Tait TM8100 mobiles Service Manual page 60

Modulation
Frequency
Generation
60 Description
band, the frequency is divided down to 72kHz. This is because the
sensitivity (K = 4MHz/V) is four times lower than that of the
VCO
transmit VCO.
The reference frequency input from the FCL is also divided down to
approximately 25kHz (30kHz for the A4 band, 72kHz receive for the
K5 band). The phase of the two signals is compared and the error translated
into a DC voltage by a programmable charge pump and dual-bandwidth
loop filter. This DC signal is used to control the VCO frequency and reduce
the initial error. The loop eventually settles to a point that minimizes the
phase error between divided down reference and VCO frequencies. The net
result is that the loop "locks" to a programmed multiple of the reference
frequency.
The FCL generates an output of 13.012MHz±4kHz. Initially, a VCXO
(voltage controlled crystal oscillator) produces a quasi-regulated frequency in
the required range. The VCXO output is fed to a mixer where it is mixed
with the 13.000MHz TCXO frequency. The mixer, after low-pass filtering
to remove unwanted products, produces a frequency of 12kHz nominally.
This is converted to digital form and transported to the frequency control
block in the custom logic.
The frequency control block compares the mixer output frequency to a
reference generated by the digital clock and creates a DC error signal.
A programmed offset is also added. This error signal is converted to analog
form and used to control the VCXO frequency and reduce the initial error.
Once settled, the loop "locks" to the TCXO frequency with a programmed
offset frequency. The FCL output therefore acquires the TCXO's
frequency stability.
The full bandwidth modulation signal is obtained from the DSP in digital
form at a sample rate of 48kHz. In traditional dual-point modulation
systems the modulation is applied, in analog form, to both the frequency
reference and the VCO in the RF PLL, combining to produce a flat
modulation response down to DC. Reference modulation is usually applied
directly to the TCXO.
In the system employed in the radio, the frequency reference is generated by
the FCL, which itself requires dual-point modulation injection to allow
modulation down to DC. With another modulation point required in the
RF PLL, this system therefore requires triple-point modulation.
The modulation signals applied to the FCL are in digital form while for the
RF PLL (VCO) the modulation signal is applied in analog form.
The modulation cross-over points occur at approximately 30 and 300Hz as
determined by the closed loop bandwidths of the FCL and RF PLL
respectively.
The RF PLL has a frequency resolution of 25kHz (30kHz for the A4 band,
72kHz receive for the K5 band). Higher resolution cannot be achieved
owing to acquisition-time requirements and so for any given frequency the
error could be as high as ±12.5kHz. This error is corrected by altering the
TM8100/TM8200 Service Manual
© Tait Electronics Limited November 2007