Service manual

5 CIRCUIT DESCRIPTIONS

Introduction

The SIMRAD RD68 consists of 3 PCB assemblies. The main Receiver /

Transmitter PCB assembly, Navico Part No. E03866, contains all the transmitter

and receiver circuitry including the synthesiser, modulator and audio power

amplifiers. The control PCB assembly, Navico Part No. E03656, houses the

microcontroller, user controls and interfaces, display module and ATIS detection

and mute circuitry. A full Class D DSC controller is also incorporated through a

V.23 modem. The second receiver, Channel 70 watchdog receiver, Navico Part

No. E03211, is fitted to the main Tx / Rx assembly, the 2 PCBs are an

interdependent matched pair.

5.1 Receiver / Transmitter PCB Assembly

Refer to drawing number E02955.

Receiver. RF from the Antenna socket passes through the low pass PA

filter to a quarter wave match / switch L5, L6 and pin diode D1. During

transmissions, D1 is switched on to protect the receiver. There then

follows an optional attenuator and power splitter, to provide RF to the

second receiver in the RT1400, which is bypassed by link LK1 in the

RT1200. C214 is a 1nF coupling capacitor and L7, L8 and the

associated capacitors form a bandpass filter at the input to the RF pre-

amplifier TR1. L9 and L11 form a further bandpass section before the

mixer TR2. The RF signal is fed to TR2 Source and the local oscillator to

Gate 1. Low side injection is used, i.e. the LO, TR105 buffered by

TR103 and TR104, is 21.4 MHz below the receive frequency. L13 forms

the drain load before matching through C25 and C26 to the 4 pole crystal

filter XTAL1 and XTAL2. The output of the filter is coupled by L14, C28

and C29 to the integrated IF amplifier / demodulator IC1. A second local

oscillator running at 21.855MHz produces the second IF of 455KHz.

XTAL4 is a 6 pole ceramic filter completing the necessary bandwidth

definition with quadrature coil L16 providing demodulation.

Audio from pin 9 of IC1 is de-emphasised by R20 and C43, buffered by

TR20 and distributed to the various audio stages on the control PCB.

The audio from pin 9 is also passed through a very high gain amplifier,

within IC1 and configured as a bandpass filter, to detector D2 to provide

a voltage proportional to the received signal strength. This level is fed to

the front panel to provide squelch control. The AF signal is returned from

the control panel after the volume and squelch controls to 2 audio power

amplifiers in bridge mode. The internal speaker output remains live

when an external speaker is connected.

Transmitter. The LO switch / buffer TR102 switches the transmit signal

for amplification by controlled transistors TR101 and TR100 to drive the

hybrid PA module IC100 which is capable of generating the required 25

Watts. The output is switched through PIN diode D100 before passing