Vector Modulation and Frequency Conversion Fundamentals. John Hansen, Agilent Technologies

Transcription

1 Vector Modulation and Frequency Conversion Fundamentals John Hansen, Agilent Technologies

2 Why Optimize I/Q Waveforms? Purpose of test equipment: Characterize design performance and verify functionality Measurement challenge: Minimize test equipment influences on the device or system measurement uncertainty Demands on test equipment: Must perform better than the device or system under test to create a objective test environment and yield accurate measurement results 2

3 Vector Signal Generation Overview Block Diagram 16 I/Q Waveform File I RAM 16 Resampling 16 FIR 16 DAC LPF LPF I I/Q modulator LO 90 RF/MW 16 Q RAM 16 Resampling Q 16 FIR 16 DAC Q ALC I 3

4 The I/Q Modulator Quadrature Power Splitter 4

5 I/Q Modulator Characteristics: Double-Sideband Suppressed Carrier (DSBSC) LO 1 = sin(ω c t) LO 2 = cos(ω c t) I = Asin(ω m t) Q = 0 RF 1 = sin(ω c t) * Asin(ω m t) = A/2 * cos((ω c ω m )t) A/2 * cos((ω c + ω m )t) and RF 2 = cos(ω c t) * 0 = 0 The output will be: RF 1 + RF 2 = A/2 * cos((ω c ω m )t) A/2 * cos((ω c + ω m )t) Lower Side Band Upper Side Band 5

6 I/Q Modulator Characteristics: Single-Sideband Suppressed Carrier (SSBSC) LO 1 = sin(ω c t) LO 2 = cos(ω c t) I = Asin(ω m t) Q = Acos(ω m t) RF 1 = sin(ω c t) * Asin(ω m t) = A/2 * cos((ω c ω m )t) A/2 * cos((ω c + ω m )t) and RF 2 = cos(ω c t) * Acos(ω m t) = A/2 * cos((ω c ω m )t) + A/2 * cos((ω c + ω m )t) The output will be: RF 1 + RF 2 = Acos((ω c ω m )t) (Lower sideband) 6

7 I/Q Modulator Imperfections 7

8 I/Q Modulator Imperfections: LO Feed Through Source of Error: LO or Carrier feed-through, sometimes called Origin Offset, can be caused by: The two mixers not being identically matched and balanced, resulting in LO leakage which is dependent on carrier frequency. DC offset at the I and/or Q inputs, resulting in LO leakage which is independent of carrier frequency. Result: LO feedthrough, in-band interference and out-of-band spurs Solutions: adjust I and Q path DC offset level and quadrature angle apply predistortion (also termed correct or calibrate the waveform) 8

9 Amplitude & Phase Correction Setup Signal generator, AWG and spectrum analyzer (or oscilloscope) are remotely controlled by a PC running an equalization routine Magnitude of each tone in the multi-tone signal is measured and frequency response stored in a file Assuming stable system behavior (no large phase discontinuities) and using Hilbert transforms, the phase response can be derived from the measured amplitude response Pre-distorted signal can be calculated and the new file downloaded to the AWG Signal Studio for pulse building Signal Studio for multitone distortion PC running Waveform equalization software Remote Control Internal or external AWG Vector Signal Generator Spectrum Analyzer IQ Tools (amplitude only) 9

10 DC or Differential Voltage Offset voltage level V I Q Time, ns 10

11 Carrier Feed Through Before DC offset adjustment Measured CF = -55 dbc After DC offset adjustment Measured CF = -66 dbc Two-tone - 2 MHz spacing Two-tone - 2 MHz spacing 11

12 I/Q Path DC Offset Adjustments Wideband I/Q path adjustment for external AWG I/Q path adjustment for internal AWG 12

13 Carrier Feed Through Baseband Predistortion / Correction Before Predistortion Measured CF = -55 dbc After Predistortion Measured CF = -73 dbc Two-tone - 2 MHz Bandwidth Two-tone - 2 MHz Bandwidth 13

14 I/Q Modulator Imperfections: LO Quadrature Error The two LO signals are not exactly 90 degrees apart RF 1 = sin(ω c t) * Asin(ω m t) = A/2 * cos((ω c ω m )t) A/2*cos((ω c + ω m )t) and RF 2 = cos(ω c t) * Acos(ω m t + α) = A/2 * cos((ω c ω m )t α) + A/2 * cos((ω c + ω m )t + α) The output will be: RF 1 + RF 2 = A cos(α/2) cos((ω c ω m )t α/2) (desired lower sideband) A sin(α/2) sin((ω c + ω m )t + α/2) (unwanted image) Caused by LO splitter phase error, or phase matching imperfections in the mixers Resulting spectrum of LO feedthrough and quadrature error 14

15 I/Q Quadrature Angle Adjustment Wideband I/Q path quadrature angle adjustment for external AWG I/Q path quadrature angle adjustment for internal AWG 15

16 I/Q Modulator Imperfections: I/Q path gain imbalance and timing skew Source of error: Mismatched I/Q path delay, magnitude & phase response skew Result: I/Q images Solution: Minimize I/Q path delay (timing skew) and/or apply predistortion V I/Q mismatch of baseband path I Q delay Time, ns 16

17 I/Q Gain Balance & Timing Skew Adjustments I/Q Gain Balance Reduces images with a more random amplitude across the modulation bandwidth I/Q Timing Skew Reduces images with the distinctive curved bat wing shape across the modulation bandwidth 17

18 I/Q Path Gain Imbalance and Timing Skew Before Gain Balance and Timing Skew Adjustments Measured images = -23 dbc After Gain Balance and Timing Skew Adjustments Measured images = -40 dbc 20 Tone Multitone Waveform - 80 MHz Bandwidth 18

19 I/Q Path Gain Imbalance and Timing Skew Before Predistortion Measured images = -23 dbc After Predistortion Measured images = -77 dbc 20 Tone Multitone Waveform - 80 MHz Bandwidth 19

20 I/Q Timing Skew Adjustment Multitone signal at 30 GHz 64 tones spaced over 80 MHz with upper side band tones suppressed Timing Skew misadjusted 400 picoseconds Timing Skew properly adjusted 20

21 Group Delay Source of error: frequency selective devices and components, such as filters Result: Added Inter-Symbol Interference(ISI) and Error Vector Magnitude (EVM) Solution: apply predistortion corrections Tradeoff: calculation time (Agilent MXG has real time corrections) Group Delay ripple indicates distortion t g t o Group delay ripple Average delay Frequency 21

22 Group Delay OFDM Signal with 400 MHz bandwidth Before Predistortion Measured EVM = -30 db (3.3%) 22

23 Group Delay OFDM Signal with 400 MHz bandwidth After Predistortion Measured EVM = -34 db (2.0%) 23

24 RF Amplitude Flatness Source of error: I/Q modulator, RF chain Result: passband tilt, ripple, and roll off Solution: apply predistortion corrections Tradeoff: calculation time (Agilent MXG has real time corrections) 24

25 RF Amplitude Flatness I/Q modulator amplitude flatness examples 25

26 RF Amplitude Flatness Before Predistortion Correction Measured flatness = 2.4 db After Predistortion Correction Measured flatness = 0.1 db 32 tone signal - 80 MHz Bandwidth 32 tone signal - 80 MHz Bandwidth 26

27 RF Amplitude Flatness Before Predistortion Measured flatness After Predistortion Measured flatness OFDM MHz Bandwidth OFDM MHz Bandwidth 27

28 Agilent MXG: Factory-Equalized Wide Bandwidth Real time corrections for 160 MHz bandwidth with <± 0.2 db flatness MXG offers the only one-box solution with factory-equalized 160 MHz BW Internal baseband processing accelerator enables real-time phase and amplitude corrections 28

29 Wideband I/Q Modulation M8190A AXIe Arbitrary Waveform Generator 29

30 Wideband I/Q Modulation Frequency Response of the Agilent PSG 30

31 Wideband I/Q Input Adjustments A wideband I/Q modulator path is available for modulating differential I/Q signals from an external AWG onto carrier frequencies up to 44 GHz

32 Advanced AWG Solutions M8190A Arbitrary Waveform Generator 14 bit up to 8 GSa/s 12 bit up to 12 GSa/s Up to 5 GHz analog bandwidth per channel Up to 2 GSa memory per channel Signal Studio for Pulse Building and SystemVue support AXIe form factor DC and AC amplifier SFDR: -80 dbc typical Harmonic distortion: -72 dbc typical Advanced sequencing scenarios define stepping, looping, and conditional jumps of waveforms or waveform sequences 2 markers per channel (does not reduce DAC resolution) ISO or Z54 calibration 32

33 M8190A AWG I/Q Timing Skew Adjustment I/Q timing skew adjustment is available within the M8190A wideband AWG Shown here is the M8190A virtual front panel 33

34 Signal Generation Setups I/Q Modulation PCIe Differential I/Q signals Modulation BW up to 2 GHz* RF up to 44 GHz *Unspecified operation >2GHz available M8190A Marker output Pulse mod. input E8267D Opt. 016 / H18 RF/IF out Direct IF/RF PCIe RF/IF out IF/RF up to 5 GHz Modulation BW up to 2 * (5 GHz IF) M8190A Upconversion solution E8257D Opt. H30* *Wideband mixer upconversion unfiltered 34 34

35 Analog/Hardware I/Q Modulation Example (1/4) Example waveform: Multi-tone signal with 20 tones spanning 2 GHz Asymmetric with respect to the carrier frequency Notice: Images Carrier feedthrough Non-Flatness Amplitude Phase 35

36 Analog I/Q Modulation Example (2/4) Adjusting the timing skew and relative amplitude (gain balance) between the I and Q signals reduces the images Typically, they can be reduced to about -30 dbc 36

37 Analog I/Q Modulation Example (3/4) Adjusting the differential offset of the I and Q signals reduces the carrier feedthrough 37

38 Analog I/Q Modulation Example (4/4) With amplitude correction, the frequency response can be adjusted to be flat within less than 0.5 db 38

39 Digital/Software I/Q Modulation (1/3) Digital I/Q modulation avoids a few of the problems associated with analog I/Q modulators: No (in-band) carrier feed-through No (in-band) images Generally less signal power due to the insertion loss of the mixer Frequency response is still not flat 39 39

40 Digital I/Q Modulation (2/3) With amplitude correction, an almost distortion-free RF signal can be generated Analog I/Q modulation has the advantage of using two AWG channels to double the available modulation bandwidth 40 40

41 RF Upconversion Mixers f RF1 = f LO - f IF f RF2 = f LO + f IF LO IF S RF Power IF RF1 RF2 S LO Frequency 3-port, non-linear device usually designed using Schottky diodes, FETs or CMOS transistors SSB mixers are available internally cancelling one of the RF sidebands Signal level at the LO port essentially turns the mixer on and off 41

42 Digital I/Q Modulation/Up-Conversion (3/3) Unwanted mixing images will need to be filtered in most applications Filtering Band pass filters Tuneable/variable Filter bank 42

43 Analog versus Digital I/Q Modulation & Upconversion Analog I/Q modulation Analog I and Q signals are generated using an AWG. A hardware I/Q modulator generates the IF or RF signal AWG Memory Memory D/A D/A Analog I/Q Modulator ~ X 90 X + Digital I/Q modulation Modulation is performed digitally either in real-time (in DSP/FPGA hardware) or prior to playback in the creation of the waveform file (in software) AWG Memory Memory ~ X 90 X + D/A Mixer / Multiplier / LO X ~ Digital signal Analog signal 43

44 Comparison of Different I/Q Modulation/Upconversion Methods Software calculates I/Q BB data Low sample rate I/Q BB data downloaded to AWG Vector PSG with wideband I/Q inputs upconversion Analog I/Q upconversion can cause distortions advantages in power and BW High sample rate Software calculates I/Q BB data Up-conversion to IF in software IF data downloaded to AWG RF Upconversion using a mixer IF in software requires high sample rate eats up memory; poor freq resolution Low sample rate High sample rate Software calculates I/Q BB data I/Q BB data downloaded to AWG Interpolation & upconversion in DAC/ASIC RF Upconversion using a mixer Digital Upconversion in hardware combines the benefits of both approaches 44

45 Real Time Digital I/Q Modulation/Upconversion & Additional Advantages Sample Memory Sequence Memory FPGA I+Q data Interpolator x3, x12, x24 or x48 Complex multiplier Numerically controlled oscillator (DDS engine) Direct mode Digital Upconversion mode DAC Agilent proprietary ASIC Conserve sample memory by interpolating a low baseband sample rate to a higher DAC sample rate Carrier frequency, phase, amplitude and frequency sweep can be controlled in real time 45

46 Frequency / Phase and Amplitude Changes Independent of Modulation Waveform With IF calculation in software, the frequency, phase and amplitude of the IF signal are folded into the modulation waveform With digital up-conversion in hardware, these parameters can be changed on the fly. Radar applications: Pulses with the same shape but different amplitude or frequency are stored only ONCE. Amplitude and frequency information is stored along with sequence information This approach allows fast changing signals (GHz pulses) to be combined with slow changes (e.g. a radar antenna scan at 15 RPM) which would otherwise use up a large amount of memory Simulated antenna scan 46

47 Summary & Conclusion Hardware I/Q modulation has advantages and disadvantages Signal corrections/predistortion is an important for today s wideband systems I/Q modulation performed in software or digitally in real time has distinct advantages Agilent has the tools you need for high performance signal simulation 47

48 Resources Archive of webcasts: For more on digital down conversion see the presentation titled: Multi-antenna Array Measurements using Digitizers Agilent Aerospace and Defense application page: More information on the PSG microwave signal generator: More information on the M8190A arbitrary waveform generator: 48

49 Thank you for Attending Questions?