Alan HP106 - 2.5.e Receiver Audio Circuit; 2.6 Signalling; 2.6.a General; 2.6.b CTCSS Tone Encoder; Digital Code Squelch (DCS) Encoder

HP406 Service Manual
Page 7 of 12
The output on IC6 Pin 14 signals the microprocessor Q17 with a low level (  0V) to unmute the radio.
The audio is unmuted by the microprocessor Q17 Pin 27 switching to a high value (  5V on SQL MUTE) thus
biasing on Q206 . The audio is then routed to the audio amplifier Q221 via the volume control S201 .
2.5.e Receiver Audio Circuit
The detector’s audio output also is fed to the tone(CTCSS and DCS) low-pass filter Q212C .
Then the output of the low-pass filter is routed to the second stage filter Q212B . The output of Q210B is applied
to the squaring circuit Q212A and finally to the microprocessor Q17 Pin 60 for decoding.
Another branching of the detector output feeds the audio high-pass filter Q208 via Q212D . The output of the
audio high-pass filter feeds the Volume Control S201 (VOL). From the wiper arm on the Volume Control, the
audio is routed to Pin 2, the input to the audio power amplifier Q221 . The output of the audio power amplifier is
routed through the earphone jack J101 to the internal speaker E101 .
2.6 Signalling
2.6.a General
The microprocessor is fitted with a ADC/DAC converter built-in, so it provides generating and decoding the tones
for selective calls, CTCSS and DCS. It can do that without using any other external ICs, but only by means of
some transistors. The deviation of the selective call can be adjusted by the trimmer RV1.
The microprocessor manages the analogue switches for the scrambler as well, which is base-band-inversion
type.
2.6.b CTCSS Tone Encoder / Digital Code Squelch (DCS) Encoder
CTCSS signals and DCS signals are synthesized by microprocessor Q17 and appear as pulse waveform on I/O
line at Pin 39 . This I/O line is a pseudo-sine wave for CTCSS or a DCS pseudo-waveform and is applied to the
transistor IQ5 which makes the signals closer to the theoretic CTCSS/DCS signals. The waveform is then
smoothed by low pass filters Q213B to produce an acceptable sine wave output. The CTCSS tone signal is
adjust to the proper level by RV202 . The DCS signal is adjust to the proper balance by RV203 . The signal is then
applied to the audio processing circuit at R305 and to the TCXO circuit at X401 .
2.6.c Selective call
Similarly to CTCSS/DCS, selcal signals are also generated and decoded by the microprocessor Q17 . The selcal
decode input is the pin 59 ( ADC_SELCALL ), whilst the TX tones are generated at pin 37 ( SELCALL_PWM ),
then fed to the transistor Q4 and associated circuitry which provides to amplify and smooth the tones in order to
make them suitable for the modulator. RV1 adjusts the level (deviation) of the tones.
2.6.d Scrambler
It’s a classic “baseband inversion” scrambler which inverts the audio baseband (300-30000 Hz). The audio
baseband AFTX_IN is mixed with a fixed tone (1300 Hz) SCRMBLR_CLK generated by the microprocessor Q17 .
The mixer’s output AFTX_OUT is a scrambled baseband which sounds garbled (not understandable) by normal
receivers. However, if the receiver is equipped with the same kind of scrambler which is properly set on the same
fixed tone, the received scrambled baseband is fed in the RX mixer which provides a complementary process
obtaining a clear (understandable) baseband at its output. If fact if the scrambled transmission is received by the
party’s HPx06 (with scrambled activated), the scrambled baseband AFRX is fed to the scrambler unit and mixed
with the same tone generated by the microprocessor. The output of the mixer AFRX_OUT is a normal
(unscrambled) baseband and can be clearly heard.
As you can see, the over stated fixed audio tone acts as a encoding/decoding key, so it must be the same both
at the TX and RX parties. As already stated, the standard version of HPx06 is fitted with a 1300 Hz
encoding/decoding key, however a different tone can be required.