User Guide

PAMS
Technical Documentation
NSB–1
System Module
Original 06/98
Page 3 – 34
Frequency Synthesizers
A 13 MHz VCTCXO module is used as a stable reference for both the RF and
BB circuitry. Temperature variations in the VCTCXO module are controlled
by an AFC voltage which is generated by a 11 bit D/A converter in COB-
BA_GJ. The output of the VCTCXO module feeds both the UHF PLL and
the VHF PLL (both of which are located in PLUSSA) and the BB circuitry for
A/D conversion. The BB uses this information for frequency compensation
algorithms.
The UHF synthesizers contains a 64/65 dual modulus prescaler, a ”N” and
”A” divider, a reference divide, a phase detector, a charge pump, a modular
VCO, a buffer circuit and a lowpass filter. The UHF and VHF PLL are con-
trolled with three serial busses; a data bus (SDATA), a serial clock bus
(SCLK) and a latch enable (SLE). The UHF LO signal is generated by the
UHF VCO module which has a tunable frequency range from 1443 MHz to
1510 MHz for the DCS1900 engine. A sample of the LO signal is fed to the
64/65 prescaler. The signal is then fed to the programmable dividers (N and
A) which are programmed via the serial bus. This output then becomes one
of the inputs to the phase detector. The other input to the phase detector is a
multiple of the 13MHz VCTCXO (reference frequency is 200 kHz). Output of
the phase detector is connected to the charge pump, which charges or dis-
charges the integrator capacitor in the loop filter depending on the phase of
the measured frequency compared to reference frequency. The loop filter
attenuates the pulses and generates a DC voltage which controls the fre-
quency of UHF VCO. This loop filter defines the step response of the PLL
(settling time), affects the stability of the loop and is used for sideband rejec-
tion. A buffer circuit is required to ensure that the impedance changes in
CRFU2a and PLUSSA do not kick the VCO off frequency
The VHF synthesizers contains a 16/17 dual modulus prescaler, a ”N” and
”A” divider, a reference divide, a phase detector, a charge pump, a modular
VCO and a lowpass filter. The frequency of the VHF VCO is 800 MHz which
is frequency divided to 400 MHz and 100 MHz. Operation of the VHF PLL is
similar to that of the UHF PLL. The VHF PLL using the 400 MHz signal as its
input frequency. The reference frequency in the VHF synthesizer is 1 MHz.