ADI 2006 RF Seminar. Chapter VI A Detailed Look at Wireless Signal Chain Architectures

Size: px

Start display at page:

Download "ADI 2006 RF Seminar. Chapter VI A Detailed Look at Wireless Signal Chain Architectures"

Vernon French
5 years ago
Views:

1 DI 2006 R Seminar Chapter VI Detailed Look at Wireless Chain rchitectures 1

2 Receiver rchitectures Receivers are designed to detect and demodulate the desired signal and remove unwanted blockers Receiver must also get rid of unwanted signals that it generates (e.g. mixer spurs) Receiver uses variable gain and power detection Most Receivers will have some form of utomatic Gain Control Diversity: Some Receiver Systems have two separate Receive Paths (ntennas separated by a quarter wavelength). Diversity Receiver will either pick the strongest signal or intelligently combine both signals to increase signal power 2

3 Blockers a closer look Transmit Out-of-Band Blocker In-Band Blockers Rx Band Desired req Blockers can be orders of magnitude larger than the desired signal Large Blockers can jam a receiver Blockers can inter-modulate with each other and produce IMD products right at the frequency of the desired signal Some Blockers can be filtered (e.g. out-of-band) but others must be tolerated. 3

4 Superheterodyne (Single Conversion) I Sampling Receiver Band/Image ilter MIXER Channel Select ilter B C D E G H VG I SMPLING I DUPLEXER RSSI /GC TRNSMITTER Mixes the received signal from R down to a single I Uses SW filters to remove blockers and unwanted mixing components Detects signal power and implements GC at the I Reduces number of down-conversions by sampling the spectrum at an Intermediate requency but requires a high performance Is the most popular architecture in non-cellular applications 4

5 I Sampling low Transmit Out-of-Band Blocker In-Band Blockers Rx Band Desired req Channel Select ilter req B Transmit In-Band Blockers Rx Band Desired Out-of-Band Blocker req H GC & Nqyuist ilter I SMPLING CLOCK S 2nd Harmonic req In-Band Blockers C/D Desired Blocker IMD Product I T Rx Band req S-I S/2 req LO - R LO + R 5 E Desired LO Leakage LO req

6 How I sampling works The receiver uses R and I filters to eliminate the transmit signal and blockers so that only the desired signal is sampled The must sample at twice the signal bandwidth to meet Nyquist criteria Oversampling can be used to improve the signal to noise ratio by 3 db for each doubling of the sample frequency Harmonics of driver amp that are not filtered will degrade performance There is usually a clock recovery loop in an PG or DSP or both that locks the sampling rate to a multiple of the symbol rate 6

7 Direct Conversion Receiver Band ilter B C D E VG IQ DEMOD 0 90 G DUPLEXER RSSI /GC TRNSMITTER Saves money by mixing R spectrum to baseband in a single step Reduces component count and eliminates I SW filters There is a reason why R engineers have not tried this sooner removing offsets at baseband 7

8 Direct Conversion Receiver Transmit In-Band Blockers Desired Blocker IMD Product In-Band Blockers Desired Rx Band req Offset from LO Self Mixing & IP2 Intermodulation BB mp Distortion S/2 S req C Desired In-Band Blockers G Desired Carrier Nqyuist ilter Transmit Rx Band req S/2 S req In-Band Blockers E Transmit Blocker IMD Product Desired req Rx Band 8

9 Direct Conversion Receiver In-Band Blockers can only be eliminated at the end of the signal chain or in the digital domain. In-Band Blockers can mix in the ront End (before mixer) to produce an unwanted product at baseband LO leakage to the R input causes self-mixing and produces an unwanted dc offset at dc (right in the middle of the desired signal) Non-Ideal 90 degree balance in the Demodulator produces unwanted images of blockers which can be close to the carrier Direct Conversion Receivers are cheaper and smaller (no I SW filters, cheaper s, only one mixer) 9

10 Transmitter rchitectures Super Heterodyne with IQ Modulator Super Heterodyne with Real I DC Synthesis Direct Conversion Low I to R Conversion 10

11 Superheterodyne Transmitter using IQ Modulator -15 dbm 380 MHz -25 dbm Gain=10dB N=12 db OIP3=20 dbm P1 db=10 dbm -15 dbm -18 dbm -3 dbm +45 dbm DC SW CTIVE MIXER BND ILTER P DRIVER P I MP Diff to SE +15dB 48 db DC TxDC B D db 0 to -20dB -3 db C D +10dB E G -5 dbm 380 MHz D4212L (Int-N) D4252 (rac-n) /-30 MHz /-30 MHz /-37.5 MHz D db RMS Detector D db RMS Detector Superheterodyne Transmitter uses one or more Intermediate requencies. DC constructs the baseband signal, centered either at dc or at a low Intermediate requency (I) Gain control and filtering may be implemented at R, I, and baseband. Lots of power back-off to avoid distortion in non-constant envelope systems 11

12 Superheterodyne Transmitter using IQ Modulator B I I E IMGE LO C I D I G 12

13 Superheterodyne Transmitter using IQ Modulator Noise and Spurs generated in the I stage can be filtered fter mix to R, band filtering removes out of band noise along with the image In-Band noise generated in mix to R cannot be removed 13

14 Example: Superhet with I Synthesis of signal in IQ format 16-bit D8345 D8343 Step ttenuator Power mplifier D Mhz 40dB 16-bit B C 380 MHz D E Band G ilter 402 MHz 1.52 GHz Driving IQ mod with a low I creates a single-sideband-like spectrum at the modulator output. Once I has been filtered (removing unwanted sideband and LO), modulation quality (EVM) is excellent. 14

15 Example: Superheterodyne Receiver with I Synthesis of signal in IQ format B C LOW I LOW I I LO LEKGE UNDESIRED UPPER SIDEBND E IMGES LO LEKGE D I G Unwanted LO leakage and Upper Sideband are filtered at I, resulting in excellent EVM If low I is high enough, do a single up-conversion to R 15

16 Direct Conversion Zero I rchitecture -15 dbm -18 dbm -3 dbm +45 dbm DC BND ILTER P DRIVER P +15dB 48 db DC 0 to -20dB -3 db D8349 D9767 TxDC -5 dbm /-30 MHz /-30 MHz /-37.5 MHz D db RMS Detector D db RMS Detector Direct Conversion mixes a base-band signal from a dual DC up to the transmission frequency in a single step. With no I, gain control, filtering, and equalization must be performed either in the digital backend, at the reconstructed analog base-band output or at R. Effects of LO leakage and Upper Sideband Leakage occur in-band potentially interfering with the signal s EVM. Dual channels are required to generate the complex signal, any channel mismatch causes In-band distortion which cannot be filtered. High quality components are required to generate an accurate signal In-Band Modulator Noise cannot be filtered Calibration of LO leakage and Quadrature balance is generally necessary P to LO leakage can modulate or pull the PLL 16

17 Example: Direct Conversion Transmitter -15 dbm -18 dbm -3 dbm +45 dbm DC BND ILTER P DRIVER P DC D9767 TxDC +15dB 48 db 0to-20dB -3 db C D E D8349 B -5 dbm /-30 MHz /-30 MHz /-37.5 MHz D db RMS Detector D db RMS Detector B req D req req C E req req 17

Poor OIP3 causes djacent Channel Leakage djacent Channel Leakage SNR Think of a broadband spectrum multiple tones inter-modulating with each

18 Poor OIP3 causes djacent Channel Leakage djacent Channel Leakage SNR Think of a broadband spectrum multiple tones inter-modulating with each other IM3 products produce djacent Channel Power/Leakage/Distortion Use 3-to-1 decay of IMD products to reduce dbc IMD but this degrades SNR 18

19 CPR and Noise vs. Output Power CP - dbc CP 2140 MHz Noise loor 2140 MHz Noise loor - dbm/hz (25 Mhz Carrier Offset) Per-Carrier Output Power - dbm CP degrades with increased output power due to IMD Noise is independent of input and output power t low power levels CP degrades because of falling SNR 19

20 Example: Low I to R Transmitter using I Synthesizing DC and Passive Mixer D9786 DC Pout -15 dbm 190 MHz NTI LIS -1 db -16 dbm I MP 10 db -6 dbm Gain= -5 db N= 5 db OIP3= +35 dbm P1 db= 25 dbm PSSIVE MIXER -11 dbm 0 to -20dB BND ILTER -3 db -14 dbm P DRIVER +15dB +1 dbm P 44 db +45 dbm +5 db -5 dbm 2.33 GHz 2.15 GHz 2.03 GHz D db RMS Detector D db RMS Detector Baseband DC, IQ Modulator and PLL are replaced by an I Synthesizing DC or DDS modulator Trade Off: High Performance DDS/DC + SW + Mixer + PLL vs. IQ DC + Modulator + PLL None of the problems typically associated with Direct Conversion Probably more expensive than Direct Conversion 20

21 Low I to R rchitecture ω LO ω I ω B ω I ω ω ω ω ω LO LO - I LO + I ω High Performance DC generates real I at a low I ( MHz) Mixer performs Double Sideband Modulation dvantage: Unwanted LO and Sideband are removed -> excellent EVM Challenge: To move unwanted LO and upper sideband out of band means that the I must be quite high 21

ADI 2006 RF Seminar. Chapter II RF/IF Components and Specifications for Receivers

ADI 2006 RF Seminar. Chapter II RF/IF Components and Specifications for Receivers ADI 2006 RF Seminar Chapter II RF/IF Components and Specifications for Receivers 1 RF/IF Components and Specifications for Receivers Fixed Gain and Variable Gain Amplifiers IQ Demodulators Analog-to-Digital