Circuit Description - Kenwood TM-281A Service Manual

Frequency Confi guration
The receiver utilizes double conversion. The first IF is
49.95MHz and the second IF is 450kHz. The fi rst local oscil-
lator signal is supplied from the PLL circuit.
The PLL circuit in the transmitter generates the neces-
sary frequencies. Figure 1 shows the frequencies.
ANT
1st MCF
MIX 49.95MHz
ANT RF
SW AMP
RX
1/2
POWER RF
AMP AMP
TX
Fig. 1 Frequency confi guration
Receiver System
The receiver is a double conversion superheterodyne.
The frequency confi guration is shown in Figure 1.
■ Front-end RF Amplifi er
An incoming signal from the antenna is applied to an RF
amplifier (Q505) after passing through a transmit/receive
switch circuit (D309, D311, D306 and D307 are off) and
a band pass fi lter (L515, L514 and varactor diodes: D509,
D508). After the signal is amplified, the signal is filtered
through a band pass fi lter (L511, L510 and varactor diodes:
D506, D505) to eliminate unwanted signals before it is
passed to the fi rst mixer.
The voltage of these diodes are controlled by tracking the
MCU (IC707) center frequency of the band pass fi lter. (See
Fig. 2.)
■ First Mixer
The signal from the RF amplifi er is heterodyned with the
fi rst local oscillator signal from the PLL frequency synthesiz-
er circuit at the fi rst mixer (Q504) to create a 49.95MHz fi rst
intermediate frequency (1st IF) signal. The fi rst IF signal is
then fed through the monolithic crystal fi lter (MCF: XF501)
to further remove spurious signals.
ANT
L515,L514
D306,D307
D509,D508
D309,D311
BPF
ANT
SW
TV
IC706 IC705
DC AMP D/A

CIRCUIT DESCRIPTION

CF 450kHz
AF
IF SYSTEM SP
PA
50.4MHz
x3
multiply
16.8MHz
TCXO
MIC
PLL/VCO MIC
AMP
L511,L510
Q505 D506,D505 Q504
RF AMP MIX
BPF
IC2
1/2 divider
MCU
1st local OSC (VCO/PLL)
Fig. 2 Receiver system
■ IF Amplifi er
The fi rst IF signal is amplifi ed by Q503, and then goes to
IC501 (FM processing IC). The signal is heterodyned again
with a second local oscillator signal within IC501 to create
a 450kHz second IF signal. The second IF signal is then
fed through a 450kHz ceramic fi lter (Wide: CF501, Narrow:
CF502) to further eliminate unwanted signals before it is
amplifi ed and FM detected in IC501.
Item
Nominal center frequency
Pass bandwidth
50dB stop bandwidth
Ripple
Insertion loss
Guaranteed attenuation
Spurious
Terminating impedance
Table 1 Crystal fi lter (L71-0655-05): XF501
Item
Nominal center frequency
6dB bandwidth
50dB bandwidth
Ripple
Insertion loss
Guaranteed attenuation
I/O matching impedance
Table 2 Ceramic fi lter (L72-1034-05): CF501
Item
Nominal center frequency
6dB bandwidth
50dB bandwidth
Ripple
Insertion loss
Guaranteed attenuation
I/O matching impedance
Table 3 Ceramic fi lter (L72-0999-05): CF502
CF501 (Wide)
CF502 (Narrow)
XF501 Q503
MCF IF AMP
IC501
FM IC
X1
Q501
x3 multiply
TCXO
TM-281A
Rating
49.95MHz
±4.0kHz or more at 3dB
±25.0kHz or less
1.0dB or less
5.0dB or less
80dB or more at fo ± 900kHz
50dB or more at fo ± 1MHz
270Ω / 8.5pF
Rating
450kHz
±7.5kHz or more
±15.0kHz or less
2.0dB or less
6.0dB or less
35.0dB or more within fo ± 100kHz
1.5kΩ
Rating
450kHz
±4.5kHz or more
±10.0kHz or less
2.0dB or less
6.0dB or less
60.0dB or more within fo ± 100kHz
2.0kΩ
IC705
D/A CONVERTER
W/NO
(EVOL2)
5