where Δf is the bandwidth of the narrowband

RF signal• Aliasing of Noise

• Digital Radio– Digitize at the RF Front End


Transmitter Architectures

• Much relaxed requirements:– Noise– Interference Rejection– Band Selectivity

• Direct Conversion Transmitters• Two-step Transmitters


Direct Conversion Transmitter

• Transmit Carrier Freq = LO Frequency– Modulation and upconversion occur in same circuit.

sinωct

cosωct

PA MatchingNetwork

Duplexer


Direct Conversion Transmitter

• Major Drawback: PA output disturbs transmit VCO

ωLO

Leakage: Injection pulling

sinωct

cosωct

PABPF


Transmitter Architecture

• Direct Conversion– Frequency Pulling

• Can be alleviated by “offsetting” LO freq (Use f1 + f2)• Use Two-step architecture

• Two-step Architecture– Quadrature mixing at low frequency

• Lesser mismatch - lesser cross-talk• Channel filter to improve adjacent channel rejection

– BPF needed to reject large unwanted side-band• High center frequency - ‘off-chip’ filtering (expensive)

– Higher Power Consumption owing to more components

04 Radio System Design 2

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