Narda Remote Spectrum Analyzer, NRA Series
Interference and Direction Analyzer, IDA Series
Technical Note TN101
Capturing IQ data with
NRA and IDA
A brief theoretical outline with
practical examples
With the Scope/IQ option, the NRA and IDA instruments allow you to
generate IQ data from the received data and display, demodulate, and
store them, and transmit them block by block or as a continuous stream
via a remote interface. What, though, is the significance of this I/Q data?
What are they used for? And, how are they obtained using NRA and
IDA?
Communications systems
To transmit data wirelessly it must be modulated onto a suitable, usually
high frequency, carrier signal. This gives a basic setup as shown in
figure 1. The demodulator must perform exactly the opposite function to
the modulator. The receiver must also be able to cope with the
impairments to the modulated signal that are added as it passes through
the communications channel.
The type of modulation has to be suitable for the signal content and the
communications channel. Various techniques are used to transmit
different signals together through the same medium and then separate
them again, e.g.:
Frequency multiplex,
Time multiplex and / or
Code multiplex
The following types of modulation are commonly used for sinusoidal
carriers:
AM, FM, PM, (angle modulation) for analog signals
ASK, FSK, PSK, APSK, QAM for digital signals
If a prominent but unknown signal in the spectrum is to be classified or
even its message content reconstructed, various parameters such as
Communications
channel
Modulator Demodulator
Transmitter Receiver
Figure 1: Basic setup for a communications system
See last page for abbreviations
TN_IDA_1068_E_IQ data 1 / 8 Subject to change
the type of modulation need to be determined before a matching
Q
receiver can be produced for the unknown transmitter. This is not
possible “on the fly”, so the NRA and IDA provide facilities for recording
the unknown signal in the form of IQ data so that it can subsequently be
subjected to a more precise investigation.
(I, Q)
M
φ
I
IQ data
Generally speaking, the IQ data consists of pairs of data describing an
instantaneous time signal in the complex plane, as shown in figure 2.
“I” stands for the in phase component, which has a phase angle of 0°
relative to the converted carrier, and which therefore forms the real
component in the complex plane. In contrast, “Q” is the quadrature
component, which is phase shifted by 90° relative to the carrier, thus
forming the imaginary component.
The values for an IQ data pair describe the instantaneous amplitude M
and the phase angle φ of a signal. Changes in the time signal are
reflected as a shift in the IQ data point in the complex plane.
IQ modulator Communications- IQ demodulator
channel
cos(ω
t + φτ) cos(ωνt + φν)
τ
Modulator Demodulator
-sin(ω
t + φτ) -sin(ωνt + φν)
τ
Complex base band signal Real Complex base band signal
band pass signal
Figure 2: Representation of an IQ data pair in the
complex plane
Figure 3: Basic representation of an IQ based
communications installation
Transmitter Receiver
TN_IDA_1068_E_IQ_data 2 / 8 Subject to change
–
–
–
The IQ data find application in the transmitters and receivers in
communications installations. They are also suitable for recording
signals from the communications channel itself.
From the point of view of a basic communications installation – as
depicted in figure 3 – the IQ data appear before conversion from the
base band into the actual transmission band in the transmitter and after
conversion from the transmission band to the base band in the receiver.
The payload signal or data stream is fed to the IQ modulator in the
transmitter in the form of IQ data. The IQ modulator limits the bandwidth
of the signal and converts it into the transmission frequency band. After
addition of the I and Q components, the real, band-limited signal can be
transmitted in a radio channel.
The received signal together with any captured interference is converted
back to the base band again and band limited by the IQ demodulator in
the receiver, so that the original payload signal or data stream together
with the interference is available at the output of the IQ demodulator as
IQ data.
The signal can be captured by the NRA and IDA during transmission
over the radio channel. In this case, the instrument takes over the role of
a receiver that can store IQ data. The IQ data represent a specific
section of the spectrum in the time domain shifted into the base band,
as depicted clearly in figure 4.
Signals that have known modulation, such as AM or FM, can often be
directly demodulated by the IDA and output to a loudspeaker or
headphones. Experts can discern other types of modulation by the
sound after AM or FM demodulation.
The IDA can also demodulate a UMTS or LTE signal and determine
certain parameters of the transmitted signal.
Things are more difficult if the modulation of the signal is unknown. The
modulation type needs to be identified, and the characteristic of the
matching receive filter, the exact carrier frequency, the phase, and the
line digit rate all need to be determined. NRA and IDA can save the
corresponding section from the spectrum in the form of IQ data for this
purpose for subsequent processing or export to another device over an
interface.
P
Signal under consideration
f
f
ν
P
0 f
Base band
Time 'I' 'Q'
0.00E+00
5.00E-08
1.00E-07
1.50E-07
2.00E-07
2.50E-07
3.00E-07
3.50E-07
4.00E-07
4.50E-07
5.00E-07
5.50E-07
6.00E-07
6.50E-07
Figure 4: Spectrum and IQ data.
Top: The entire spectrum,
Center: Spectrum of signal converted to the base
band,
Bottom: A time section of the signal converted to the
base band expressed as IQ data.
0.00208341 -0.00078946
0.00243203 -0.00051186
0.00280739 -0.0002444
0.00316431 2.40E-05
0.00353968 0.00029236
0.00384864 0.00053031
0.00414745 0.00076456
0.00442414 0.00097576
0.00461874 0.001151
0.00479581 0.00128104
0.00490279 0.00140462
0.00493231 0.00146272
0.00490372 0.00151068
0.0048161 0.00151898
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