Maxon Telecom SP210 - Manual (Page 32)

Maxon SP200/210 Radio SP200/210
DETAILED FUNCTIONAL DESCRIPTION Issue 1.0
Page 5-4 03/01
5.4 Audio/SAT Circuits
CTCSS / DCS Decoder Circuits
Discriminator audio from pin 9 IC5 is applied, via IC408D, to pin 3 of the audio ASIC. The audio
frequencies of the signal are filterecd out by the 7
th
order Elliptic Low Pass Filter, leaving only the SAT
audio tones. The level is then set by the ASIC and is routed via IC407A back to the ASIC for
comparison with a reference voltage. This produces the logic data signal. This logic signal on pin 21
(IC406) is fed into the microcontroller (IC403) where it is matched with a programmed frequency. If
there is a successful match, decode occurs, which is shown by a green LED on the top of the radio
and audio is heard. If there is an invalid decode, the LED will light yellow (BUSY)
CTCSS / DCS Encoder Circuits
During TX encode the tone squelch digital signal is produced as a 3-bit parallel word at pins 48, 49
and 50 of the micro controller (IC403). The 3-bit digital signal is converted to an analogue signal by
resistors R420, 421 and 423. The analogue signal is fed into IC406 pin 4 where it is filtered and its
deviation level is set.
The filtered encode output from pin 23 (IC406) is fed back into pin 22 (IC406) via IC407a (LM358).
The filtered encode signal is mixed with the audio signal from TXTRIM1 by TXSUM. Via ATTN, the
mixed signals are applied to TXTRIM2/TXTRIM3, which controls the modulation to the VCO and
TCXO respectively (see RF description).
External Mic / PTT Control Circuit
The external microphone is connected via a 3.5 mm stereo jack socket on the right-hand side of the
radio. The internal mic and speaker are disabled when the external connector is plugged the
SPK/MIC jack socket.
When the external PTT is depressed, a low impedance is presented to the base of Q403 (below 20k
ohm). Q402 and Q403 switch on and so Q402 collector is low (normally tied high). This point is
connected to IC403 (microcontroller) pin 43 (PTT). The same point is connected to the normal PTT
button, SW403, which is earthed when depressed.
The mic audio itself is switched through IC401A by the Tx enable line to IC408D amplifier stage and
through to the audio ASIC for filtering and deviation level setting. As described previously, this audio
signal is added to the SAT and routed to the TCXO and VCO.
Rx Audio
The discriminator audio is fed directly into the audio ASIC from the RF section (see later). The ASIC
sets the gain and filters out any SAT. The output is fed to the volume control (VR5). The alert tones
and switch on melody is routed from the microcontroller via link LK27 to the same point to the volume
control.
The AF from the volume control is fed to the audio amplifier, a TDA7233, which is activated by the
mute circuit. The output is then fed to the internal or external speaker.
Mute Circuit
The microcontroller (IC403) has a mute circuit which is output on pin 13 (7233EN). The operation of
the mute depends upon a number of factors, such as is the demodulator receiving a mute signal from
the FM detector and does the signal have the correct CTCSS / DCS. The microcontroller will also
change the state of the mute if the monitor button is depressed (SW402), assuming that this function
has been enabled in the programming
Pin 13 is connected to Q414, Q415 via R446, which mutes the TDA7233IC.