Tait TM8100 mobiles - Page 61

TM8100/TM8200 Service Manual Description 61
© Tait Electronics Limited November 2007
reference frequency to the RF PLL. The FCL supplies the reference
frequency and is able to adjust it up to ±300ppm with better than 0.1ppm
resolution (equivalent to better than 50Hz resolution at the RF frequency).
The FCL offset will usually be different for receive and transmit modes.
Fast Frequency
Settling
Both the FCL and RF PLL employ frequency-acquisition speed-up
techniques to achieve fast frequency settling. The frequency-acquisition
process of the FCL and RF PLL is able to occur concurrently with minimal
loop interaction owing to the very large difference in frequency step size
between the loops.
Frequency
Acquisition of
RF PLL
In the RF PLL the loop bandwidth is initially set to high by increasing the
charge-pump current and reducing time constants in the loop filter.
As a result, settling to within 1kHz of the final value occurs in under 4ms.
In order to meet noise performance requirements the loop parameters are
then switched to reduce the loop bandwidth. There is a small frequency kick
as the loop bandwidth is reduced. Total settling time is under 4.5ms.
Frequency
Acquisition of FCL
The FCL utilizes self-calibration techniques that enable it to rapidly settle
close to the final value while the loop is open. The loop is then closed and
settling to the final value occurs with an associated reduction in noise.
The total settling time is typically less than 4ms.
Calibration The following items are calibrated in the frequency synthesizer:
■ nominal frequency
■ KVCO
■ KVCXO
■ VCO deviation.
Calibration of the nominal frequency is achieved by adding a fixed offset to
the FCL nominal frequency; the TCXO frequency itself is not adjusted.
The items KVCO and KVCXO are the control sensitivities of the RF VCO
(in MHz/V) and VCXO (in kHz/V) respectively. The latter has
temperature compensation.