Kenwood TM-V708A Service Manual page 12

TM-V708A
7. Data Terminal and Peripheral Circuits
J700 (data terminal) is the data communications terminal
on the front. It handles transmission control, data input/
output, and squelch signals.
There are two data communications modes: 9600bps
mode and 1200bps mode. 9600bps mode communications
are FAST FM mode of SSTV, GMSK and G3RUH packet
communications. Unlike with 1200bps AFSK, with this type
of high-speed modulation, frequency modulation is carried
out after the digital base band signals (rectangular wave) are
passed through a band limiting filter. For 9600bps GMSK for
example, compared to 4800Hz signals (nearly sine wave
signals passed through a filter), these signals have a hissing
sound like digital modulation when listened to by ear.
7-1. Transmission signals
Transmission modulation signals enter from PKD of the
data tenninals (J700). The path to the modulation depends on
whether communications are 1200bps or 9600bps mode.
For 1200bps mode, the transmission modulation signals
pass through the analog switch (IC803), and are input to
IC801 (Pin 6). The signals pass through the pre-emphasis
(Q801, Q802), are adjusted by the electronic volume, and are
input to the VCO.
PKD
SQC
PR1
PR9
PKS
IC703
BUFFER
12
CIRCUIT DESCRIPTION
ANALOG SW
IC803
MICRO
COMPUTER
MICRO
COMPUTER
AMP IC801
BUFFER
Q701
2SC4617
(R)
BUFFER
Q700
2SC4617
(R)
Fig. 16 Transmission signals
Different types of modulation, such as GMSK and G3RUH,
are distinguished by the type of band limiting filter.
Pin Pin
No. name
1 PKD bps switching
Modulation input
Frequency shift
4 PR9 Out level 500mVp-p/10kΩ
Always output during reception
5 PR1 Output level 500mVp-p/10kΩ
Not output when squelch off
Table6 DATA terminal input/output level
For 9600bps mode, the transmission modulation signals
pass through IC803 and amplified by the tone amp (IC802).
Then it is adjusted by the electronic volume, and are input to
the VCO.
The frequency shift depends on the input signal level, so
there is an amplitude limiting circuit (D702, D703) to hold the
signal below 4 Vp-p to avoid extreme shifts.
Thanks to this circuit, the PKD signal does not go above 4
Vp-p and the frequency shift does not fluctuate extremely.
PRE-EMPHASIS
Q802
Q801
2SC4617
2SC4617
(S)
(S)
RA
RD
Specification
1200bps 9600bps
40mVp-p 2Vp-p
3±0.5kHz 2.2±0.5kHz
IC802
TA75SOIF
ELECTRONIC
VOLUME
IC804
VCO
VCO