Refers to any reduction in the strength of a signal (loss). It is usually measured in decibels (dB) which is a logarithmic term that ratios the output voltage or power as a function of input voltage or power. As an example, 3dB loss is equivalent to 30% loss of voltage and 6dB loss is equivalent to 50% loss of voltage. In the case of power, 1.5dB loss is equivalent to 30% loss of power and 3dB loss is equivalent to 50% loss of power. Typically, cables are rated in dB loss per 100 feet.
AP = 10log (POUT /PIN ), where AP = power attenuation
AV = 20log (VOUT /VIN ), where AV = voltage attenuation
The bit error rate (BER) is the percentage of bits that have errors relative to the total number of bits received in a transmission, usually expressed as ten to a negative power. For example, a transmission might have a BER of 1 x 10-6, meaning that, out of 1,000,000 bits transmitted, one bit was in error. The BER is an indication of how often a packet or other data unit has to be retransmitted because of an error. A high BER may indicate that a slower data rate would actually improve overall transmission time for a given amount of transmitted data since the BER might be reduced, lowering the number of packets that had to be resent. A BERT (bit error rate test or tester) is a procedure or device that measures the BER for a given transmission.
Refers to the ability of a device to store energy in the form of an electrostatic field. In its simplest form, a capacitor is a pair of parallel plates spaced apart with a dielectric material between them. Coax cables have a certain capacitance per foot. The dielectric materials in a cable affect the capacitance of the cable. If two cables are identical in geometry, the cable with the higher dielectric constant insulator will have the higher capacitance per foot and, thus, the lower impedance. The standard unit of capacitance is the farad, abbreviated F. This is a large unit; more common units are the microfarad, abbreviated µF (1 µF = 10-6 F) and the picofarad, abbreviated pF (1 pF = 10-12 F). The reactance of a capacitor decreases as frequency increases. XC = 1/(2πfC), where XC = capacitive reactance f=frequency and C=capacitance
Uses two conductors to carry the signal, and one shield to provide the return path. The signal conductors are driven by opposite polarity signals. Typical examples include twinax, shielded twisted pair, and ribbon cable.
The name given to the “picture” or pattern produced when multiple traces are recorded on an oscilloscope by sending multiple 1’s and 0’s down a cable assembly and “looking” at the far end of the cable. The 1’s and 0’s are randomly generated so that many different sequences of highs and lows are examined by the eye pattern.
This test is an excellent way to look at how the cable assembly will perform in the “real world”. It encompasses attenuation and risetime degradation (reduction in amplitude and risetime as a function of frequency components of the pulse and bit pattern sequence), dispersion (jitter due to differing speeds of signal transmission as a function of frequency) and within-pair skew (offset of differential signal due to differing electrical lengths of the wires within the differential pair). The “eye” of the input signal is compared to the “eye” of the signal at the far end . The height and width of the “eye” (referred to as the “eye opening”) show how the signal has been distorted and can be used to analyze if the user’s receiving circuit can reliably distinguish the logic 1’s and 0’s emerging from the cable assembly. Ideally, the eye pattern would be a square that is the same height and width as the input signal. The X-axis plots time and the Y-axis plots amplitude. Referring to the illustration, 1,2,.,8 are “snapshots” of the signal at equal lengths of time apart (X number of clock cycles) and these “snapshots” are superimposed one on top of another yielding the eye-pattern in the lower left of the illustration. (Illustration courtesy of Agilent Technologies and Electronic Design)
Hertz refers to frequency at which a signal repeats itself; Mbps refers to how many bits per second are transferred; MBps refers to how many bytes per second are transferred. Generally a byte is 8 or 16 bits long. In general, MHz equals one-half of Mbps, (Therefore 400 MHz = 800 Mbps. If a byte is 8 bits long, then 800 Mbps = 100 MBps.)
A specific transmission line on a PCB where the signal trace is on an outside surface of the PCB and is spaced above a ground plane by the dielectric material, such as FR-4.
Refers to the creation and simulation of electronic circuits on a computer. SPICE is the most common circuit simulator in use. IBIS is a newer version that semiconductor manufacturers prefer to use because it protects their proprietary designs. The models can be created using various methods including actual measurements and computer programs. A model of a connector may contain the inductance, capacitance, impedance and time delay of the connector. These numbers can be used in SPICE (and other) circuit simulation models to simulate how the connector will perform in the user’s circuit. A “lumped” model uses R, L, and C components to model the behavior of the device. A lumped model is usually accurate then the device being modeled is much “shorter” than the rise time of the signal. A “distributed” model is a higher order approximation that can more accurately simulate the real world device.
Refers to the driven end of a transmission line.
The speed or rate with which a signal travels between the input and output of a transmission line, usually expressed in nanoseconds per foot. The materials surrounding the conductor affect this speed. Space or vacuum is the fastest medium (1 nsec/ft), whereas air is 1.0167 nsec/ft. Teflon is slower than air (1.45 nsec/ft), and FR-4 is slower than Teflon (2.12 nsec/ft). See also electrical length.
PD (nsec/ft) = (√K)/Co , where PD = propagation delay
K = dielectric constant and Co = speed of light » 1 ft/nsec
Refers to the increase in the risetime and falltime of a pulse as it travels through a transmission line. This means that a pulse with a 100 psec risetime entering a cable or connector will exit that cable or connector with a risetime longer than 100 psec.
Amplitude – indicates voltage (or current) levels (“height” of signal)
Pulse Width – length of time that digital signal is high (or low) measured at 50% of amplitude
Risetime – length of time for digital signal to go from low to high; usually defined as the Period of time that it takes to go 10-90% or 20-80% of the high level
Rule of thumb : 10-90% risetime » 1.33 times the 20-80% risetime
Frequency – number of times per second that a repeating signal (i.e. sine wave, clock) repeats itself; usually expressed as KHz (thousands of cycles per second), MHz (millions of cycles per second) or GHz (billions of cycles per second)
Period – length of time before a repeating signal (i.e., sine wave, clock) repeats itself. Period=1/frequency
Data Transfer Rate – see bit rate
Wavelength – the physical length of a period of a signal. In air, the formula is l = c/f, where l = wavelength, f = frequency and c = the speed of light in air. In another dielectric, the wavelength scales as 1/ Ö K, where K = dielectric constant. Therefore, in any dielectric, l = c/(f x Ö K). Wavelength in air at 100 MHz = 120 inches; at 1 GHz = 12 inches; at 5 GHz = 2.4 inches
Also called single mode, uses two conductors, one carries the signal, and one provides the return path. Typical examples: coax, twin lead, twisted pair, ribbon cable
A specific transmission line on a PCB where the signal trace is buried within the PCB and is spaced above and below a ground plane by the dielectric material, such as FR-4.
The actual speed at which the signal travels from one point to another along a conductor. The materials surrounding the conductor affect this speed. For example, in air, the velocity of propagation is 300 million meters per second and in FR-4 printed circuit boards it is about half that speed. It is usually expressed as a percentage of the speed of light. Air is the fastest medium (100%), Teflon is slower than air (70%), and FR-4 is slower than Teflon (47%). Typically connectors are faster than printed circuit boards; those connectors with the most air being fastest. Velocity of propagation affects skew. See dielectric constant.
VP (%) = 1/(√K) x 100
where VP = velocity of propagation and K = dielectric constant