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
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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
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The I/Q Modulator
Quadrature
Power
Splitter
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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
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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)
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I/Q Modulator Imperfections
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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)
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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)
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DC or Differential Voltage Offset
V
voltage level
I
Q
Time, ns
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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
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I/Q Path DC Offset Adjustments
Wideband I/Q path
adjustment for
external AWG
I/Q path
adjustment for
internal AWG
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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
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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
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I/Q Quadrature Angle Adjustment
Wideband I/Q path
quadrature angle
adjustment for
external AWG
I/Q path quadrature
angle adjustment for
internal AWG
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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
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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
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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
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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
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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
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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
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Group Delay
OFDM Signal with 400 MHz bandwidth
Before Predistortion Measured EVM = -30 dB (3.3%)
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Group Delay
OFDM Signal with 400 MHz bandwidth
After Predistortion Measured EVM = -34 dB (2.0%)
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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
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RF Amplitude Flatness
I/Q modulator amplitude flatness examples
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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
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RF Amplitude Flatness
Before Predistortion
Measured flatness
OFDM - 500 MHz Bandwidth
After Predistortion
Measured flatness
OFDM - 500 MHz Bandwidth
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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
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Wideband I/Q Modulation
M8190A AXIe Arbitrary Waveform Generator
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Wideband I/Q Modulation Frequency
Response of the Agilent PSG
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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
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© 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
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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
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© Agilent Technologies 2013
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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
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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
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Analog I/Q Modulation Example (3/4)
Adjusting the
differential offset
of the I and Q
signals reduces
the carrier feedthrough
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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
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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
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© Agilent Technologies 2013
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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
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© Agilent Technologies 2013
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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
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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
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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
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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
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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
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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
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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
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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
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Thank you for Attending
Questions?
© Agilent Technologies 2013