Kenwood TM 271 A Service Manual

Page 2

TM-271A/271 E

CIRCUIT DESCRIPTION

Frequency Configuration

The receiver utilizes double conversion. The first IF is
49.95MHZ and the second [F is 450kHz. The first local oscile
lator signal is supplied from the PLL circuit.

The PLL circuit in the transmitter generates the necessary
frequencies. Figure 1 shows the frequencies.

ANT CF 450kHZ

lst MCF
MIX 4995MHZ

ANT FIF AF
sw ->AMP \/ -- IF SYSTEM -> PA +EGSP

ISO/(MHZ
x3
multiply

l

TCXO

16.8MHZ

RX I MIC
POWER RF 1/2 _ PLLNCO «AMP <-G MIC

AMP AMP TX
Frequency configuration

Fig. 1

Receiver System
The receiver is double conversion superheterodyne. The
frequency configuration is shown in Figure 1.

I Front-end RF Amplifier

An incoming signal from the antenna is applied to an RF
amplifier (0353) after passing through a transmit/receive
switch circuit (D603, D605 are off) and a band pass filter
(L357, L356 and varactor diodes : D353, D354). After the
signal is amplified (0353), the signal is filtered through a band
pass filter (L354, L355 and varactor diodes: D351, D352) to
eliminate unwanted signals before it is passed to the first
mixer.

The voltage of these diodes are controlled by tracking the
CPU (IC101) center frequency of the band pass filter. (See
Fig. 2.)

I First Mixer

The signal from the RF amplifier is heterodyned with the
first local oscillator signal from the PLL frequency synthesizer
circuit at the first mixer (0352) to create a 49.95MHZ first
intermediate frequency (1st IF) signal. The first IF signal is
then fed through one pair of monolithic crystal filter (MCF :
XF351) to further remove spurious signals.

ANT
D602 L357,356 L354,355
D603 D353,354 0353 D351,352 0352
D605 BPF RF AMP BPF MIX

XF351 0351
MCF IF AMP

I IF Amplifier

The first IF signal is amplified by 0351, and then goes to
IC301 (FM processing IC). The signal is heterodyned again
with a second local oscillator signal within IC301 to create a
450kHz second IF signal. The second IF signal is then fed
through a 450kHz ceramic filter (Wide : CF301, Narrow :
CF302) to further eliminate unwanted signals before it is am-
plified and FM detected in IC301.

Item Rating
49.95MH2

15.0kHz or more at 3dB
iZODKHz or less

1.0dB or less

5.0dB or less

80dB or more at fo:1MHz
40dB or more

Terminal impedance 3509/55pF

Table 1 Crystal filter (L71-0620-05) : XF351

Nominal center frequency
Pass bandwidth
35dB stop bandwrdth

Ripple

Insertion loss

Guaranteed attenuation

Spurious

Item Rating
450kHz

:6.0kHz or more

Nominal center frequency
BdB bandwidth
50dB bandwidth

i12.5kHz or less

Ripple 2.0dB or less

Insertion loss 6.0dB or less

Guaranteed attenuation 35.0dB or more within foiIOOkHz
Terminal impedance 2.0KQ

Table 2 Ceramic filter (L72-0993-05) : CF301

Item Rating
450kHz

Nominal center frequency
BdB bandwidth
50dB bandwidth

:4.5I :10.0l

Ripple 2.0dB or less

Insertion loss 6.0dB or less

Guaranteed attenuation 60.0dB or more within foi100l Terminal impedance 2.0KQ

Table 3 Ceramic filter (L72-0999-05) : CF302

CF301 (Wide)

CF302 (Narrow)

|C161
D/A CONVERTER

OSC (VCO/PLL)

ANT | IC301
SW /%\ | /%\ Ig fl _.>_> IF system W/NO
In. I l l (EVOL2)
IC402 x401 0302
TV 1/2 divider TCXO _ X3 multiply
IC203 IC161
DC AMP _ D/A CPU 1st [teal

Fig. 2 Receiver system