X-Series Spectrum Analyzers Seminar Page 1

Transcription

1 Digital Signal Analysis Page 1

2 Objectives - Provide a digital modulation overview - Review common digital radio impairments Page 2

3 Digital it Modulation Overview 3

4 Vector Signal Analysis on one page 4

5 Transmitter Overview 5

6 Receiver Overview 6

7 Signal Characteristics to Modify 7

8 Polar Display / IQ Relationship 8

9 Quadrature Amplitude Modulation QPSK 2 bits /symbol Symbol Rate = 1/2 bit rate 64 QAM 6 bits /symbol Symbol Rate = 1/6 bit rate 16 QAM 4 bits /symbol Symbol Rate = 1/4 bit rate 32 QAM 5 bits /symbol Symbol Rate = 1/5 bit rate 128 QAM 7 bits /symbol Symbol Rate = 1/7 bit rate Page 9

10 Comparing OFDM and SC-FDMA QPSK example using N=4 subcarriers The following graphs show how this sequence of QPSK symbols is 1, 1-1,-1-1, 1 1, -1 1, 1-1,-1-1, 1 1, -1 represented in frequency and time V V CP CP f c OFDMA 15 khz Data symbols occupy 15 khz for one OFDMA symbol period Frequency 60 khz Frequency f c SC-FDMA Data symbols occupy N*15 khz for 1/N SC-FDMA symbol periods 10

11 IQ Modulator 1 0 0,1 0,0 1,1 1,0 0 symbol 1 11

12 IQ Demodulation 12

13 Error Vector Concept 13

14 How EVM is calculated 14

15 Digital Radio Impairments and Measurements 15

16 Common Radio Impairments Transmitter I IQ Imbalance I/Q Modulator Compression Symbol Encoder BB Filter DAC 0d deg IF LO 90 deg + IF Filter PA Q DAC/DSP Error Phase Noise Spurious RF LO Incorrect Coefficients I/Q Demodulator I Receiver It Interferers RF Filter LNA IF Amp IF Filter 0 deg IF LO ADC BB Filter 90 deg Symbol Decoder Thermal Noise RF LO Ripple & Tilt Q 16

17 Wrong filter coefficients I I/Q Modulator Symbol Encoder BB Filter DAC 0 deg IF LO 90 deg + IF Filter PA 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. 17

18 Amplifier Nonlinearity Transmitter I I/Q Modulator Compression Symbol Encoder BB Filter DAC 0 deg IF LO 90 deg + IF Filter PA Q RF LO Traditional Two-Tone Test Testing a Digitally Modulated Signal IMD With compression f 1 f 2 Without compression 18

19 Amplifier Nonlinearity - CCDF What are the other observable bl effects of Compression? CCDF Measurements AWGN Signal as a reference Compressed QPSK Signal probability Non-compressed QPSK Signal db above the average 19

20 Amplifier Nonlinearity - Demodulation What are the other observable effects of Compression? Vector (IQ) Diagram Without compression Increased EVM With compression 20

21 I/Q Impairments Imbalances I I/Q Modulator 0 deg IF Filter Symbol Encoder BB Filter DAC IF LO 90 deg + PA Q Gain Imbalance Quadrature Error DC Offset Path Difference - Delay RF LO 21

22 I/Q Impairments Gain Imbalance IQ Constellation QPSK Summary Table Gain Imb. = 1.02 db (Ideally 0 db) Ideal (square) Measured (rectangle) 22

23 I/Q Impairments Quadrature Error IQ Constellation QPSK Summary Table Quad. Error = 5.9 deg. (Ideally 0 deg*) Ideal Measured (square) (parallelogram) * meaning that I and Q are ideally 90 deg. apart 23

24 What is causing this problem? I/Q Demodulator I Receiver Interferers RF Filter IF Amp IF Filter LNA 0 deg IF LO ADC BB Filter 90 deg Symbol Decoder Q RF LO Spectrum A Spectrum B No Spurious Signals ACP Looks Good 24

25 Signal A: Demodulation is Good Spectrum A Demod Points randomly distributed EVM Low (0.4%) 25

26 Signal B: Demodulation Spectrum B Demod Points are not randomly distributed EVM High (3.6%) Mag & Phase Errors High and comparable 26

27 Signal B: EVM Spectrum Shows Spur EVM Spectrum Spurious Signal -36dBc spur was buried under the modulated carrier Spectrum B 27

28 Infamous V-shape Receiver RF Filter IF Amp IF Filter LNA I/Q Demodulator 0 deg I IF LO ADC BB Filter 90 deg Symbol Decoder RF LO Q Symbol Rate 28

29 Digital Modulation Measurements Constellation Errors Ideal Symbol Point Random Noise 64 QAM Constellation ti Q Phase Noise AM Distortion I PM Distortion Delay Distortion/ISI Interference 29

30 Demonstration Page 30

31 THANK YOU! Page 31