Sailor RT2047 Manuel de service télécharger pdf (Page 43)

2 CIRCUIT DESCRIPTION RT2047 DSC - PART II

2.3.4 TX-VCO

The TX-VCO comprises a Field Effect Transistor T311 (oscillator transistor), two coaxial coils L309 and

L310, the capacitors C343 and C345, and a variocapacitor diode D302.

The frequency is mainly determined by L309, L310, C345, and D302. The TX-VCO is a Voltage Controlled

Oscillator, where the control voltage from the loop filter determines the frequency by means of the

variocapacitor diode D302. A high voltage to D302 means a smaller capacitor in D302 and again a higher

VCO frequency. In the opposite way a small control voltage means a smaller frequency.

From drain of transistor T311 the signal is led to a buffer amplifier T310 via a tuned filter consisting of L308,

R342, and C342. The ferrite bead is blocking for UHF oscillating. In the collector of T310 there is a tuned

filter consisting of L307 and C340. From here the signal for two circuits is taken.

From an output on coil L307 the signal to the TX-buffer is taken through capacitor C338. Here the TX-

VCO signal is amplified in transistor T309. In the drain of T309 there is a tuned filter consisting of L306,

C332, C333, R332, and R333, which gives a 50 ohm generator for the Power Amplifier Unit. The output

power from the TX-buffer is 25 mW and is adjusted by trimmer potentiometer R341.

Stop and start of both TX-buffer and TX-VCO are controlled from the microcomputer by controlling the

supply for the transistor. This secures that the frequency is correct before the transmitter is started. From

the collector of T310 the signal for the 16.8 MHz Mixer is taken via capacitor C341.

2.3.5 LOOP-FILTER

The Loop-filter amplifier is a differential amplifier (OP-Amp.) with the inverted input at the base of

transistor T314, where also the phase detector is connected. The base of transistor T313, which is the

non inverted input, is connected to +5V by means of R349 and R350. Transistor T312 is the output stage,

and the control voltage for TX-VCO is taken over the resistor R346 and is filtered in the ripple-filter

consisting of R345, C348, C347, and R344. At the input R351 and C352 are working as a filter against

the higher harmonics in the phase detector pulse.

We assume that the system is in lock. While the detector pulse is low a current is flowing out of the

capacitor C351 and the output voltage over R346 increases. The TX-VCO frequency is also increasing

until the detector pulse goes high again. Because the diode D303 is blocking, a current flows into C351

through the resistors R354 and R355. The control voltage over R346 decreases and the frequency is also

decreasing. The current which is flowing in and out of C351 is equal when the system is in lock. For

principal understanding accept that C353 and R352 have no influence on the current to C351.

If the TX-VCO frequency is too high, the detector pulse gets smaller and that means that more current

is flowing into C351 than out. That means again that the control voltage and the frequency will decrease

until the frequency is correct again.

The opposite process will happen if the frequency is too low. Even though the system is in lock there is

always a little phase error. The AF modulation signal is led into the Loop-filter via resistor R356. The

control voltage is then modulated which will modulate the phase in the TX-VCO again.

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Sailor RT2047 Manuel de service Page 43