Maxon Telecom SP210, SP200 service manual Audio/SAT Circuits

Maxon SP200/210 Radio SP200/210

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 7th 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.

DETAILED FUNCTIONAL DESCRIPTION	Issue 1.0
Page 5-4	03/01