TRANSCEIVER
ARCHITECTURES
Transceiver
Receivers
1. Heterodyne
2. Direct conversion
3. Image Reject
Receiver receives RF signal → converted to Baseband
Two-stages
RF→ IF
IF → Baseband
Hostile environment
Wireless standards
Global System for Mobile Communication (GSM)
IS-95 CDMA
Wideband CDMA
Bluetooth
IEEE802.11a/b/g
Rising complexity
Constraints in design
Shannon’s theorem :
the achievable data rate of a communication channel is equal to B
log2(1 +SNR)
limited channel bandwidth – hostile constraint in transceiver design
Cascaded stages : increasing stages → nonlinearity, noise
Presence of interferers
Requirement of highly linear receiver
Difficulty in filtering interferes
1. interferer falls within vicinity of desired channel → high selective
filters → prohibitively high Q factor
2. Allocation of different carrier frequency
Limited channel bandwidth
Both transmitter- receiver
During transmission → minimum leakage to adjacent channels
Requires narrowband modulation-amplification from transmitter
Receiver side:
Desired channel interferes
Proper selection-rejection capacity
leakage
narrow channel bandwidth
transmitter must employ narrowband modulation
and amplification
avoid leakage to adjacent channels
Receiver must be able to process the desired channel
Sufficient rejection of strong in-band and out-of-band
interferers
1. Small in-band loss Trade-off of BPF
2. Large attenuation for interferer
• linearity of a receiver must be high enough
• interferers causes compression or significant
intermodulation
• Problem of filtering interferers
• Interferer falls only one/two channels away from desired channel
→ very high selectivity
• interferer level 50–60 dB above the desired
signal level → required value of Q reaches
prohibitively high
• Different carrier frequency allocation to user → variable center
frequency
Channel selectivity
Quality factor Q
Channel selection : selects only desired channel
Rejects all interferers from adjacent channel
amplifier, filter, mixer, attenuator, detector
Requirement of high linearity- channel selection
Front end : BPF , Amplifier, Local oscillator, Mixer
band-select filter
In-band interferer out-of-band interferer
components generated by users that do not belong to the standard of
interest
Front- end Band –select filter → trade-off : selectivity-in
band loss
Sharp frequency response → increasing order of filter
Number of stages in cascade
Noise factor
Desirable small loss 1dB- frequency selectivity
Band are allocated to different applications like GSM, Wifi, TV
channels represent users
At high carrier frequency channel selection proves to be difficult → unreasonable
high value of Q
Translate desired channel → lower centre frequency
Application of Mixer
Multiplication by A0 cosωLOt
multiplication in the time domain corresponds to convolution in the frequency
domain
Desired channel shifted to ±(ωin ± ωLO)
Removes undesired
components
Down-conversion → receiver
Requirement of LNA to remove noise
ωin - ωLO → Intermediate Frequency (IF) : low freq
Mixer : shift the frequency of an electromagnetic signal without
changing characteristics like phase and amplitude of the initial signal
How does a heterodyne receiver cover a given frequency band?
Method 1
LO frequency is constant
each RF channel is down converted to a different IF channel
channels within RF range from Wch1 to Wchn
IF it ranges from WIF1 to WIFn
WIF1 = Wch1 – WLO
tuneable filter to choose each of these channels in IF
high Q band pass filter with a tuning part covering the range
of IF
Method 2:
Variable LO frequency
all the RF channels within the band are translated to a single value
different WLO ranging from WLO1 to WLOn
translated to a single IF value
different RF signals will be down converted to the
same IF
Requirement of tuneable oscillator
Effect of Image
• major drawback or problem with the heterodyne receiver
• Users in other standards transmit undesirable signals that
act as interferers
• Down-conversion process → local oscillator at a distance
of wIF away from the center frequency of the desired
channel
• It is also away from some other signal called the image
signal with frequency wim
Refers to the frequency of the local oscillator
high-frequency design issues of the LO
strength of the interferers
High-side injection
choose the frequency of the local oscillator (wLO)
above the desired channel
WIF = WLO-W1
Low-side injection
frequency of the local oscillator is selected below the desired
channel in down conversion
WIF = W1-WLO
2ωIF
two spectra located symmetrically around ωLO are
downconverted to the IF
component at ωim is called the image of the desired signal
ωim = ωin + 2ωIF = 2ωLO - ωin
Constraints of emission within band
image power can be much higher than that of the desired
signal
Image rejection
LO frequency → image frequency in a interference band
suppress the image
precede the mixer → image-reject filter
Filter exhibits a relatively small loss in the desired
band
Large attenuation in the image band
Sufficiently large 2ωIF
