Fgu; Working Principle Of Pll; Figure 8-4 Diagram Of Fgu - RCA RDR9000 Service Manual

Service Manual

8.3 FGU

The Frequency Generation Unit (FGU) comprises VCO and PLL. It is the core module of the
whole TX-RX system. This circuit provides accurate carrier frequency during transmission,
and stable LO signal during reception. It has a direct influence on the performance of the
system.
MOD-VCO
OMAP

8.3.1 Working Principle of PLL

The 19.2 MHz frequency generated by the reference crystal oscillator goes to PLL for division,
generating the reference frequency (i.e. step frequency f1). Meanwhile, the frequency
generated by VCO generates another frequency (f2) through the frequency divider in PLL.
Then frequencies f1 and f2 are compared in the phase detector (PD), to generate continuous
pulse current. The current goes to the loop filter for RC integration, and is then converted to
CV voltage. Then the CV voltage is sent to the varactor diode of VCO. It adjusts the output
frequency of VCO directly until the CV voltage becomes constant. Then the PLL is locked,
and the stable frequency output by VCO goes to the TX-RX channel after passing through
two buffer amplifiers.
TX_VCO
4CH DAC
RX_VCO
4-Pole RC Filter
CV Buffer
Adapt
OP
SW
Adapt
Control
Reference OSC Signal

Figure 8-4 Diagram of FGU

TX VCO
TX VCO
Amplifier
Buffer
TX BPF
RX VCO
RX VCO
Amplifier
Buffer
RX BPF
Frequency
Synthesizer
From OMAP
Ref. Osc
19.2 MHz
CV
CV OUT
SW
CV-Read
Control
236
8. VHF (136-174 MHz) Information
LPF
TX VCO
OUTPUT
LPF
RX VCO
OUTPUT