Anti-alias Filter with respect to digital bit synchronizers is that analog function responsible for eliminating noise signals greater than ½ the analog-to-digital sampling rate. Anti-alias filters can also affect the data detection performance of a bit sync, particularly at symbol rates approaching the cut-off frequency.

BER (Bit Error Rate) Performance is a fractional statistical measurement in bit errors per bit relative to Eb/No. Graphically presented with BER on the Y-axis with a log scale and Eb/No on the X-axis in dB. Random errors from gausian white noise can create a maximum BER of 0.5.

BERT (Bit Error Rate Tester)is any device which can simultaneously transmit and receive a digital signal and derive a BER measurement by comparing the received with the transmitted signal. More sophisticated BERTs can add gausian noise and can include many other features helpful in testing bit syncs.

Biphase-L, -M, -S-Biphase –Level, -Mark, -Space are a set of PCM codes where there is at least one transition, but no more than two transitions in a bit period. In biphase-L a ‘1’ is represented by a falling edge in the middle of the bit period and a ‘0’ is represented by a rising edge. Unlike NRZ signals, biphase signals have no significant spectrum near DC and so are desirable when using media where DC response is not possible.

Bit Synchronizer is a device that establishes a series of clock pulses which are synchronous to an incoming signal. It then identifies each bit in the signal. The desired condition of a bit sync (i.e. lock) when the output data and clock signals match those of the transmitted signal. The bit rate of the output will average to the same as the transmitted signal and the phase will remain within 180 degrees.

Bit Acquisition is a statistical measurement of the number of bit periods between the beginning of a transmission and the last bit slip (if any) at the bit sync output.

Bit Slip is the occurrence of a 360 degree or more phase change at the output relative to the input.

Clock phase is a measurement of the phase relationship between a bit sync’s data and clock outputs. Usually measured in degrees from a data transition to the rising edge of the clock where 360 degrees is 1 bit period.

Commutator is a device used to accomplish time-division multiplexing (TDM) by repetitive sequential switching.

Convolutional Encoding/Decoding is a technique using feedback shift registers to generate redundancy in a data stream for forward error correction. The original data is encoded into symbols at a multiplied rate where the higher the multiple, the better the error correction capability.

CRC is the acronym for cyclic redundancy check. A procedure used in checking for errors in data transmission. CRC error checking uses a complex polynomial to generate a number based on the data transmitted. The sending device performs the calculation before transmission and sends its result to the receiving device. The receiving device repeats the same calculation after transmission. If both devices obtain the same result, it is assumed that the transmission was error-free. The procedure is known as a redundancy check because each transmission includes not only data but extra (redundant) error-checking values.

Data Detection is the function in a digital receiver where data symbols are identified.

Decommutator is a unit that reverses the commutation process; separation of information in a commutated data stream into as many independent information channels as were originally commutated. A device that separates (or demultiplexes) commutated signals into its constituent measurands and data.

Derandomizer is a function that decodes randomized signals. See Randomizer.

Derived Parameter is a parameter that is not part an original telemetry stream’s measurands, but is derived by processing the values of multiple measurands.

Differential-Data is extracted from the level of difference between two signals. Also called RS-422. Compare to Single-Ended.

Display is the visual output device of a computer, which is commonly a flat panel or CRT-based video display.

Display Object is the basic real-time dynamic unit within a window. For example: strip charts, bar charts, vertical meters, round gauges, cross plots, annunciators, tabular, orientation, and bit maps.

DM-M,-S-Delayed (or Miller) Modulation, -Mark, -Space are a set of PCM codes where there is at least one transition in each 1 ½ bit period and no more than one transition in each 1 bit period.

Eb/No-Energy per bit to noise density ratio is a measure of signal-to-noise ratio of a digital communications channel. Usually measured in dB. (0 dB means the signal and noise power levels are equal and a 3 dB increment doubles the signal relative to the noise.)

Encoder is an electronic version of a commutator.

Engineering Units (EU) are units of data measurement (e.g., degrees, Celsius, pounds, grams).

Filter-sample is a type of Matched-Filter implementation that takes into account the effects of past and/or future symbol levels in a band-limited channel.

Flash RAM is a form of non-volatile read/write memory.

Flywheel is the ability to generate a clock even without any input signal transitions. See Retention.

Forward Error Correction (FEC) is any technique used to transmit redundancy in a digital signal for the purpose of correcting random bit errors at the receiver without the need for a reverse data link.

FPGA-Field Programmable Gate Array is an integrated circuit where the functions of, and interconnections between, gates are programmable.

Frame in time-division multiplexing is one complete commutator resolution, including a single synchronizing signal or code.

Frame Synchronization Code is a unique 3- to 33-bit code, coded pulse, or interval that marks the end of a commutation frame period.

Frame Synchronizer is hardware that recognizes the unique signal that indicates the beginning of a frame of data. A typical frame synchronizer "searches" for the code, "checks" the recurrence of the code in the same position for several frame periods, and then "locks" on the code.

Input impedance is the load placed on the device driving an input. Ideally the impedance should be "matched" (the same as) the cable over which it is driven to prevent signal reflections that can degrade BER performance.

Integrate-Dump is a type of Matched-Filter implementation where signal energy is accumulated during a bit period. This technique matches "square wave" signals and is best used in non-band-limited channels.

IRIG for Inter-Range Instrumentation Group standard for telemetry on U.S. Government test ranges.

Jitter-Phase variation is usually measured statistically as a RMS deviation from a center value in degrees or radians. Usually caused in the input signal by noise but can be caused by encoders, modulators and channel distortion. Jitter is reduced by a bit sync depending on its loop bandwidth.

Loop Bandwidth is a characteristic of a phase locked loop (PLL) that limits the spectral response of a PLL. Noise at the input is generally translated to phase jitter at the output. By limiting the loop bandwidth, high frequency noise can be rejected by the bit sync, reducing the phase jitter. By increasing loop bandwidth, a bit sync is better able to acquire and track a time-varying PCM input bit rate. Loop bandwidth for PCM signals is usually expressed as a fraction of bit rate or transition rate.

Major Frame in telemetry formats, the time period during which all data (excluding sub-sub frame measurands) of a multiplex is sampled at least once. Includes one or more minor frames. Major Frame length is determined as (N) (Z) words. Where: N = no words per minor (prime) frame and Z = The number of words in the longest sub-multiple frame.

Matched Filter is the part of an ideal data detector implementation in which the receiver is looking for precisely the symbol waveform that was expected. Ideally taking into account the transmitted waveform and any predictable channel distortion. Also called correlation or auto-correlation.

Measurands are the physical or electrical quantity, property or condition which is measured. The term "measurand" is preferred over prime parameter to be measured.

Minor Frame is the period between frame synchronization words that includes one complete cycle of a commutator having the highest rate.

Modulation is the process of impressing information on a carrier for transmission. Various types include Amplitude Modulation (AM), Phase Modulation (PM), and Frequency Modulation (FM).

Multiplexer is multiple input digital device that can select one of a number of inputs and pass the logic level of that input to the output. Multiplexer size is normally defined by the number of bits of a single parallel input plus the ratio of the inputs to the output.

NRZ-L, -M,- S-Non-return-to-zero-Level, -Mark, -Space are a set of PCM codes where there is either zero or one transitions in a bit period. In NRZ-L a high level represents a ‘1’ and a low level represents a ‘0’. In NRZ-M a ‘1’ is represented by a transition and a ‘0’ by no transition in the bit period. NRZ-S is the inverse of NRZ-M. Mark and Space codes are used when a polarity neutral signal is desired, for example when transmitting encrypted data.

PCM (Pulse Code Modulation) is a class of digital baseband signals transformed to pulse waveforms. The timing and direction of level transitions in the waveform contain the digital information.

Phase Detector is the part of a PLL that measures the difference between the time of input transitions with the expected time.

Phase Locked Loop is a negative phase feedback technique for extracting a synchronous clock from an input signal. Implementations range from purely analog to various combinations of analog with digital and software. Second order loops are generally used in PCM bit syncs because of their strong retention and stability characteristics.

Processed Parameter is the resultant of the algorithmic manipulation of a single measurand (e.g., EU)

Pseudo-noise (PN) is usually meant to be a sequence of 1’s and 0’s generated by a feedback shift register and used to represent something resembling a random pattern. PN patterns are used in bit error testing in data communication channels because they simulate random data, and, are easy to generate at the transmitter and at the receiver for bit comparison. PN codes are generally identified by the length of the shift register. The larger the code the longer the sequence of non-repeating bits (2ⁿ-1).

PRN-Pseudo-random noise or number. See PN.

Randomization is a technique in which a feedback shift register adds a PN sequence to the original data. This is used to add transitions to long sequences of 1’s or 0’s in the original data. The receiver subtracts out the PN sequence using a derandomizer, thereby recovering the original data. Randomized codes are generally identified by the length of the shift register. The IRIG-106 standard of 15 bit randomized NRZ-L code is usually employed.

Real-Time is the notion of completing a computing task before the next task arrives as in algorithm processing

Retention-(Bit rate retention) is a characteristic of a bit sync in which after acquiring an NRZ signal, the bit sync will correctly time bits during long sequences of no transitions without any bit slips. Usually measured by a statistical count of the number of bits of no transitions before a bit slip occurs within a otherwise random sequence transmitted every n bits.

Single-ended-A signal requiring only one wire for transmission. Referenced to ground.

Soft bit decision-An output of a matched filter that measures the relative (to other near-by bits) signal strength of a particular bit. Used as inputs to a convolutional and PCM decoder functions.

Subcommutation is commutation of a number of channels with the output applied to an individual channel of the primary commutator; subcommutation is synchronous if its rate is a submultiple of that of the primary commutator. Unique identification must be provided for the subcommutation frame pulse.

Sub-Frame A multiplex generated at a slower rate than a frame, and input to the frame through one of the channels.

Sub-Sub-Frame A multiplex generated at a slower rate than a frame, and input to the frame through one of the channels.

Supercommutation(1) Commutation at a higher rate than once per commutator cycle. Accomplished by connecting a single data input source to equally spaced contracts of the commutator (cross-patching). Corresponding cross-patching is required at the decommutator. (2) The technique of "strapping" commutator inputs to the same measurement point, such that it is sampled two or more times per minor frame.

Symbol-The shortest unit of transmission in a digital channel consisting of one of n possible waveforms. There may be more than one symbol per bit as in a convolutional code, or more than one bit per symbol as in QPSK.

Sync threshold is the minimum Eb/No at which a bit sync will acquire the input signal.

Tape output normally the PCM output of a bit sync that tracks the PCM input but optionally with a different code. The function generating a tape output is sometimes called a "code converter." Normally intended to be used to record a better quality signal on an instrumentation tape.

Telemetry is the science of measuring quantities, transmitting the results to a distant station, and interpreting, indicating, and/or recording the quantities measured.

Tracking is a characteristic of a bit sync where the output bit rate varies with (follows) the input bit rate. Track range is a measurement of tracking and defines the range in percent of bit rate from a nominal value in which tracking will be maintained without bit slips.

Transition Density is the statistical rate at which the signal varies from high to low and low to high during a specified number of bits. In NRZ-L, alternating 1’s and 0’s have a transition density of 100%, random data has a transition density of 50% and constant 1’s or 0’s have a transition density of 0%.

Viterbi Decoder-An efficient algorithm for decoding convolutionally encoded FEC signals. Developed by Andrew Viterbi.

View (see window)

Window is a portion of (or the entire) display {hot link} that contains its own document or display objects in an applications or graphical interface. The display can contain multiple windows either by stacking (only the top one is entirely visible) or tiling (all are visible) or a combination of both.

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