FLY SP-770, As Manual

Page: 11 / 77

1.2.4.3 Synthesizer Frequency Programming for TX Mode
The frequency ranges of the synthesizer for TX mode are
TX mode GSM850 1813 MHz ~ 1868 MHz
DCS1800 1881 MHz ~ 2008 MHz
PCS1900 2035 MHz ~ 2149 MHz
And the divider number N can be decided by the following procedure.
1. Set the divider ratio D1 of TX reference divider = 11
2. Calculate LO frequency fVCO from TX channel frequency fCH
fVCO = 2 * D1 * fCH / (D1-1) for GSM850
fVCO = D1 * fCH / (D1-1) for DCS1800 and PCS1900
3. Calculate Nint and Nfrac
N = 64 + Nint + Nfrac/5200 = fVCO/26M Nint and Nfrac are integers
0 ≤ Nfrac < 5200
4. If Nfra < 400 or Nfra > 4800, re-set D1 = 9 and repeat Step 2 and 3 to get new
Nint and Nfrac.
5. Use the binary equivalents of Nint and Nfrac to program registers CW1-N_INT
and CW1-N_FRA.
1.2.4.4 Digital Calibration Loop
The MT6120 uses a digital calibration technique to reduce the PLL settling time.
Once the RF synthesizer is programmed through a 3-wire serial interface, the
calibration loop is activated. The main function of the calibration loop is to preset
the RF VCO to the vicinity of the desired frequency quickly and correctly, thus
aiding the PLL to settle faster.
On the other hand, since a large portion of initial frequency error is dealt with by
the integrated calibration loop, the overall locking time can be drastically reduced,
irrespective of the desired frequency.
1.2.4.5 Fast-Acquisition System
After the digital calibration loop presets the RFVCO, the RF synthesizer reverts to
the PLL operation and a fast-acquisition system is activated. For faster settling, the
charge pump current is set to a higher current than normal setting for a period of
time, typically, 20 µs or 60 µs.
1.2.4.5 Voltage Control Crystal Oscillator
Voltage Control Crystal Oscillator (VCXO) consists of an amplifier, a buffer, and a
programmable capacitor array. The VCXO provides the MT6120 with a selectable
reference frequency of either 13 MHz or 26 MHz.
The amplifier is designed to be in series resonance with a standard 26 MHz crystal.
The crystal is connected from the input pin XTAL of amplifier to ground through a
series load capacitance. The buffer provides a typical 600 mVpp voltage swing at
either 13 MHz or 26 MHz. It is designed to drive a tuned load to improve harmonic
contents and reduce the oscillator current consumption. The capacitor array, from
0.0625 pF to 4 pF in steps of 0.0625 pF, is used to shunt the series load capacitor
for coarse tuning and remove any fixed offsets due to crystal manufacturing
variations. An internal varactor that provides fine tuning combines with the