Sailor RT2047 Service Manual download pdf (Page 42)

2 CIRCUIT DESCRIPTION RT2047 DSC - PART II

2.3 TX-EXCITER UNIT MODULE 300

The TX exciter unit contains the following circuits:

2.3.1 INSULATION BUFFER AND 16.8 MHZ MIXER

From RX-VC0 the signal is led to transistor T301 and from TX-VC0 the signal is led to transistor T302.

The transistors are not coupled with common base and act as buffer amplifiers. The mixed signal over

resistor T305 is led to the mixer amplifier T303 via capacitor C305. The differential signal which is the TX-

VC0 frequency minus the RX-VC0 frequency is led to the amplifier transistor T304 through the low-pass

filter consisting of C310, L301 and C311. In transistor T305 the signal is amplified to TTL level. The diode

D301 works as a base clamp diode.

2.3.2 THE 21 MHZ OSCILLATOR

The oscillator is a Colpitt type and the oscillator transistor T308 is oscillating by means of a 21 MHz crystal

X301.

The trimmer capacitor C331 is for fine adjustment of the oscillator frequency.

RF signal for the reference divider is led to transistor T307 for amplifying. In transistor T306 the signal

is amplified to TTL level.

2.3.3 PHASE/FREQUENCY DETECTOR, 32 COUNTER AND REF. DIVIDER

The crystal frequency is led to IC306 pin 10 which is a decade counter. From output pin 5 of this, the

frequency is divided by 5. This frequency is led to flip-flop IC305a, where the frequency is divided by 2.

From IC305a pin 5, the 2.1 MHz is taken and this is the clock frequency for the microcomputer and the

RX synthesizer. The resistor R322 is for pull-up and the ferrite bead FP301 is blocking for higher

harmonics of the clock frequency.

From IC306 pin 8, the 21 MHz is also divided by 10. The frequency is divided by 2 in flip-flop IC305b and

divided by 2 in IC304a. At IC304b pin 13, the 21 MHz is then divided by 40 and the frequency is therefore

525 KHz at which the phase detector is working.

From the other side the mixed signal of 16.8 MHz is led to the binary divider IC301 pin 10 where it is divided

by 16. In flip-flop IC302b the signal is divided by 2. From IC320b pin 8 the 16.8 MHz is then divided by

32 giving 525 KHz.

The phase/frequency detector consists of IC302a, IC303, and IC304b. From IC304b pin 9 the detector

is connected to the loop filter via the pull-up resistor R321 and the diode D303. The phase corrections

pulse from the detector is active low and when the phase locked loop is in lock the duty cycle of the

correction pulse is about 30 per cent.

THE CORRECTION PULSES:

After the count down of the 21 MHz the IC304b pin 9 goes low. Pin 8 goes high and this is connected to

IC303 pin 2. When the 16.8 MHz is divided by 32 the IC302a pin 6 and IC303 pin 1 go high. When IC303

pin 13 is always high the IC303 pin 8 will go low and preset IC302a and IC304b; the correction pulse will

then go high and so on. The propagation delay in the two inserters IC303 determines the size of the preset

pulse.

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Sailor RT2047 Service Manual Page 42