DP990/DP995service manual
The amplified signals the IF amplifier outputs enter the IF subsystem AD9864 (U1201) through its 47th pin for a
second frequency mixing, upconverting the first IF signals to the second IF signals (2.25 MHz). The second IF
signals are converted to digital signals through ADC sampling, and output through the SSI. The signals enter a DSP
(OMAP-L138) for demodulation.
Note:
The IF subsystem AD9864 (U1201) has a reference frequency of 12.8 MHz.
The VCO is composed of an external oscillation transistor, a varactor diode and relevant components, providing the
second local oscillator signals (UHF: 71.1 MHZ; VHF: 49.4 MHz).
The clock frequency is 18MHz generated by an external LC resonance circuit.
3.4.Frequency Synthesizer Circuit
As the key module of the Tx/Rx system, this circuit is composed of a VCO and a PLL. In Tx, this module provides
precise carrier; in Rx, it provides stable local oscillator signals.
PLL
The 12.8 MHz frequency generated by a reference crystal oscillator enters the frequency divider of a PLL chip for
frequency division, generating reference frequency (i.e. step frequency F1). The frequency generated by a VCO
passes an LPF to filter second harmonic, and then enters the PLL chip for frequency division to generate frequency
F2. Phase comparison between F2 and F1 is performed in a phase detector (PD), generating continuous pulse
current. When the current passes a loop filter, its RC integral converts to CV. The CV is sent to the varactor diode of
the VCO to adjust the output power of the VCO until the CV becomes constant. At this moment, the PLL is locked,
and the stable frequency the VCO outputs passes two buffer amplifiers and enters the Tx/Rx channel.
VCO
A VCO employs an oscillator circuit of three-point capacitance to get different output frequency through changing
the control voltage (i.e. CV) of a varactor diode.
Local oscillator signals are provided to the Rx VCO by D1001, D1002, D1003, D1004, L1005, C1044 and Q1001
of the oscillator circuit; carrier is provided to the Tx VCO by D1005, D1006, D1007, D1008, L1012, C1084 and
Q1002.
Two-Point Modulation
In Tx, MOD-VCD and MOD-XO respectively send modulation signals to the VCO modulation end and to the PLL
reference crystal oscillator for modulation.
To Next Page
Loading...
Need help?
General
