Here is a list of conversion formulas and miscellaneous technical information. Please feel free to bookmark this page for your quick reference. It will be continually updated. Any suggestions will be appreciated.
HOW CHARACTERISTICS ARE MEASURED FOR Intercon 1 CABLES (sample):
- Characteristic Impedance TDR Single End - This test measures impedance of a transmission line using Time Domain Reflections. The equipment commonly used is a Tektronics 11801A Digital Oscilloscope. A signal is sent down the line, the reflected signal coefficient is recorded at a point 4.7 Ns down the length of the cable. The measurement is the reflection coefficient and is used to calibrate the impedance by use of the following formula.
|
z =
|
1 + p
1 - p
|
Where
|
Z = Impedance
R = 50 ohms
p = Reflection Coefficient |
- Capacitance Single End - Using a HP 4262A Digital LCR Meter (or similar) single end capacitance is measured. Readings come directly from the meter.
- Conductor Resistance - Using the HP 4262A Digital LCR Meter (or similar), the resistance of a conductor is measured in a 1,000 ft cable. Max allowed based on cable requirements.
- Insulation Resistance - This measures the insulation resistivity using a Hypotronics HM3A Megometer (or similar). The calculation is as follows:
|
A x L
B
|
Where |
B = unit in feet
L = sample length
A = Deflection from meter x multiplier |
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Impedance
Impedance is the total opposition in a system to alternating current. It is important to match a cableÕs impedance to the instrument or device being used. Due to low level power in electronics or communication devices, unmatched impedanceÕs will cause power or signal strength loses. Proper impedance matching allows for optimum energy transfer conditions. When testing the cables for impedance there will usually be a specific impedance value to look for with a tolerance of + or - a certain range in ohms. The value is determined from what the cable applications will be as well as what kind of transmissions into the cable are expected. Normally, Intercon 1 cable will have a tolerance of + / - 4 ohms. For some cables the tolerance is + / - 3 ohms.
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Capacitance is a charge storing property of an element. In our case its the charge storing property of cables. Capacitance can effect the shape of transmitted signal waveforms. If the capacitance changes enough the results can be in the form of phase lagging, decreased amplitude, and/or signal filtering. Again when tested there will usually be a specific value with a tolerance range for the requirements which will be determined by the cable application.
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Conductor Resistance
Conductor Resistance is simply the resistance to current flow in the conductor. The higher the resistance the less conductivity in the wire. Maximum resistance is usually in ohms/1,000 ft.
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Insulation Resistance
Insulation Resistance is simply the resistance to current flow in the conductor. The higher the resistance the less conductivity in the wire. Maximum resistance is usually in ohms/1,000 ft.
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GENERAL COAXIAL CABLE DATA
A coaxial cable consists of a center conductor surrounded by an outer, tubular conductor, a dielectric that separates the conductors, and a jacket that protects the parts. The inner conductor typically is a solid or stranded wire while the outer conductor generally is a single - or double - braided shield or tubing.
Similar to shielded wire in construction, the essential difference between coaxial cabling and shielded wire is that coaxial cable transmits a signal with as little loss as possible, with little or no distortion, and minimum radiation. In shielded wire, the shield is merely used to confine the signal or to screen it from external excitation: It does not have the controlled characteristics of coaxial cable.
The four basic parameters of coaxial cable are:
Impedance
Capacitance
Attenuation
Velocity of propagation
All are interrelated and depend on the properties of the dielectric and the cable dimensions. Additionally, when specifying coaxial cable, the voltage standing wave ratio (VSWR) and the dielectric constant must also be considered.
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General Coax Connector Data
50 ohm vs. 75 ohm
Historically 50 ohm BNC connectors have been used with 75 ohm cable for analog video equipment with little distortion effect on the signal at frequencies below 300 MHz. However, digital signals in video applications have necessitated the usage of 75 ohm connectors with 75 ohm cable. Mismatched connectors cause attenuation of the digital signal resulting in slower rise time of square waves. This distortion of the signal can cause transmission errors.
Also, impedance mismatched connections cause reflections of the signal returning to the source known as return loss. Often one mismatched connection will not have a noticeable effect on system performance, but multiple mismatched connections in the link between the source and destination have a cumulative effect causing possible distortion of the transmitted signal.
As a standard practice, Intercon 1 uses only 75 ohm connectors with gold plated terminals (source Connex or Tyco) with 75 ohm cable.
BNC Background: In the early 1940's, it is generally agreed that a group of three individuals (Bayonet, Neill and Councelman) developed a new coaxial cable connector with the express purpose of providing a secure interface and an easy to use locking mechanism. These connectors were used to link equipment incorporating an electronic vacuum tube platform. The new connector was referred to as the BNC recognizing the origin of its development.
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