Glossary

NI IF Digitizers 14.0 Help

Edition Date: September 2014

Part Number: 374716A-01

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Prefixes

Symbol Prefix Value
n nano 10 -9
µ micro 10 -6
m milli 10 -3
k kilo 10 3
M mega 10 6
G giga 10 9

Numbers/Symbols

nV nanovolts 10-9 volts
µV microvolts 10-6 volts
µΩ microohms 10-6 ohms
mΩ milliohms 10-3 ohms
MΩ megaohms 106 ohms
pA picoamps 10-12 amperes
nA nanoamps 10-9 amperes
µA microamps 10-6 amperes
mA milliamps 10-3 amperes

A

ADC analog-to-digital converter—A hardware component that converts analog voltages to digitized values. An ADC can convert an analog signal to a digital signal representing equivalent information.
ADC Clock Clock signal to the ADC chip of the IF digitizer. On the NI 5624R, one ADC sample (conversion) occurs on each ADC Clock.
ADEapplication development environment—A software environment incorporating the development, debug, and analysis tools for software development. LabVIEW, Measurement Studio, and Visual Studio are examples.
APIapplication programming interface—Programming interface for controlling some software packages, such as the instrument design libraries.
analog A signal with an amplitude that can have a continuous range of values.

B

bandwidth The measure of a circuit or transmission channel to pass a signal without significant attenuation over a range of frequencies. Bandwidth can also refer to the information rate (in bits per second) that can pass through a circuit or transmission channel.
basebandThe range in the frequency spectrum occupied by a DC-centered signal. Both the information signal and the downconverted complex I/Q signal are referred to as baseband signals.

C

carrier The signal that carries the information encoded or modulated on it. Typically, the carrier is a fixed frequency sine wave, which can be amplitude, phase, or frequency modulated. In advanced forms of telecommunication systems, the carrier may be a moving signal, called a spread spectrum. As long as the characteristics of the carrier signal are deterministic and known by the receiver, virtually any type of carrier signal can be used.
carrier frequency The frequency of the carrier signal that is a sinusoidal signal upon which the desired signal to be transmitted is modulated. The sinusoidal signal "carries" the modulation.
center frequency The middle frequency of the channel bandwidth.
complex baseband bandwidth The combined bandwidth of I and Q channels.
component The real and imaginary parts of a complex number (specifically, S = I + jQ).

For example, you can represent a two-dimensional vector of length S as a complex number, where I and Q are the vector x- and y-components. The real part of the vector corresponds to the x-component (I), while the imaginary part corresponds to the y-component (Q).
CW continuous wave—A continuous sine wave that is the carrier wave in a radio transmission.

D

Data Clock Clock signal used within the FPGA to process input data. On the NI 5624R, the Data Clock is the ADC Clock divided by 16. The FPGA can process 16 ADC samples on each Data Clock.
dBFS Decibels relative to full scale—A measure of decibel amplitude level in a digital system that has a defined maximum available peak level.
DDC digital downconverter—A hardware component that performs IF to I/Q conversion in the digital domain. The DDC frequency-translates RF signals to center around a specified IF frequency, while filtering certain frequency bands and reducing the signal sampling rate.
demodulation Describes the recovery, from a modulated wave, of a signal having the same characteristics as the original message signal.
digital modulation A technique for encoding digital data on a carrier frequency.
DMA direct memory access—Method by which data can be transferred to/from computer memory from/to a device or memory on the bus while the processor performs another task.
downconverter A signal conditioning device that converts a specific band of high-frequency (RF) signals to lower, more manageable IF frequencies that can be digitized.
DSP digital signal processing
DUT device under test
dynamic range The range between the minimum detectable signal, typically the noise floor, and the -1 dB compression point.

E

EVM error vector magnitude—A measurement of demodulator performance in the presence of impairments. The soft symbol decisions obtained after decimating the recovered waveform at the demodulator output are compared against the ideal symbol locations. The root mean square (RMS) error vector magnitude and phase error are used to determine the EVM measurement over a window of N demodulated symbols.

F

FFT fast Fourier transform—An efficient mathematical algorithm used for spectrum analysis. Refer to Fourier transform.
Fourier transform A mathematical technique that resolves a given signal into the sum of sines and cosines. Widely used as the FFT (fast Fourier transform), which is the basis for spectrum analysis.
FPGA A semiconductor device that contains a large quantity of gates (logic devices) that are not interconnected. The wiring list for the gates is downloaded to the FPGA and determines the gates' function. The wiring list determines how the gates are interconnected, and this interconnection is performed dynamically by turning semiconductor switches on or off to enable the different connections.
frequency The number of cycles per unit time. The International System of Units has set hertz (Hz) as the standard unit of measurement for frequency, where 1 Hz equals one cycle per second.

G

gain Amplification of a signal. A gain of two corresponds to a doubling of the signal level.

H

harmonic In a complex signal, a component with a frequency that is a multiple of the fundamental frequency.

I

IF intermediate frequency—The intermediate signal that an RF signal is converted down to or up to for processing. IF signals occupy a frequency between baseband and RF.
I/Q data The translation of the magnitude and phase data of a signal from a polar coordinate system to a complex Cartesian (X,Y) coordinate system.
I/Q signal A control signal for changing an RF carrier signal.

L

linearity For signal measurement systems, linearity refers to mechanical response of hardware to signals. A linear system will respond in a determinable way to signals. A nonlinear system contains components that do not respond to signals in a determinable way for a variety of reasons that include, but are not limited to: temperature susceptibility, improperly terminated connections, and inherent device design. This nonlinear response to signals contributes noise to a system. If a system is nonlinear, the system response to signals produces noisy results.
lowpass filter A circuit that attenuates the high-frequency components in an analog signal and only passes low frequencies. For imaging, a lowpass filter removes detail and blurs the image.

M

modulation A process, or the result of a process, by which characteristics of a carrier wave are altered according to information in the baseband signal to generate a modulated wave that is transmitted.
module Refers to the NI 5624R hardware.
MXI Multisystem Xtension Interface—A high-speed serial interface bus designed to connect computers to PXI, or to connect multiple PXI chassis. In this document, MXI refers to NI MXI-3 or MXI-4 systems.

N

NI 5624R NI 5624R refers to the NI PXIe-5624R IF digitizer.
Nyquist Theorem A theorem that states that a signal must be sampled at least twice as fast as the bandwidth of the signal to accurately reconstruct the waveform; otherwise, the high-frequency content will alias at a frequency inside the spectrum of interest (passband). An alias is a false lower frequency component that appears in sampled data acquired at too low a sampling rate (FS). The following figure shows a 5 MHz sine wave digitized by a 6 MS/s ADC. The dotted line indicates the aliased signal recorded by the ADC at that sample rate.

P

passband The range of frequencies which a device can properly propagate or measure.
passband flatness A measure of the amplitude accuracy of the frequency response with respect to frequency. Passband flatness is typically specified in ±dB and referenced to the amplitude of the frequency response at a designated frequency.
PCI Express peripheral component interconnect express—A version of PCI that maintains the PCI software usage model and replaces the physical bus with a high-speed serial bus serving multiple lanes.
peer-to-peer stream The flow of data in a peer-to-peer system. The peer-to-peer stream acts like a single, unidirectional pipe from which data can flow directly from one device to another.
peer-to-peer streaming A data streaming architecture used to transfer data between hardware devices. Use of this architecture allows two or more devices to transfer data directly to each other without first going through the host processor.
PFI Programmable Function Interface—I/O channels to the digital waveform generator/analyzer. Functionality and specifications vary by device and operation.
phase noise Noise in a carrier signal due to phase and frequency modulation in the signal. Phase noise is normally very close to the carrier, and it is measured in decibels relative to the carrier frequency (dBc).
pipelining An implementation technique that allows use of single-cycle timed loops to accomplish operations requiring multiple clock cycles.

Most applications require some signal processing before writing to the analog output or after reading from the analog input. If the signal processing is inside a Single-Cycle Timed Loop, a single clock cycle may not provide enough time for the FPGA to perform all necessary computations.

To resolve this problem you can pipeline your signal processing. Pipelining divides the Single-Cycle Timed Loop into segments separated by registers, which can be implemented using shift registers, feedback nodes, or local variables. Each segment executes a portion of the computation that can be completed within a single clock cycle. At each clock cycle, every segment executes and writes its results to a feedback node. On the next iteration of the loop, the stored result is fed to the next segment. The multiple segments overlap in execution, so each segment processes new data at every clock cycle.
PLL phase-locked loop—An electronic circuit that controls an oscillator so that the circuit maintains a constant phase angle relative to a reference signal.
PXI PCI eXtensions for Instrumentation—Rugged, open system for modular instrumentation based on CompactPCI, with special mechanical, electrical, and software features.
PXI Express (PXIe) PCI Express eXtensions for Instrumentation—The PXI implementation of PCI Express, a scalable full-simplex serial bus standard that operates at 2.5 Gbps and offers both asynchronous and isochronous data transfers.
PXI Express-Compatible Module A modified PXI module that is compatible with existing PXI chassis slots and PXI hybrid chassis slots. PXI Express-compatible modules preserve hardware and software compatibility, with the exception of local bus features.
PXI trigger bus PXI equivalent of the RTSI bus, with additional timing and synchronization capabilities.

Q

quadrature modulation Any modulation scheme that uses two carrier waves out of phase by 90° that are modulated by separate information signals.

R

record A group of samples. Acquired data is stored into device onboard memory as a record. When configuring an acquisition session, you can determine how many samples are stored in a record.
Reference Clock Clock to which a device phase locks another, usually faster, clock. A common source for the Reference Clock is the 10 MHz oscillator present on the PXI backplane.
reference level The reference level represents the maximum expected power of an input RF signal.
resolution The smallest signal increment that can be detected by a measurement system. Resolution can be expressed in bits, in proportions, or in percent of full scale. For example, a system has 12-bit resolution, one part in 4,096 resolution, and 0.0244 percent of full scale.
RF The radio frequency range. RF is often used to describe a range of sub-infrared frequencies from sub-MHz to several GHz.
RMS root–mean–square—The square root of the average value of the square of the instantaneous signal amplitude; a measure of signal amplitude. The rms voltage of a signal is computed by squaring the instantaneous voltage, integrating over the desired time, and taking the square root.

S

Sample Clock See ADC Clock.
sample rate The rate at which a device acquires an analog signal, expressed in samples per second (S/s). The sample rate is typically the clock speed of the analog-to-digital converter (ADC).
SFDR spurious free dynamic range—The separation or distance, expressed in dB, from the amplitude of the fundamental frequency and the next highest spur.
signal bandwidthTwice the maximum baseband signal deviation from 0 Hz. Usually, the baseband signal center frequency is 0 Hz. In such cases, the signal bandwidth is simply the baseband signal's minimum frequency subtracted from its maximum frequency, or fmax – fmin.
SINAD signal-to-noise and distortion ratio—A measure of the quality of a signal, defined as the ratio of Signal plus Noise plus Distortion and Noise plus Distortion.
sinusoid A mathematical function that describes a repetitive oscillation. Sinusoid is synonymous with sine wave.
SMA A small type of threaded coaxial signal connector typically used in higher frequency applications.
SNRsignal-to-noise ratio—The ratio of the desired signal amplitude to the noise signal amplitude at a given point in time. SNR is expressed as 20 times the logarithm of the amplitude ratio, or 10 times the logarithm of the power ratio. SNR is usually expressed in dB and in terms of peak values for impulse noise and root-mean-square values for random noise. In defining or specifying the SNR, specify the signal and noise characterizations, for example, peak-signal-to-peak-noise ratio to avoid ambiguity.
spurs spurious effects—Undesired signals. Spurious signals can be considered in two classes: 1) Stationary, that is, their frequency and amplitude do not change significantly, 2) Non-stationary, where the frequency and/or amplitude change significantly over short time periods.

T

THD total harmonic distortion—The ratio of the sum of the powers of the first five harmonics above the measured fundamental frequency to the power of the fundamental frequency. THD is usually expressed in dB or dBc. Measurements for calculating the THD are made at the output of a device under specified conditions.
transient A brief oscillation resulting from a sudden change of voltage, current, or load.
trigger Any event that causes, starts, or stops some form of data capture. An external stimulus that initiates one or more instrument functions. Trigger stimuli include a front panel button, an external input voltage pulse, or a bus trigger command. The trigger may also be derived from attributes of the actual signal to be acquired, such as the level and slope of the signal.
TTL Transistor-Transistor Logic

V

VCO voltage-controlled oscillator—An electronic oscillator that can be controlled over a wide range of frequencies by applying voltage.
VSA vector signal analyzer
VSWRvoltage standing wave ratio—The loss due to the mismatch introduced into the signal by the load or source signal path characteristics. The ratio of the highest voltage to the lowest voltage found in the signal.

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