60 General Description TM8100 Mobile Radio Service Manual
May 2004 © Tait Electronics Limited
Frequency Generation The RF PLL has a frequency resolution of 25 kHz. Higher resolution
cannot be achieved owing to acquisition-time requirements and so for
any given frequency the error could be as high as ±12.5 kHz. This error
is corrected by altering the reference frequency to the RF PLL. The
FCL supplies the reference frequency and is able to adjust it up to
±300 ppm with better than 0.1 ppm resolution (equivalent to better
than 50 Hz 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 high by increasing the
charge pump current and reducing time constants in the loop filter. As
a result settling to within 1 kHz of the final value occurs in under 4 ms.
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 4.5 ms.
Frequency Acquisition
of FCL
The FCL utilises 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 4 ms.
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.
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