Introduction to GSM Fiber Repeater RF Test (Part 3)

    In the previous issue, we mainly introduced the test items such as nominal maximum output power, out-of-band gain per carrier frequency, out-of-band spurious emission, vector amplitude error, uplink noise factor, etc. In this issue, we will give a detailed description of all other RF indicators such as GSM repeater delay test, in-band intermodulation value test and uplink noise floor suppression function test.

1. GSM repeater delay test

    Index requirements and standards:

    The delay of the entire working frequency band is required to be ≤23us, and the test reference standard is YD/T1337-2005 "900/1800MHz TDMA digital cellular mobile communication network repeater technical requirements and test methods"/6.10 transmission delay.

    Test connection diagram:

Downlink: Network analyzer (port 1) -- connected to isolator -- connected to near-end machine (input) -- optical fiber -- remote machine
      remote machine (output) -- connected to attenuator (40-50dB) -- connected to network analyzer (port 2)

Uplink: Network analyzer (port 2) -- connected to -- isolator -- connected to -- remote unit (input) -- optical fiber -- near-end unit
      Near-end unit (input) -- connected to -- attenuator (appropriate attenuation) -- connected to -- network analyzer (port 1)

Note: The function of the isolator is to protect the signal source and prevent the DUT from self-excitation and damage to the instrument. The function of the attenuator is to protect the network analyzer and prevent the input signal power of the network analyzer from being too large.

    Test steps:

    1. Make the connections as shown in the figure;
    2. Turn off ALC and adjust the gain of the repeater to the maximum gain; use a network analyzer to test the transmission delay of the repeater;
    3. Network analyzer settings: start/stop frequency, such as: test GSM900 such as 800Mhz---900Mhz, test GSM1800 such as 1700Mhz-2000Mhz;
    4. Set the number of TRACE points as large as possible, for example: 10000 Select the delay item in the format item;
    5. Select the S21 channel to test the downlink, and select the S12 channel to test the uplink;
    6. Read the delay value from the network analyzer.

2. In-band intermodulation value test

    Index requirements and standards:

    The indicator requires that the difference between the peak signal and the intermodulation product is ≤-36dBc. The test reference standard is YD/T1337-2005 "900/1800MHz TDMA Digital Cellular Mobile Communication Network Repeater Technical Requirements and Test Methods"/6.12 Intermodulation Products
Test connection diagram:

    Test steps:

    1. Connect the test system as shown in the figure;
    2. Set the gain of the repeater to the maximum value specified by the manufacturer;
    3. Add the two-tone signal of the corresponding frequency point to make the device start to control 10dB deeply;
    4. Set the signal source two-tone signal, press Mode〉Two Tone to on, and set Freq Separation to 600kHz.
    5. The two signals deviate 300KHz from the center frequency respectively, and the signal power starts to control 10dB.
    6. Adjust the center frequency of the spectrum analyzer to the center frequency of the repeater under test, adjust the sweep width to see the intermodulation products on both sides,
and input the total loss of the power attenuator and the connecting cable as the bias input to the spectrum analyzer;
    7. Adjust the spectrum, set SPAN to 2M, and read the difference between the peak signal and the intermodulation products on both sides.

3. Uplink noise suppression function test

    Functional description:

    The GSM network wireless radio remote system has the function of eliminating noise interference. Usually in star and daisy chain networking, the uplink noise floor output by the near-end machine will increase linearly with the increase in the number of remote machines. Therefore, the system receiving sensitivity is affected by the number of remote machines. If the noise interference elimination technology is used, this problem can be solved. The principle of eliminating noise interference is: using software radio frequency selection method, it can distinguish between noise and useful signals, amplify useful signals, and suppress noise.

    Test connection diagram:

 

    Test Methods:

    Connect a remote device and a local device according to the above pattern. Assuming the GSM900 frequency band is tested, input a CW signal with a strength of -50dBm and a frequency of 895MHz at any remote device's RF port, adjust the uplink gain to 45dB, and test the RF output signal of the local device port; do not perform any operation on the device, turn off the CW signal, and test the uplink noise level of the 885~909MHz frequency band at the local device's RF port.

    Test result requirements:

    When the CW signal is turned on, the DUT port RF output signal: f=895MHz CW signal, the strength is -5~ -4dBm; when the CW signal is turned off, the DUT RF port tests the 885~909MHz frequency band ≤-87dBm/200kHz. 200kHz refers to the RBW resolution bandwidth setting in the spectrum analyzer. The principle of testing the uplink noise suppression of the GSM1800 frequency band is the same as the test method of GSM900, and the RF port tests the 1710~1785MHz. Frequency band ≤-87dBm/200kHz. Note that in this mode, the uplink noise received by the base station is -132dBm/200kHz after the 45dB coupler.

    Through three issues, the common RF tests of fiber optic repeater stations have been introduced.