Modulation; Frequency Generation; Fast Frequency Settling - Tait TM9100 Service Manual

The FCL generates an output of 13.012MHz±4kHz. Initially, a voltage
controlled crystal oscillator (VCXO) 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

Modulation

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 TM9100 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. Higher resolution
Frequency
Generation
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 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.
Both the FCL and RF PLL employ frequency-acquisition speed-up
Fast Frequency
Settling
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.
58 Description TM9100 Service Manual
© Tait Electronics Limited August 2005