DAS Fundamentals
What is a BDA? What does it do?
Bi-Directional Amplifier. RF Amplifier which selects the frequencies that are to be amplified in the downlink and uplink paths (increase RF signal strength in both directions). The FCC calls these "signal boosters." There are Class A and Class B BDAs. Class A is channelized, Class B is not.
What is a decibel? Why do we use them, what is an example?
Decibel is a practical unit of power that compresses large ranges of numbers into smaller, more manageable numbers. We use decibels as a way to shorten the length of the value -90dBm instead of .0000000001 milliwatt and 37dBm instead of 5000 milliwatt or 5 watts. Test results need to be stronger than -95dBm or 0.00000000003mW.
DAS
Distributed Antenna System
What are the components in passive DAS?
Donor antenna, coaxial cable, BDA, splitters/tappers/couplers, server antennas
What is Automatic Gain Control? AGC.
Most RF Boosters employ an AGC circuit that automatically limits the RF output power to the "rated" power of the booster. The rated output power is the maximum output power that ensures that all transmitted spurious signals and intermodulation products will be less than the FCC limit. This level is typically approximately 10 dB lower than the output 1 dB compression point of the booster.
PIM
Passive interface modulation. (PIM) is a form of intermodulation distortion that occurs in passive components such as antennas, cables, connectors, or duplexers with two or more high-power input signals.
What are splitters and tappers? What do they do?
Splitters take the RF and split it into two streams. This loses at least 3dB of power. Tappers are used to drop the power as to not over power an antenna or section or a building. Normal loss is .1-.4dB of power.
What is a spectrum analyzer used for?
Testing the signal strength of RF in the immediate area.
What is Oscillation? What is the isolation needed in a DAS?
The DAS will go into feedback oscillation similar to holding a microphone too close to the PA speaker. The rule of thumb is to ensure the total isolation is at least G + 15 dB where G is the gain of the RF booster.
BDA Gain.
The amount of dB gain set in the BDA. This can be set differently for uplink and downlink. Most BDAs are either 90 or 95dBm.
What is the UHF frequency range? What are it's qualities?
Ultra High Frequency. 440MHz through 480MHz. Large wave length, good distance traveled and material penetration.
What is the VHF frequency range? What are it's qualities?
Verh High Frequency. 156MHz through 174MHz. Large wave length, best distance traveled and material penetration.
What is a link budget?
A link budget is accounting of all of the gains and losses from the transmitter, through the medium (free space, cable, waveguide, fiber, etc.) to the receiver.
What is the difference between passive and active DAS?
Active DAS uses a RF over fiber technology to overcome loss due to long coax cable runs.
