Vector Modulation and Frequency Conversion Fundamentals John Hansen, Agilent Technologies © Agilent Technologies 2013 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 © Agilent Technologies 2013 2 Vector Signal Generation Overview Block Diagram LPF 16 I RAM 16 Re- 16 I 16 FIR sampling DAC RF/MW I/Q Waveform File 16 LPF Q RAM 16 Re- 16 sampling I/Q modulator 16 FIR LO 90 ALC DAC Q Q I © Agilent Technologies 2013 3 The I/Q Modulator Quadrature Power Splitter © Agilent Technologies 2013 4 I/Q Modulator Characteristics: Double-Sideband Suppressed Carrier (DSBSC) LO1= sin(ωct) LO2= cos(ωct) I = Asin(ωmt) Q=0 RF1 = sin(ωct) * Asin(ωmt) = A/2 * cos((ωc– ωm)t) – A/2 * cos((ωc+ ωm)t) and RF2 = cos(ωct) * 0 = 0 The output will be: RF1 + RF2 = A/2 * cos((ωc– ωm)t) – A/2 * cos((ωc+ ωm)t) Lower Side Band Upper Side Band © Agilent Technologies 2013 5 I/Q Modulator Characteristics: Single-Sideband Suppressed Carrier (SSBSC) LO1= sin(ωct) LO2 = cos(ωct) I = Asin(ωmt) Q = Acos(ωmt) RF1 = sin(ωct) * Asin(ωmt) = A/2 * cos((ωc– ωm)t) – A/2 * cos((ωc+ ωm)t) and RF2 = cos(ωct) * Acos(ωmt) = A/2 * cos((ωc– ωm)t) + A/2 * cos((ωc+ ωm)t) The output will be: RF1 + RF2 = Acos((ωc– ωm)t) (Lower sideband) © Agilent Technologies 2013 6 I/Q Modulator Imperfections © Agilent Technologies 2013 7 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) © Agilent Technologies 2013 8 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 Internal or external AWG Remote Control PC running Waveform equalization software Vector Signal Generator Spectrum Analyzer IQ Tools (amplitude only) © Agilent Technologies 2013 9 DC or Differential Voltage Offset V voltage level I Q Time, ns © Agilent Technologies 2013 10 Carrier Feed Through Before DC offset adjustment Measured CF = -55 dBc Two-tone - 2 MHz spacing After DC offset adjustment Measured CF = -66 dBc Two-tone - 2 MHz spacing © Agilent Technologies 2013 11 I/Q Path DC Offset Adjustments Wideband I/Q path adjustment for external AWG I/Q path adjustment for internal AWG © Agilent Technologies 2013 12 Carrier Feed Through Baseband Predistortion / Correction Before Predistortion Measured CF = -55 dBc Two-tone - 2 MHz Bandwidth After Predistortion Measured CF = -73 dBc Two-tone - 2 MHz Bandwidth © Agilent Technologies 2013 13 I/Q Modulator Imperfections: LO Quadrature Error The two LO signals are not exactly 90 degrees apart RF1 = sin(ωct) * Asin(ωmt) = A/2 * cos((ωc – ωm)t) – A/2*cos((ωc+ ωm)t) and Caused by LO splitter phase error, or phase matching imperfections in the mixers RF2 = cos(ωct) * Acos(ωmt + α) = A/2 * cos((ωc– ωm)t – α) + A/2 * cos((ωc+ ωm)t + α) The output will be: RF1 + RF2 = A cos(α/2) cos((ωc– ωm)t – α/2) (desired lower sideband) –A sin(α/2) sin((ωc+ ωm)t + α/2) (unwanted image) Resulting spectrum of LO feedthrough and quadrature error © Agilent Technologies 2013 14 I/Q Quadrature Angle Adjustment Wideband I/Q path quadrature angle adjustment for external AWG I/Q path quadrature angle adjustment for internal AWG © Agilent Technologies 2013 15 I/Q Modulator Imperfections: I/Q path gain imbalance and timing skew Source of error: Mismatched I/Q path V delay, magnitude & phase response skew Result: I/Q images Solution: Minimize I/Q path delay (timing skew) and/or apply predistortion I/Q mismatch of baseband path I Q delay Time, ns © Agilent Technologies 2013 16 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 © Agilent Technologies 2013 17 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 © Agilent Technologies 2013 18 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 © Agilent Technologies 2013 19 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 © Agilent Technologies 2013 20 Group Delay Source of error: frequency selective devices and components, such as filters Group Delay ripple indicates distortion tg Group delay ripple Result: Added Inter-Symbol Interference(ISI) and Error Vector Magnitude (EVM) to Solution: apply predistortion corrections Average delay Tradeoff: calculation time (Agilent MXG has real time corrections) Frequency © Agilent Technologies 2013 21 Group Delay OFDM Signal with 400 MHz bandwidth Before Predistortion Measured EVM = -30 dB (3.3%) © Agilent Technologies 2013 22 Group Delay OFDM Signal with 400 MHz bandwidth After Predistortion Measured EVM = -34 dB (2.0%) © Agilent Technologies 2013 23 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) © Agilent Technologies 2013 24 RF Amplitude Flatness I/Q modulator amplitude flatness examples © Agilent Technologies 2013 25 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 © Agilent Technologies 2013 26 RF Amplitude Flatness Before Predistortion Measured flatness OFDM - 500 MHz Bandwidth After Predistortion Measured flatness OFDM - 500 MHz Bandwidth © Agilent Technologies 2013 27 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 © Agilent Technologies 2013 28 Wideband I/Q Modulation M8190A AXIe Arbitrary Waveform Generator © Agilent Technologies 2013 29 Wideband I/Q Modulation Frequency Response of the Agilent PSG © Agilent Technologies 2013 30 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 © Agilent Technologies 2013 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 17025 or Z54 calibration www.agilent.com/find/M8190 32 © Agilent Technologies 2013 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 © Agilent Technologies 2013 33 Signal Generation Setups I/Q Modulation Differential I/Q signals Modulation BW up to 2 GHz* RF up to 44 GHz *Unspecified operation >2GHz available PCIe M8190A Marker output Pulse mod. input Direct IF/RF RF/IF out PCIe M8190A E8267D Opt. 016 / H18 RF/IF out IF/RF up to 5 GHz Modulation BW up to 2 * (5 GHz – IF) Upconversion solution E8257D Opt. H30* *Wideband mixer upconversion unfiltered 34 © Agilent Technologies 2013 34 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 © Agilent Technologies 2013 35 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 © Agilent Technologies 2013 36 Analog I/Q Modulation Example (3/4) Adjusting the differential offset of the I and Q signals reduces the carrier feedthrough © Agilent Technologies 2013 37 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 © Agilent Technologies 2013 38 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 © Agilent Technologies 2013 39 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 © Agilent Technologies 2013 40 RF Upconversion Mixers fRF1 = fLO - fIF fRF2 = fLO + fIF Power IF RF S LO IF S RF1 RF2 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 © Agilent Technologies 2013 41 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 © Agilent Technologies 2013 42 Analog versus Digital I/Q Modulation & Upconversion AWG 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 Analog I/Q Modulator Memory ~ 90° D/A Memory 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) X D/A X AWG Memory + Mixer / Multiplier / LO X ~ 90° Memory + D/A X ~ X Digital signal Analog signal © Agilent Technologies 2013 43 Comparison of Different I/Q Modulation/Upconversion Methods Low sample rate Software calculates I/Q BB data 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 Software calculates I/Q BB data Low sample rate High sample rate 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 © Agilent Technologies 2013 44 Real Time Digital I/Q Modulation/Upconversion & Additional Advantages Sample Memory FPGA Sequence Memory Interpolator x3, I+Q x12, data x24 or x48 Complex multiplier Direct mode DAC Numerically controlled oscillator (DDS engine) Digital Upconversion mode Agilent proprietary ASIC Conserve sample memory by interpolating a low baseband sample rate to a higher DAC sample rate 45 Carrier frequency, phase, amplitude and frequency sweep can be controlled in real time © Agilent Technologies 2013 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 © Agilent Technologies 2013 46 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 © Agilent Technologies 2013 47 Resources Archive of webcasts: www.agilent.com/find/AeroDefWebcasts For more on digital down conversion see the presentation titled: “Multi-antenna Array Measurements using Digitizers” Agilent Aerospace and Defense application page: www.agilent.com/find/ad More information on the PSG microwave signal generator: www.agilent.com/find/PSG More information on the M8190A arbitrary waveform generator: www.agilent.com/find/M8190 © Agilent Technologies 2013 48 Thank you for Attending Questions? © Agilent Technologies 2013