TM-281A
5
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
SW
RF
AMP
1st
MIX
AF
PA
TCXO
MIC
AMP
x3
multiply
RF
AMP
POWER
AMP
CF 450kHz
MCF
49.95MHz
IF SYSTEM
PLL/VCO
16.8MHz
50.4MHz
ANT
RX
TX
SP
MIC
1/2
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.
■
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
Rating
Nominal center frequency
49.95MHz
Pass bandwidth
±4.0kHz or more at 3dB
50dB stop bandwidth
±25.0kHz or less
Ripple
1.0dB or less
Insertion loss
5.0dB or less
Guaranteed attenuation
80dB or more at fo ± 900kHz
Spurious
50dB or more at fo ± 1MHz
Terminating impedance
270
Ω
/ 8.5pF
Table 1 Crystal fi lter (L71-0655-05): XF501
Item
Rating
Nominal center frequency
450kHz
6dB bandwidth
±7.5kHz or more
50dB bandwidth
±15.0kHz or less
Ripple
2.0dB or less
Insertion loss
6.0dB or less
Guaranteed attenuation
35.0dB or more within fo ± 100kHz
I/O matching impedance
1.5k
Ω
Table 2 Ceramic fi lter (L72-1034-05): CF501
Item
Rating
Nominal center frequency
450kHz
6dB bandwidth
±4.5kHz or more
50dB bandwidth
±10.0kHz or less
Ripple
2.0dB or less
Insertion loss
6.0dB or less
Guaranteed attenuation
60.0dB or more within fo ± 100kHz
I/O matching impedance
2.0k
Ω
Table 3 Ceramic fi lter (L72-0999-05): CF502
ANT
L515,L514
D509,D508
BPF
Q505
RF AMP
Q503
IF AMP
IC705
D/A CONVERTER
Q504
MIX
XF501
MCF
D306,D307
D309,D311
ANT
SW
IC705
D/A
IC706
DC AMP
Q501
x3 multiply
IC2
1/2 divider
X1
TCXO
IC501
FM IC
1st local OSC (VCO/PLL)
W/NO
(EVOL2)
CF501 (Wide)
CF502 (Narrow)
TV
MCU
L511,L510
D506,D505
BPF
Fig. 2 Receiver system
CIRCUIT DESCRIPTION