Kenwood TK-780 series Service Manual Supplement Download Page 28

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TK-780/H

Frequency Configuration

The TX-RX unit (A/2) incorporates a VCO, based on a frac-

tional N type PLL synthesizer system, that allows a channel
step of 2.5, 5, 6.25, 7.5kHz to be selected. The incoming
signal from the antenna is mixed with a first local oscillation
frequency to produce a first intermediate frequency of
44.85MHz.

The signal is then mixed with a second local oscillation

frequency of 44.395MHz to produce a second intermediate
frequency of 455kHz. This is called a double-conversion
system. The TX-RX unit (A/2) contains a wide/narrow MCF
and CFs. The transmit signal is produced by the PLL circuit
for direction oscillation and division. The signal output from
the VCO is amplified by a straight amplifier and transmitted.

ANT

AMP

1st

MIX

AMP

POWER

AMP

CF 455kHz

(Wide)

CF 455kHz

(Narrow)

MCF

44.85MHz

–

MIX/IF/DET

190.85~218.85MHz

: K,HK

180.85~206.85MHz

: K2,HK2

PLL/VCO

VCXO

16.8MHz

146~174MHz : K,HK
136~162MHz : K2,HK2

44.395MHz

AFO

Fig. 1

Frequency configuration

Receiver System

■

Outline

The incoming signal from the antenna passes through a

low-pass filter and a transmission/reception selection diode
switch (D211, D212) and goes to the front end of the re-
ceiver. The front-end filter is a BPF, a variable BPF consist-
ing of three coils and three varicap diodes (D206, D207,
D208) to eliminate unwanted out-of-band signal compo-
nents. The low-noise amplifier (LNA) (Q202) uses a bipolar
transistor to achieve wide-band and low-distortion amplifica-
tion.

The signal passes through the BPF and is down-con-

verted with the first local signal by IC202 to produce the first

IF signal of 44.85 MHz. The first local signal passes through
an LPF and an attenuator to eliminate unwanted harmonics
components and implement the optimum input level to the
mixer, then enters IC202. A DBM is used as a mixer to
achieve a high potential.

The signal output from the mixer passes through two

MCFs (XF1). The signal is amplified by an intermediate fre-
quency amplifier and input to the FM IF IC (IC11).

The first intermediate frequency signal is mixed with the

second local signal of 44.395MHz to produce the second IF
signal of 455kHz.

The unwanted near-by signal components are then elimi-

nated by a wide ceramic filter (CF1) or a narrow ceramic fil-
ter (CF2) and the resulting signal goes back to the FM IF IC.
The signal is quadrature-detected in the IC to produce an
audio signal, which is amplified by a DET amplifier (IC2) and
output to the TX-RX unit (B/2).

■

Wide/Narrow Changeover Circuit

The W/N port (pin 11) of the shift register (IC7) is used to

switch between ceramic filters. When the W/N port is high,
Q24 turns on and the ceramic filter SW diode (D22, D23)
CF1 turns on to receive a Wide signal. At the same time,
Q16 turns on and one of the filters is selected so that the
wide and narrow audio output levels are equal.

When the W/N port is low, Q23 turns on and the ceramic

filter SW diode (D22, D23) CF2 turns on to receive a Narrow
signal.

LPF

ANT

BPF

L202~204

BPF

Q202

AMP

Q15

AMP

IC2