Circuit Description; Main Receiver Block; Frequency Configuration; Reference Signal Generator - Kenwood TS-990S Service Manual

TS-990S

1. Main Receiver Block

1-1. Frequency Confi guration
While receiving with the main receiver, the 1st IF oper-
ates in 8.248MHz double conversion. In modes other than
ANT

1-2. Reference Signal Generator

■ With Internal Reference Oscillation Signal Se-
lected
The TCXO (X1) generates a 19.2MHz reference frequen-
cy for controlling the local oscillator frequency. The 19.2MHz
signal is applied to the DDS (IC13) via the amplifi er (Q10),
and approximately 8.4MHz is generated. A signal at approxi-
mately 8.4MHz generated by the DDS (IC13) is applied to
the PLL (IC18) via the ceramic fi lter (CF1). The frequency of
the PLL (IC18) input is divided inside the PLL (IC18) and is
used as a comparison frequency (fø).
The VCXO (X2) is a Voltage-Controlled Crystal Oscillator
which generates 20MHz. The signal output from the VCXO
(X2) is applied to D49 via a buffer (Q78), amplifi er (Q79) and
LPF, and is frequency-doubled by D49 to generate 40MHz.
The 40MHz signal passes through the amplifi er (Q82) and
LPF, and is applied to the PLL (IC18) with the frequency of
the signal divided by N in the PLL (IC18). The signal with
its frequency divided by N and the fø are compared by the
phase comparator inside the PLL and the 40MHz is phase-
locked. The phase-locked 40MHz is used as a reference sig-
nal for each local oscillator block.
4

CIRCUIT DESCRIPTION

RX MIX
1st IF
8.248MHz
Divide
VCO
PLL
ADF4111
VCO
DDS
PLL
AD9951 ADF4111
Fig. 1 Frequency confi guration of the main receiver block
FM, 24kHz is used as the 2nd IF frequency. The 2nd IF is A/
D converted and is applied to the DSP. In FM mode, the 1st
IF signal (8.248MHz) is discriminated to an audio signal by
the FM IC. It is then A/D converted and applied to the DSP.
To Scope unit
2nd IF
455kHz
FM
FM IC
except
RX MIX
FM
24kHz
TCXO
19.2MHz
DDS
DDS
AD9835
AD9835
PLL
ADF4001
VCXO
x2
20MHz
■ With External Reference Oscillation Signal Se-
lected
The external 10MHz reference signal supplied from
CN222 on the DC-DC unit (X43-322 A/3) to CN33 on the
PLL (MAIN LO) unit (X50-322), is applied to the PLL (IC18)
via the buffer (Q5) and AGC amplifi er (Q11). When the ex-
ternal reference is selected, the external 10MHz is used as
a reference signal for the PLL (IC18).
■ External Output of 10MHz Reference Signal
The 40MHz which is generated for each local oscillator
is applied to the frequency dividers (IC24 and IC25) via the
amplifier (Q81) and LPF, and is frequency-divided by 4 to
generate 10MHz. The obtained 10MHz reference signal is
supplied to CN222 on the DC-DC unit via the amplifi er (Q51),
and is sent from CN220 to the external terminal (REF I/O
(10MHz)).
■ Frequency Adjustment
Frequency adjustment is made by fi ne-tuning the approxi-
mately 8.4MHz generated from the DDS (IC13).
DSP
AF
AF OUT
ADC/DAC