Theory of Operation Marine Radio

MR F55 & MR F75

Total pages: 4

Functional Description

The Cobra models MRF55 and MRF75 are Marine band FM Radio transceivers operated in the frequency range 156.025MHz to 163.275MHz with WX and DSC. Functional as follows:
1. MRF55: Channel Up / Down, Emergency channel key CH16/9, WX/UIC key, Channel Scan key, TX power HI/Low key, Dual watch channel key, Call/set key, Distress key, Squelch control volume, DC power ON/Off and Audio Volume control, External Speaker Jack and GPS Jack, PTT key. 2. MRF75: Channel Up / Down, Emergency channel key CH16/9, WX/UIC key, MEMO Channel Scan key, TX power HI/Low key, Tri watch channel key, Call/set key, Distress key, Squelch control volume, DC power ON/Off and Audio Volume Up/Down control, PA Function, External Speaker & PA Jack and GPS Jack, PTT key.

MCU Description

The MCU uses Toshibas TMP86CM47U micro-controller, there are 32k X 8bits ROM and 1k X 8bits RAM, LQFP44 pins package. These pin assignment as below table: PIN Description Vss = GND X-in, crystal 8 MHz X-out, crystal 8 MHz Test, connect to GND. Vdd, +5VDC input DC power on control TX on/off control (on = low) Reset pin DC power on key scan LCD backlight control LCD backlight control NMEA (GPS data) input EEprom data pin Volume down key scan Volume up key scan Squelch output control Inhibit audio when DSC on
Control LCD driver A0 data pin
PLL STB. HI=F75, Low=F55 PLL clock data output Beep tone generator Control LCD CS1 pin
Model 55/75 55/75 55/75 55/75 55/75 F75 55/75 55/75 F75 55/75 55/75 55/75 55/75 F75 F75 55/75 55/75 55/75 55/75 55/75 55/75 55/75

PIN 43 44

Description Control LCD SDI data pin WX alert data pin detect TX Hi / low power control Keys scan pin Keys scan pin Beep tone Hi/low control Digital volume data pin Digital volume clock pin Open EEprom clock pin RX/TX signal indication Pull high to +5VDC AVDD (connect +5VDC) AVSS (connect to GND)
Model 55/75 55/75 55/75 55/75 55/75 55/75 F75 F75 55/75 55/75 55/75 55/75 55/75 55/75
Pin 37 to 41 for 1300Hz and 2100Hz FSK generator. 55/75
PLL data output DSC detect pin Squelch input detect

55/75 55/75 55/75

Remarks: MCU pin19 (BUS14) for MRF55 and MRF75 models select by resistor R371 and R340 control. R371 open for MRF55 model, R340 open for MRF75 model selection.

Key Scan

Key scan by MCU pin26 (BUS01) and pin27 (BUS02) control, apply difference step DC voltage input to A/D pin26 and pin27. ----- MRF55/75 MRF75 only --- Digital volume rotary operation by encoder K300 control MCU pin14 (bus15) and pin15 (bus16) realization. The DC power operation by encoder K300 push switch control MCU pin9 (bus11) achieved.

DC Power Controller

The unit DC power supplies is 13.8V and path as follows: MRF55: T102 L124 SW100 (Rotary switch) IC111 (LM7805) VCC+5V output. MRF75: T102 L124 -- R125 IC108 (LM7805) -- +5V output to MCU (CPU-DC). Q124 (KTA1276Y) -- IC111 (LM7805) VCC+5V output. 55/75 RX: Vcc+5V Q122 E Q122 C RX-Vcc+5V output. 55/75 TX: While the PTT key was press down, the TX-ON control pin of MCU is low, so, Q120 will working and the Q122 cut-off. Vcc+5V Q120 E Q120 C TX-DC-5V output.

VCO & PLL

1. The VCO oscillator stage by Q112, L115, VD100 and some resistors and capacitors component buildup. TX and RX VCO change by Q111 control. 2. The PLL IC used Toshiba brand TB31202FN and the local frequency is 20.95MHz.

TX Pre-amplifier

TX pre-amplifier stage by RF transistors Q127, Q110 and coils L106, L114 and external some resistors and capacitors buildup the 160MHz amplifier.
TX Driver & Final Amplifier
TX driver and Final amplifier stage by Mitsubishis RF power module RA35H1516M buildup. The pin1 is RF signal input, the pin2 is Gate control, the pin3 is DC power supplies input, the pin4 is RF power output and the heat sink is ground.

High low TX power control circuit
The IC112 LM358M, transistor Q108 and Q109, diode D102 and external resistors and capacitors buildup the Automatic Power Controller. TX High power output 25 watts @ 50 loading: High power mode the CPU pin25 output is low, the transistor Q105 is rest. Then adjust the VR101 to change IC112 pin3s voltage and the Q116 collect output can automatically to control the RF power module pin2s Gate voltage to control the power gain output. TX Low power output 1 watt @ 50 loading: Low power mode the CPU pin25 output is high, the transistor Q105 is working. Then adjust the VR101 to change IC112 pin3s voltage and the Q116 collect output can automatically to control the RF power module pin2s Gate voltage to control the power gain output.

Low Pass Filter

The Low pass filter is by three group LC buildup. There are L107 match C100, L100 match C102 and L101 match C103.

TX modulation path

TX modulation audio amplifier used low power quad operation amplifier IC107 LM324 buildup. IC107 pin1 to pin3 is amplifier, other three group make the limiter for maximum modulation.

RX Receiver

The RF amplifier stage used double turn circuit to achieved. The main part are L116, L117, Q114, L119 and L120. RX receiver is dual conversion super-heterodyne type with the first IF 21.4MHz and the second IF 450KHz. PLL supplies the local frequencies below 21.4MHz to convert 21.4MHz signals. Q114 amplifies the incoming RX signals. Q115 mixes the incoming RX signal and local to generate IF frequency possessing audio information. There stage 450KHz amplifier circuit IC104 amplify the 450KHz signal to make enough level for discriminate the audio signal. IFT L123 and R171 are working as a FM detector. The RX audio through Volume control coupling the audio amplifier IC TDA2003 output to speaker.