Kenwood TM 521 A Service Manual

Page 2

TM-521A/ E

CIRCUIT DESCRIPTION

MODEL I"
_ TM<521A TM-521 E
Unit Name l
Final Unit X4573200700 X45-3200700
Control Unit X53v3160-11 X53-3160~61
TXVRX Unit L X57-3240A11 X57»3240-61

Table 1

Frequency Configuration

The TM7521A/E utilizes a PLL synthesizer system and digital
VFO. The VFO is capable of tuning in 10 kHz, 125 kHz,
20 kHz, and 25 kHz steps.

The receiver system configuration is based upon double super,
heterodyne principles with a first intermediate frequency (IF)
of 59.7 MHZ and a second intermediate frequency of 455 kHz.
The transmit signal generated by the PLL (Phase Locked Loop)
circuit, oscillating at one half the fundamental frequency is direct,
ly modulated, amplified and applied to the antenna

455ka

12w~1299.aoMHz CFWM455E
lsr Mix MCF , -I
ANT sw RF AMP mx'w'DET AF AMP
55.7MHZ
= 59 ZGSMHZ
naoao<
izw.25MHz
(- _ - "l
PLL I
SYSTEM
PA AMP TX AMP G-I I
L _ _J
1240~ L- M (2 MP
izseesmi-iz ' A
Fig. 1 Frequency configuration diagram

Receiver System

' General

incoming Signals from the antenna pass through a low pass
filter circuit in the transmitters final stage, then through the
transmit/receive switching diodes, and then to the front end
of the receiver,

The incoming signals are amplified by a microwave GaAs
(Gallium Arsenidel PET and enter a two-pole helical
reasonator. The Signals are then passed through an additional
microwave GaAs PET and two-pole helical reasonator in order
to remove any remaining undesirable components. The signal
is then applied to the first mixer. The front end of this
transceiver is matched thru the use of a microstrip line in
order to obtain high sensitivity and reliability The first mixer
employs a GaAs FET that provides excellent tworsignal
characteristics. The incoming Signal is combined With the first
local oscillator signal from the PLL unit and converted into the
first IF signal of 59.7 MHz. Undesirable harmonics are
removed from the IF signal by a two stage MCF (Monolithic
Crystal Filter).

MIC

The first IF signal is amplified and applied to the narrow-band
FM IC (TA7761Fl The incoming lF signal is mixed With the
second local oscillator frequency (597 MHz) to obtain the
second IF frequency of 455 kHz, This Signal is then applied to
a Six element ceramic filter (CFWM455El to sharpen the
Signal quality, The second if is then amplified by a 5 stage
limiting amplifier and applied to the quadrature detector in
order to obtain an audio frequency signal. UndeSirable high-
frequency components in the detected audio are removed by
an active low pass filter The audio signal is then applied to
the volume control, amplified and is finally applied to the
speaker. The srgnal path after detection is shown in Fig. 2.

0 Squelch Circuit

The horse components from the detector output are filtered to
remove the 455 kHz component and are then applied to a
two stage amplifier, This amplified Signal is then applied to the
squelch detector The Signal obtained from this DC detector
passes through the squelch control and is then used to control
the audio muting circuits

- S-Meter Circuit

A portion of the output from the TA7761F is applied to the
SrMeter control Circuits The output from this circuit is applied
to an analog to digital (AID) converSion circuit in the
microprocessor and is used to control the LCD bargraph

Item Standard

59.700 MHZ
$12.5 kHz or more at 3 dB

Center Frequency
Passband width

Attenuation 135 kHz or less at 25 dB
bandwidth i 100 kHz or less at 60 dB
Guaranteed 70 dB or more within FDi'I MHz

attenuation 80 dB at Fott890~930l kHz

Spurious 40 dB or more within Fo~ Fo-i- 500 kHz
Ripple 2 dB or less. Minimum loss 4 dB or less
Impedance Input/output 560 ohms i 5%

Input/output 1.5 pFiOJ pF

Table 2 MCF (L71-0280v05) characteristics
(TX-RX unit L8)

r
i

Item Standard

455 kHz

i 75 kHz or more (at 455 kHz)
115 kHz or less (at 455 kHz)
3 dB or less

Nominal center frequency
6 dB bandwidth

50 dB bandwidth

Ripple (within 455 2 kHz)

Insertion loss (at the max-

imum output point) 6 dB or less

Guaranteed attenuation
(within 4551100 kHz)
Input/Output matching imv
pedance

35 dB or more

1.5kl'1

Table 3 Ceramic filter CFWM455E (L72-0366-05I
Characteristics (TX-RX unit L13)