MAXON SM-2000 MOBILE
THEORY OF OPERATION
EEPROM
RX/TX channels, CTCSS/DCS as well as other data from the programmer are stored in the EEPROM. The data stored is retained without power supplied. This is a non-volatile memory. The EEPROM may have information re-programmed or erased. IC408 is an EEPROM with 2048 (256x 8) bit capacity and data is written and read serially, with the Programming cable through the front panel microphone connector.
High Pass Filter
The high pass filter is an 8-pole .1dB chebeyshev active filter that comprises IC410 and associated components. The de-emphasis is provided by resistor R451 and capacitor C452. Receive audio is passed to IC410 by Pin 4 of IC411 where sub-audible tones below 250Hz are removed. Mic audio is also fed into IC410 via IC411 (Pin 4) where sub-audible voice products below 250Hz are also removed.
Mute (Squelch) Circuit
The mute circuit which is controlled by the output of IC409 (micro) pin 77 (386) is connected to Q409 via R438 which opens analog gate IC401D and mutes the audio path to the AF amplifier, in conjunction with Q137 shorting the input to audio amp IC108.
TX Audio and Filter Circuits
Microphone audio from front panel (via IC412), is fed into IC401A, and into IC404B (Pin 6). The TX audio is output on Pin 7 (IC404B) and into the high pass filter (IC410), where sub-audible voice products are removed. The TX audio output from IC410 is fed into IC404A&B which with associated parts form a pre-emphasis mic amplifier and limiter. The output from Pin 1 IC404A is fed to RV402 (TX Modulation Level Adjust) and fed into IC404C&D with associated parts to form a 3kHz low pass filter. The output of IC404D (Pin 14) is then fed into the Audio Mixer Circuit.
Audio Mixer and Inverter
IC405A is an audio mixer where audio and sub-audible tones are combined. RV403 is used as a level control for the Sub-audible tones. This level is summed with amplified, pre-emphasized, and limited audio and feed to IC405A, which is an inverting amplifier. The output is fed to the VCO and TCXO to provide the necessary modulation signal.
Analog Ground Supply
IC406D supplies analog ground (2.0V) to all operational amplifier circuits except for IC405A and IC405D.