Introduction to GSM Fiber Optic Repeater RF Test (I)

     The biggest difference between fiber optic repeaters and wireless repeaters is the transmission method of the signal from the donor base station. Wireless repeaters amplify the wireless signals transmitted in space, thereby expanding the coverage of the base station. Fiber optic repeaters transmit through optical fibers, using optical signal receivers and converters to connect remote areas. Compared with wireless repeaters, fiber optic repeaters are relatively expensive and require the laying of optical fibers, which is more difficult to design and construct. However, compared with wireless repeaters, fiber optic repeaters have incomparable advantages. Fiber optic repeaters are mainly used in the following situations where there are no conditions for installing wireless repeaters:
      1. The coverage area is far away from the base station, and the base station wireless coverage signal cannot be obtained in this area.
      2. The wireless environment in the coverage target area is very bad, and an antenna array is needed to cover the area. When it is impossible to lay a thick feeder in this area, a fiber optic repeater is used to lay optical fiber transmission signals for easy design and construction. Moore Lab recently organized relevant personnel to study the testing of fiber optic repeaters, and also shared the testing experience with readers as soon as possible. The tests of optical fiber repeater mainly include the following contents:
      working frequency band, system gain, number of carrier channels, adjustable gain range, in-band fluctuation, nominal maximum output power, out-of-band gain per carrier, spurious emission, uplink noise coefficient, vector amplitude error, transmission delay, input standing wave ratio, in-band intermodulation, out-of-band intermodulation, uplink noise floor suppression function, network management function, environmental test, power adaptability, safety test, electromagnetic compatibility test. This issue starts with the introduction of RF test of optical fiber repeater. 1: Working frequency band
       index requirements and standards:
       Uplink: 885～909/1710～1785MHz; Downlink: 930～954/1805～1880MHzTest
       connection diagram: Test steps: 900MHz: Uplink:       1. Connect the instrument and equipment according to the diagram;       2. Set the channel numbers of 885MHz, 890 MHz, 897MHz, and 909MHz on the equipment in sequence;       3. Add 885MHz, 890 MHz, 897MHz, and 909MHz signals from the signal source in sequence, and observe whether there is signal output on the spectrum analyzer. Downlink:       1. Connect the instrument and equipment according to the diagram;       2. Set the channel numbers of 930MHz, 938MHz, 946MHz, and 954MHz on the equipment in sequence;       3. Add 930MHz, 938MHz, 946MHz, and 954MHz signals from the signal source in sequence, and observe whether there is signal output on the spectrum analyzer. 1800MHz: Uplink:       1. Connect the instrument and equipment according to the diagram;       2. Set the channel numbers of 1710MHz, 1725MHz, 1740MHz, 1755MHz, 1770MHz, and 1785MHz on the equipment in sequence;        3. Add 1710MHz, 1725MHz, 1740MHz, 1755MHz, 1770MHz, and 1785MHz signals from the signal source in sequence, and observe whether there is signal output on the spectrum analyzer. Downlink:       1. Connect the instrument and equipment according to the diagram;       2. Set the channel numbers of 1805MHz, 1820MHz, 1835MHz, 1850MHz, 1865MHz, and 1880MHz on the equipment in sequence;       Add 1805MHz, 1820MHz, 1835MHz, 1850MHz, 1865MHz, and 1880MHz signals from the signal source in sequence, and observe whether there is signal output on the spectrum analyzer. 2: System gain       index requirements and standards:           Maximum gain 50dB; (according to the manufacturer)           Maximum gain error does not exceed ±2dBTest connection diagram:  Test steps: 1. Connect the test system as shown in the figure; 2. Connect the signal source output to the input port of the device under test through a cable, and then input the total loss value of the power attenuator and the connecting cable into the spectrum analyzer as the bias input; 3. Turn off the reverse link (measure the forward output power) or turn off the forward link (measure the reverse output power); 4. Set the GSM signal source to the center frequency within the operating frequency range of the repeater or the center frequency of the assigned channel, and adjust the gain of the repeater under test to the maximum; 5. Adjust the signal source output level until the ALC control point is reached. The power per channel directly displayed on the spectrum analyzer should be within the tolerance range of the maximum output power declared by the manufacturer of the repeater under test; 6. Record the output power level Lout (dBm) and input level (GSM signal source output level minus the loss value of the connecting cable) Lin (dBm) of the repeater under test. 3: Carrier frequency channel number     index requirements and standards: The maximum number of carrier frequency channels that the system can support; Requirements: At least 24 carrier frequencies can be supported (or defined by the manufacturer). Test connection diagram:   Test steps:       1. Connect the test system as shown in the figure; (You can also use a power meter to replace the spectrum analyzer for testing)       2. Turn off the forward link;       3. Set the GSM signal source to the center frequency within the operating frequency range of the repeater, and make it generate a CW signal;       4. Set the device channel numbers in sequence at intervals of 600kHz, and turn on all channel switches;       5. Adjust the level of the GSM signal source until the output power of the repeater is the maximum output power -5dB;       6. Add a sweep signal, sweep from 885MHz to 909MHz, and set the spectrum analyzer to MaxHold mode until the spectrum is stable;       7. Observe the spectrum to see the number of carrier frequencies. 4: Gain adjustable range Indicator requirements and standards:       0~30dB continuously adjustable, gain adjustment step ≤1dB (or specified by the manufacturer) Test connection diagram: Test steps:        1. Connect the test system as shown in the figure;        2. Adjust the gain of the tested repeater to the minimum, and read the output power level Loutmin of the tested repeater from the spectrum analyzer;        3. Adjust the gain of the tested repeater to the maximum, and read the output power level Loutmax of the tested repeater from the spectrum analyzer;        4. Gain adjustment range ΔG=Loutmax-Loutmin (dB); 5: In-band fluctuation Indicator requirements and standards:











































 






         ≤3dB (peak-to-peak)
test connection diagram: Test steps:        1. Connect the test system as shown in the figure;        2. Set the repeater gain to the maximum value specified by the manufacturer;        3. Set the start and end frequencies of the signal source to be wider than the working frequency bandwidth of the repeater under test, and adjust the level to 5dB lower than the ALC level;        4. Test the output power levels in the high, medium and low channels respectively, and the difference between the maximum power level and the minimum power level is regarded as the in-band fluctuation. The above is a brief introduction to the RF test of GSM fiber repeaters. In the next issue, we will continue to introduce the test methods of the nominal maximum output power, out-of-band gain per carrier frequency, spurious emission, uplink noise coefficient, and vector amplitude error.