Instruction manual

Connect Systems Inc. -Model TP-154 Page 21
CIRCUIT DESCRIPTION
The incoming audio is buffered, de-emphasized and amplified to the
amount set by the PREAMP control by input op amps U1. The audio
then proceeds to a six pole low pass filter and a six pole high
pass filter. The low pass filter U2 removes voice audio and
extracts the DCS/CTCSS signal for detection at the adaptive
differential comparator. The comparator output is fed to the
microcomputer U12 pin 12 for software DCS/CTCSS detection. The six
pole high pass filter (U3) removes CTCSS and DCS from the incoming
audio to avoid retransmission. The audio from the high pass filter
continues through the squelch switch U18 and on to the audio
output amplifier.
The COS input is buffered and compared to a reference voltage
which is set by the COS threshold pot. The output then proceeds to
the polarity select strap JP-10 and then to the microcomputer U12
pin 6.
DTMF is decoded and regenerated by U11 with some assistance from
the microcomputer U12.
DCS is generated directly by the microcomputer U12. CTCSS is
generated by waveform generator U16. The microcomputer U12
controls the U16 waveform generator via pins 34,35 and 36. DCS and
CTCSS have separate level controls the output of each is fed to
summation amplifier and four pole low pass filter U4. The output
of U4 feeds the transmitter modulator with SUBTONE.
The PTT circuit consists of Q6, Q14 and Q20. This circuit is
controlled by the microcomputer U12 via pin 7. A polarity select
strap gives the choice of ground keying or plus keying.
A power on reset circuit consisting of Q21 and associated
components monitors the input voltage and quickly halts the
microcomputer if the applied input voltage is too low. The power
on reset circuit provides proper computer start up after power is
applied and protects the EE memory U13 on power down.
Counters U22 and U23 divide the 448 KHz clock from U11 and allow
remote re-booting of the microprocessor when any incoming DTMF
digit exceeds 9 seconds in duration. At the end of nine seconds
U22 pin 15 turns on transistor Q9 which in turn enables the power
on reset circuit.
The front panel digital display U6-U9 are controlled by the
display driver U5. The microcomputer U12 talks to U5 via pins
37,38 and 39.
Incoming power is fed through reverse polarity protect diode D8
and then fuse F1. From there the +V voltage feeds a +5 volt
regulator U20 used by the digital components. Another smaller +5
volt regulator U21 is used to supply bias and reference voltage to
the analog op-amps etc.