Digital Signal Analysis Digital Signal Analysis X-Series Spectrum Analyzers Seminar 2010 Page 1
Objectives - Provide a digital modulation overview - Review common digital radio impairments X-Series Spectrum Analyzers Seminar 2010 Page 2
Di it l M d l ti Digital Modulation Overview O i X-Series Spectrum Analyzers Seminar 2010 3
Vector Signal Analysis on one page X-Series Spectrum Analyzers Seminar 2010 4
Transmitter Overview X-Series Spectrum Analyzers Seminar 2010 5
Receiver Overview X-Series Spectrum Analyzers Seminar 2010 6
Signal Characteristics to Modify X-Series Spectrum Analyzers Seminar 2010 7
Polar Display / IQ Relationship X-Series Spectrum Analyzers Seminar 2010 8
Quadrature Amplitude Modulation QPSK 2 bits /symbol Symbol Rate = 1/2 bit Symbol Rate 1/2 bit 64 QAM 64 QAM rate 6 bits /symbol Symbol Rate = 1/6 bit rate 16 QAM 4 bits /symbol Symbol Rate = 1/4 bit rate 128 QAM 32 QAM 7 bits /symbol 5 bits /symbol Symbol Rate = 1/7 bit Symbol Rate = 1/7 bit Symbol Rate = 1/5 bit rate rate X-Series Spectrum Analyzers Seminar 2010 Page 9
Comparing OFDM and SC-FDMA QPSK example using N=4 subcarriers QPSK example using N=4 subcarriers The following graphs show how 1, 1 -1,-1 -1, 1 1, -1 1, 1 -1,-1 -1, 1 1, -1 this sequence of QPSK symbols is this sequence of QPSK symbols is represented in frequency and time V V CP CP CP Frequency Frequency 60 kHz 60 kHz Frequency F f c f c 15 kHz SC-FDMA OFDMA Data symbols occupy 15 kHz for Data symbols occupy N*15 kHz for one OFDMA symbol period one OFDMA symbol period 1/N SC FDMA 1/N SC-FDMA symbol periods b l i d X-Series Spectrum Analyzers Seminar 2010 10
IQ Modulator 1 0 0,0 0,1 1,1 1,0 symbol symbol 0 0 1 1 X-Series Spectrum Analyzers Seminar 2010 11
IQ Demodulation X-Series Spectrum Analyzers Seminar 2010 12
Error Vector Concept X-Series Spectrum Analyzers Seminar 2010 13
How EVM is calculated X-Series Spectrum Analyzers Seminar 2010 14
Digital Radio Impairments and Digital Radio Impairments and Measurements X-Series Spectrum Analyzers Seminar 2010 15
Common Radio Impairments IQ Imbalance IQ Imbalance Transmitter Compression I/Q Modulator I IF Filter 0 d 0 deg Symbol + BB Filter PA DAC IF LO Encoder 90 deg Q Q Phase Noise RF LO DAC/DSP Error Spurious Spurious Incorrect Coefficients Receiver I/Q Demodulator I I IF Filter RF Filter IF Amp 0 deg Symbol LNA ADC BB Filter IF LO Decoder I t f Interferers 90 d 90 deg Q Thermal Noise Ripple & Tilt Ripple & Tilt RF LO RF LO X-Series Spectrum Analyzers Seminar 2010 16
Wrong filter coefficients I/Q Modulator I IF Filter 0 deg Symbol Symbol + BB Filter PA DAC IF LO Encoder 90 deg Q Baseband Filter Impulse Response RF LO Overshoot from incorrect alpha Baseband Filter Freq. Response For this example, the alphas for the Nyquist filter were 0.2 in the transmitter and 0.35 in the receiver. X-Series Spectrum Analyzers Seminar 2010 17
Amplifier Nonlinearity Transmitter Compression I/Q Modulator I IF Filter IF Filter 0 deg Symbol + BB Filter PA DAC IF LO Encoder 90 deg Q Q RF LO Traditional Two-Tone Test Testing a Digitally Modulated Signal With compression IMD f 1 f 1 f 2 f 2 Without compression Without compression X-Series Spectrum Analyzers Seminar 2010 18
Amplifier Nonlinearity - CCDF What are the other observable effects of Compression? Wh t th th b bl ff t f C i ? CCDF Measurements AWGN Signal as AWGN Si l a reference y probability p Compressed QPSK Signal Non-compressed QPSK Signal dB above the average g X-Series Spectrum Analyzers Seminar 2010 19
Amplifier Nonlinearity - Demodulation What are the other observable effects of Compression? Vector (IQ) Diagram ( ) g Without compression Increased EVM With compression X-Series Spectrum Analyzers Seminar 2010 20
I/Q Impairments Imbalances I/Q Modulator I IF Filter 0 deg Symbol + BB Filter PA DAC IF LO Encoder 90 deg Q RF LO • Gain Imbalance • Quadrature Error Quadrature Error • DC Offset • Path Difference - Delay P th Diff D l X-Series Spectrum Analyzers Seminar 2010 21
I/Q Impairments – Gain Imbalance IQ Constellation QPSK Summary Table QPSK Summary Table Gain Imb. = 1.02 dB (Ideally 0 dB) ( y ) Measured Ideal Ideal (rectangle) (square) X-Series Spectrum Analyzers Seminar 2010 22
I/Q Impairments – Quadrature Error IQ Constellation QPSK Summary Table QPSK Summary Table Quad. Error = 5.9 deg. ( (Ideally 0 deg*) y g ) Ideal Ideal M Measured d (square) (parallelogram) * meaning that I and Q are ideally 90 deg. apart X-Series Spectrum Analyzers Seminar 2010 23
What is causing this problem? Receiver I/Q Demodulator I IF Filter RF Filter IF Amp 0 deg S Symbol b l LNA ADC BB Filter IF LO Decoder Interferers 90 deg Q RF LO Spectrum A Spectrum A Spectrum B Spectrum B No Spurious Signals ACP Looks Good X-Series Spectrum Analyzers Seminar 2010 24
Signal A: Demodulation is Good S Spectrum A A Points randomly Demod distributed EVM Low (0.4%) ( ) X-Series Spectrum Analyzers Seminar 2010 25
Signal B: Demodulation S Spectrum B B Points are not Demod randomly distributed EVM High (3.6%) g ( ) Mag & Phase Errors High and comparable and comparable X-Series Spectrum Analyzers Seminar 2010 26
Signal B: EVM Spectrum Shows Spur EVM Spectrum S Spurious Signal i Si l -36dBc spur was buried under the modulated carrier Spectrum B X-Series Spectrum Analyzers Seminar 2010 27
Infamous V-shape I/Q Demodulator I Receiver IF Filter RF Filter IF Amp 0 deg S Symbol b l LNA ADC BB Filter IF LO Decoder 90 deg Q RF LO Symbol Rate X-Series Spectrum Analyzers Seminar 2010 28
Digital Modulation Measurements Constellation Errors Constellation Errors Ideal Symbol Point Ideal Symbol Point 64 QAM C 64 QAM Constellation t ll ti Q Random Noise Phase Noise AM Distortion I PM Distortion D l Delay Distortion/ISI Di i /ISI Interference X-Series Spectrum Analyzers Seminar 2010 29
Demonstration X-Series Spectrum Analyzers Seminar 2010 Page 30
THANK YOU! THANK YOU! X-Series Spectrum Analyzers Seminar 2010 Page 31
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