Kenwood TH-K20A, Service Manual

Page: 4 / 40

TH-K20A/K20E
4
1. Frequency Confi guration
The receiver utilizes double conversion. The first IF is
38.85MHz and the second IF is 450kHz. The fi rst Local os-
cillator is supplied from the PLL circuit.
The PLL circuit in the transmitter generates the neces-
sary frequencies.
Fig. 1 Frequency confi guration
Fig. 2 Receiver system
2. Receiver System
The receiver system is shown in Figure 2.
2-1. RF Amplifi er Circuit
The signal coming from the antenna passes through
the transmit/receive switching diode circuit (D101,
D102,D103,D104 and D105) and a BPF (L213,L214), and is
then amplifi ed by the RF amplifi er (Q204).
The resulting signal passes through a BPF (L209,L211
and L217) and goes to the mixer. These BPFs are adjusted
by variable capacitors (D200,D201 and D204). The input
voltage to the variable capacitor is a regulated voltage out-
put from the DAC (IC300).
2-2. First Mixer
The signal from the front end is mixed with the fi rst local
oscillator signal generated in the PLL circuit by Q203 to pro-
duce a fi rst IF frequency of 38.85 MHz.
The resulting signal passes through the XF200 MCF to
cut the adjacent spurious and provide the optimum charac-
teristics, such as adjacent frequency selectivity.
RX: 136~173.995MHz(K,E,M)
174.85~
212.845MHz
1st MIX MCF SP
MIC
CF
450kHz
ANT
ANT
SW
RF
AMP
IF
SYSTEM
X2
doubler
PLL
VCO
38.85MHz
38.4MHz
TX
AMP
T/R
SW
AF
AMP
MIC
AMP
AF
filter
TCXO 19.2MHz
RF
AMP
TX: 136~173.995MHz(M)
144~147.995MHz(K)
144~145.995MHz(E)
1st Lo:
TX: 136~173.995MHz(M)
144~147.995MHz(K)
144~145.995MHz(E)
SP
ANT
IC200
IF,MIX,DET
Q203
1st MIX
IC303
CF200
ANT
SW
Q201
doubler
2nd Local
X1
TCXO
19.2MHz
BPF
BPF
Q204
RF AMP XF200
MCF
Q202
IF AMP
IC307
AF PA
BPF 1st Local
BPF AF filter
VR300
CIRCUIT DESCRIPTION
Rating
Item
Nominal center frequency (fo)
Pass bandwidth
Attenuation bandwidth
Ultimate attenuation 70dB or more (fo –900 kHz)
Spurious response
1dB or less Ripple
Insertion loss
Terminating impedanc 610
:
// 3pF
MCF (L71-0669-05) characteristics (TX-RX unit XF200)
Nominal center frequency (fo)
3dB bandwidth
50dB bandwidth
3.0kHz) ± o f ( Ripple
Guaranteed attenuation
50dB or more (fo
±
100kHz)
Insertion loss
I/O matching impedance
Ceramic filter (L72-1042-05) characteristics (TX-RX unit CF200)
5dB or less
±11.0kHz or less
38.850MHz
450kHz
6dB or less
1.5k
:
Item
Rating
40dB or more (fo +1MHz)
±25.0kHz or less at 45dB
±5.0kHz or more at 3dB
±4.5kHz or more
2dB or less