Direct Digital Synthesis (DDS) Glossary
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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Active Filter
An active filter is one that uses active devices such as operational amplifiers to synthesize the filter response function. This technique has an advantage at high speeds because the need for inductors (with their poor high-frequency characteristics) is eliminated.
Aliased Imaging
This is a technique, commonly applied to Direct Digital Synthesis (DDS), for using intentional aliasing as a source of high-frequency signals.
Aliasing
In a sampled data system, the analog input must be sampled at a rate of FS>2FA in order to avoid loss of data (Nyquist Theorem). Adhering to the Nyquist Theorem prevents in-band "alias" signals, which are beat frequencies between the analog signal and the sampling clock that inherently occur at FS± FA. As the Nyquist limit is exceeded, the aliased signals move within the band of the analog input (DC - FS/2) and create distortion. Likewise, high-frequency noise can also be aliased into the input signal range, which mandates low-pass filtering, or anti-alias filtering, on the input of a sampled system. See also Aliased Imaging.
Analog Ground
In high-speed data acquisition applications, system ground is generally physically separated into "analog" and "digital" grounds in an attempt to supress digital switching noise and minimize its effect on noise-sensitive analog signal processing circuitry. Input signal conditioners, amplifiers, references, and A/D converters are usually connected to analog ground.
Aperture Jitter
Uncertainty, or sample-to-sample of variation, in the aperture delay time. Aperture jitter is a source of error in a sampling system, and it determines the maximum slew rate limitation of the sampled analog input signal for a given system resolution.
A digital communications application that allows for up to 7 MBPS of Subscriber Line data transmission capacity over conventional twisted pair telephone lines. ADSL is a contender for a major piece of the "information highway" pie and it promises to deliver telephone, TV, and data services to your home over the existing telephone line.
Asynchronous
A multiplexing and switching technique that organizes information Transfer Mode into fixed-length cells consisting of an identification header field and an information field. The transfer rate is asynchronous in that the recurrence of cells depends on the instantaneously required bit rate.
Autocorrelation
Multiplication of a signal with a time-delayed replica of itself.
Automatic Frequency Sweep
An implementation of DDS where the output frequency is automatically swept across a band of frequencies. This is particularly useful in radar applications.
Baseband
The frequency bandwidth of the fundamental signal of interest, i.e., the voice, audio, or video signal bandwidth within a communication system.
Base Station
The central transmitter in a communications system that acts as the cell hub for communicating with handsets and/or mobile units.
Baud Rate
The speed at which data is transmitted measured in symbols-per-second.
Bit Rate
The rate of transfer of information necessary to insure satisfactory reproduction of the information at the receiver.
Cable Telephony
This is the idea of using digital communications techniques to provide enhanced home telephone service via the exisiting home cable-TV connections. The bandwidth of cable is high enough to simultaneously support interactive cable-TV, telephone communications, and on-line data services. In this scenario, the cable connection becomes the primary link to the information highway vs. twisted pair telephone wire or a wireless connection.
Chip
A single frequency output from a frequency hopping signal source.
Chip Rate
In spread spectrum systems, this is the rate at which the pseudo-random noise code is applied. In frequency hopping systems, chip rate is the inverse of the dwell time which the output frequency occupies a single carrier frequency. Also called "chipping rate".
Chirp
Pulsed frequency modulation scheme in which a carrier is swept over a wide frequency band during a given pulse interval.
Comparator
On chip comparator allows square wave generation for clocking purposes.
Cross-correlation
The degree of agreement between two unlike signals.
D/A Converter (also D/A or DAC)
Short for digital-to-analog converter, this is a device that changes a digitally-coded word into its "equivalent" quantized analog voltage or current. Just like the A/D device, there are very high-speed D/A's available, capable of converting at data rates up to 1 GHz.
Differential Nonlinearity (also DNL)
In an ideal D/A and A/D converter, any two adjacent digital codes should result in measured output (or input) values that are exactly one LSB apart. Any positive or negative deviation in the measured "step" from the ideal differences is called differential non-linearity. It's expressed in (sub) multiples of 1 LSB. DNL errors more negative than -1 LSB can result in a non-monotonic response in a D/A and missing codes in an A/D.
Digital Downconversion (also direct-IF-to-digital conversion)
This refers to a demodulation technique for sampling an intermediate frequency (IF) signal with a wide-bandwidth A/D whose sampling rate is equivalent to the local oscillator frequency (< IF frequency). In this super-Nyquist application, the A/D serves as the mixer stage and its digital output data is a beat frequency; the modulation data can be recovered with a DSP stage.
Digital Filtering
The process of smoothing, spectrally shaping, or removing noise from a signal has traditionally been accomplished with analog components. With the advent of high-speed DSP products, now filtering can effectively and economically be accomplished in the digital domain. Digital filters are basically mathematical functions that are performed on the digital data stream and their characteristics can be altered under software control, which adds to their overall flexibility. Finite Impulse Response (FIR) and Infinite Inpulse Response (IIR) are examples of digital filter functions.
Direct Digital Synthesis (DDS)
A process by which you can digitally generate a frequency-agile, highly-pure sinewave, or arbitrary waveform, from an accurate reference clock. The digital output waveform is typically tuned by a 32-bit digital word that allows sub-Hz frequency agility. The DDS's frequency output is normally reconstructed with a high-speed, high- performance D/A to generate an analog output signal. The ability to add internal functions such as phase modulation, amplitude modulation, digital filtering, and I&Q outputs, are making DDS devices attractive for digital communication applications. They serve in capacities such as modulators, local oscillators, and clock detect/recovery circuits.
Dither
The technique of adding controlled amounts of noise to a signal to improve overall system loop control, or to smear quantizing error in an A/D converter application.
Dynamic Range
The ratio of the maximum output signal to the smallest output signal that can be processed in a system, usually expressed logarithmically in dB. Dynamic range can be specified in terms of harmonic distortion, signal-to-noise ratio, spurious-free dynamic range, or other AC input-based performance criteria
FFT Fast Fourier Transform
A computationally efficient mathematical technique, which converts digital information from the time domain to the frequency domain for rapid spectral analysis. FFTs generally utilize a "time weighting" function to compensate for data records with a non-integer number of samples; some popular weighting functions are Hanning Window and 4-term Blackman-Harris.
Frequency Hopping System
Carrier frequency shifting in discrete increments in a pattern dictated by a code sequence. The transmitter jumps from frequency-to-frequency within some predetermined set: the order of frequency hops is determined by a code sequence, which, in turn, is detected and followed by the receiver.
Frequency Shift Keying
A modulation scheme that shifts between two frequencies to represent a "1" or "0" state of data transmission.
Glitch
A spike caused by the skew (difference in turn-on/turn-off time) of switches or logic. Glitches are a troublesome source of error in high-speed D/A converters, and they are most prevalent at the mid-scale switching location when all digital input bits are switching. Glitch energy is specified in picovolt-seconds, which describes the area under the voltage-time curve at its worst case occurence.
Group Delay
Distortion resulting from non-uniform speed of transmission of the various frequency components of a signal through a transmission medium, specifically the propagation delay of a lower frequency is different from that of a higher frequency. This creates a time-related "delay distortion" error.
Heterodyne
A process by which two signals are mixed for the purpose of cross-frequency translation. Integral Nonlinearity (INL) This term describes the absolute accuracy of a converter. It is the maximum deviation, at any point in the transfer function, of the converter's output from its ideal value.
I & Q
In-phase and Quadrature - A modulation technique whereby signal information is derived from a carrier frequency at its 0° and 90° phase angles.
Jitter
Unwanted variations in the frequency or phase of a digital or analog signal.
Mixer
Circuit block used to translate signals from one frequency to another.
Multipath Propagation
A transmission path anomaly that acts as a time-varying source of signal non-linearity. Multipath can distort or reduce a received signal to the point of unreliable reception. In television, multipath is manifested as image "ghosting".
Nyquist Theorum
This theorum says that if a continuous bandwidth-limited signal contains no frequency components higher than fC then the original signal can be recovered without distortion if it is sampled at a rate of at least 2 fC. This theorum applies to A/D converter applications as well as data transmission density over limited-bandwidth channels.
Orthogonal
Term used to signify that two signals (or signal attributes) are mutually transparent and non-interfering with each other. Frequency and amplitude modulation are orthogonal signal attributes.
Packet
A digital communications technique involving the transmission of short bursts of data in a protocol format that contain addressing, control, and error-checking information, along with the field information, in each transmission burst.
Phase Locked Loop (PLL)
A circuit containing a voltage-controlled oscillator whose output phase or frequency can be "steered" to keep it in sync with a reference source. A PLL circuit is generally used to lock onto and "up-convert" the frequency of a stable source.
Phase Noise
The amount of phase noise energy contained in a frequency carrier. Specified in dB/Hz, phase noise amplitude is usually characterized and plotted in 1 Hz increments, offset from the carrier.
Programmable Gain (PGA)
An amplifier with an analog- or digitally-controlled gain amplifier function. A PGA can be used in front of an A/D converter to effectively increase its dynamic range.
Pulse Code Modulation
A method of quantizing audio-range analog signals into a digital form for transmission in digital communications systems or for processing in DSP. Effectively the same as Analog-to-Digital conversion.
Pseudo-noise
Any group of code sequences that exhibit a noise-like characteristic.
Phase Shift Keying (PSK)
A digital modulation technique whereby the phase of a carrier frequency is shifted to represent a digital "1" or "0" state. In "Quadri-phase-shift keying" systems, the phase angle locations of 0°, ±90°, and 180° are used as reference points to represent sixteen possible digital states (24).
Quadrature Amplitude (QAM)
This communication scheme involves the modulation of a carrier by two different signals. One signal modulates the carrier (I) and the other signal modulates the carrier shifted by 90° (Q). The two modulated carriers are then summed and transmitted as a single I&Q modulated carrier. The receiver decodes the I&Q channels and demodulates them 90° apart. QAM lends itself to the transmission of data in a digital format by assigning discrete levels to the two signal inputs which creates a "constellation" of possible digital word combinations on the I&Q graph. The following illustrates a 16-QAM constellation (4 levels of input on the I channel/4 levels of input on the Q channel).
Signal to Noise Ratio (SNR)
SNR is the ratio of the RMS value of the measured output signal to the RMS sum of all other spectral components below the Nyquist frequency, excluding the first six harmonics and DC. The value is expressed in decibels.
Sin(X)/X
The output of a D/A converter is a series of quantized levels that represent an analog signal whose amplitude is determined by the sin (X)/X response. At higher output frequencies, a D/A converter application may require a sin(X)/X compensation filter to normalize its output amplitude.
Spread Spectrum
This communications technique has been used in secure military systems for a number of years and is now becoming popular in commercial systems. This format involves transmitting information, which has been multiplied by a pseudo-random noise (PN) sequence which essentially "spreads" it over a relatively wide frequency bandwidth. The receiver detects and uses the same PN sequence to "despread" the frequency bandwidth and decode the transmitted information. This communications technique allows greater signal density within a given transmission bandwidth and provides a high degree of signal encrytion and security in the process.
Spurious-free Dynamic (SFDR)
This refers to the range between the highest level of the fundamental Range frequency and the highest level of any spurious, or harmonically- related, signal. SFDR is expressed in dB.
Wavelet
A mathematical algorithm that is used to efficiently compress and decompress the phase & frequency information that is contained in a transmitted signal.
Wireless Telephony
The idea of replacing the conventional twisted pair telephone service to the home with a wireless RF connection. High-speed digital communications techniques would be utilized to allow enhanced telephone, multimedia, and data services to be transmitted over the airwaves to the home subscriber. In this scenario, the link to the information highway will be wireless.
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