AFC Calibration process AFC calibration process
Because GSM mobile phone uses time division multiple access (TDMA) technology, with different times the instantaneous gap, to differentiate the user, therefore the handset and the system maintains the time synchronization to appear very important. If the phone clock is out of sync with the system clock, it will cause the phone to not communicate properly with the system. In the GSM system, there is a public broadcast control channel (BCCH), which contains frequency correction information and synchronization information. When the phone is turned on, it will scan the channel under the control of logic circuit, and obtain the synchronization and frequency correction information, for example, the mobile phone system detects the clock and the system is out of sync, and the logic circuit of the mobile phone will adjust the control voltage of the oscillation circuit. It changes the inverse bias voltage at both ends of the VCO in the 13/26mhz oscillation circuit, thus making the output frequency of the VCO circuit change, thus ensuring that the handset is synchronized with the system. The handset's AFC (Auto Frequency control) calibration is divided into two types: one type is crystal and the other is VC-TCXO.
⑴. A type of crystal crystal refers to crystals
Calibration process of Crystal AFC;
①. Calibrating the CAP ID first
With the AFC DAC unchanged, a CAP ID is selected between 0-63 (127) and the standard is the minimum frequency offset of its corresponding transmit signal, and then validation of the CAP ID is in the correct range for the 6225 platform. This capid is usually between 30-45.
②. Then the school AFC SLOP.
The AFC DAC takes 3800 (3900)-4200 (4300), has two frequency offset values, and then the two sets of values to calculate slope, then the verification, according to the slope value of the frequency offset in the 0 of the DAC value, write in the test again, it is 0 or so, indicating that the calibration was successful. General AFC School However, the main check TRS there is no problem.
③ the TX AFC offset calibration (a bit unclear here, please expert guidance ...). ）
AFC is primarily designed to ensure that the Target's clock frequency and network are properly synchronized. We know that the DAC (digital-to-analog converter) and Frequency offset (clock frequency offset) have an approximate linear relationship, the dac-frequency offset curve is determined by two elements, one is the base value (nominal value), and the other is the slope of the curve ( Slope). The so-called reference value refers to the value of the DAC when the Frequency offset value is zero, and if you know the value of Slope, you can calculate the corresponding DAC value based on any one of the Frequency offset.
⑵. One type is tcvcxo (temperature compensated voltage controlled crystal oscillator circuit)
The correspondence between the DAC value and the TCVCXO output frequency (13/26mhz) makes the frequency error of the test receive signal within the allowable range.
Calibration steps:
1. Control the tester Agilent 8960 or R&s CMU200 set in the BCCH (broadcast control channel) one of the channels arfcn_c0_gsm can be one of 1-124, by the board test software initially set), and set the transmitting power to PDL (DBM) (initially set by the Board test software);
2. Set the receiving gain of the if part of the mobile phone is: -35-PDL (DB), Afc_dac value is DAC1 (by the Board test software initial set), the software issued AFC testing request, on the ARFCN_C0_GSM channel to get N_AFC sampling value;
3. Wait for the CPU to calculate the average frequency error of the received I/Q signal: F1;
4. Set the receiving gain for the if part of the mobile phone is: -35-PDL (DB), the AFC_DAC value is DAC2 (initially set by the Board test software), here Dac2>dac1, the software issued AFC test request, on the measurement channel to N_AFC sampling value;
5. Wait for the CPU to calculate the average frequency error of the received I/Q signal: F2;
6. Calculate the AFC DAC slope as: slope= (f1-F2)/(DAC2-DAC1);
The initial ADC value is computed by the resulting slope value and DAC1: Init_afc_dac:
Use Default value= f1/slope+dac1;
Judging whether the test result is passed, that is to see the measured result value: Slope, INIT_AFC_DAC is within the upper and lower limits. The result is then written to the NVRAM area.
⑶. Similarities and differences of Tvccxo AFC and Crystal AFC in MTK Platform calibration test
①tvccxo AFC
AFC Automatic frequency control, now used by the Vc-tcxo 13/26m, the purpose of the communication process is to adjust the VC-TCXO frequency in real-time, so that the cell phone and the base station frequency consistency, such as <0.1ppm. The frequency reference of mobile phone is vc-tcxo, so the purpose is also VC-TCXO frequency offset <0.1ppm. The VC-TCXO is controlled by voltage, typically a DAC digital-to-analog output that controls the VC-TCXO voltage, such as the DAC12 BIT. That output range is 0-(2^12-1).
The purpose of AFC calibration is to give an initial DAC value, and also to calculate the slope, which is how much the DAC frequency changes 1HZ. Hz/dac. The initial DAC value is when the AFC is too large to adjust. The purpose of the slope is to calculate the frequency offset of the baseband and then adjust the DAC based on the frequency offset.
②crystal AFC
In order to reduce the cost, now the mobile phone useful dcxo to replace the traditional vc-tcxo, but because the DCXO itself does not have a frequency adjustment mechanism, it is necessary to real-time baseband demodulator detection DCXO output of the main clock frequency difference and appropriate adjustment. The adjustment of the whole dcxo is divided into the fine tuning of the demodulator carrier and the clock recovery module (supplemented by AFC Automatic Frequency control algorithm) with the instrument in the factory. Factory coarse tune very good understanding, for fine tune, now know only the following some superficial information:
In the GSM system, a string of all 0 special messages (equivalent to the non-modulated pure carrier) is sent through the FCCH message, and the demodulator on the mobile phone detects the frequency difference of the local DCXO output clock frequency based on the implied base station clock frequency information (pure carrier) of the output intermediate frequency signal, and then according to the preset " The frequency difference-crystal control voltage mapping table outputs the corresponding control voltage to adjust the oscillation frequency of the dcxo.
Crystal/dcxo (Crystal) and Tc-vcxo (crystal oscillator) calibration process
