Sailor RE2100 Technical Manual page 124

5 CIRCUIT DESCRIPTION AND SCHEMATIC DIAGRAMS RE2100
MODE SELECTOR
Only two bits from the shift register U07 (pin 15 and pin 1) are used to control the mode selector U08 to
select the right combinations of carrier level and modulation level for the mode in question. The outputs
from U08 are forward biasing or reverse biasing the switch diodes in the carrier level amplifier and in the
modulation level amplifier.
CARRIER INSERTION AND RF LIMITER
The SSB signal from the crystal filter FL01 is led to transistor Q11. The wanted amount of carrier signal
from transistor Q10 is added to the modulated SSB signal on the collector of Q11. The output from the
transformer TR02 is connected to a peak to peak RF limiter network, consisting of the diodes D19 and
D20 biased by the zener diode D21. The signal from TR02 is led to the buffer amplifier Q19 at the input
to the step attenuator.
STEP ATTENUATOR AND 1st MIXER
The step attenuator is divided in two parts. Part one consisting of the transistors Q20, Q21, Q22 and Q23
is divided in steps of 0.3 dB, and part two, consisting of the transistors Q14, Q15, and Q16 is divided in
steps of 4.5 dB.
Part one of the step attenuator is working as a voltage divider, where the SSB signal on the emitter of Q19
is divided by R148 and the parallel connection of the resistors R151, R153, R173, and R175, depending
on which one is grounded.
The grounding of the resistors in this attenuator takes place in normal binary code, meaning (0000) is
equal to non attenuation and (1111) is equal to full attenuation.
In part two of the attenuator the voltage gain of transistor Q12 will be changed according to the wanted
attenuation step. From attenuator step 16, R130 will be grounded, and from attenuator step 48, R113,
R140, and R145 will be grounded. The wanted attenuator step is controlled by U09, which converts the
received input serial code to an output parallel code which controls the grounding of the transistors in the
attenuator.
The gain in the amplifier Q12 is adjusted with R120. The potentiometer R159 on the basis of the buffer
transistor Q18 is used to adjust the attenuation at step 16 to be 0.3 dB higher than the attenuation at step
15.
The output from Q12 is in U10 mixed with the 1st local oscillator to give an intermediate frequency of 70
MHz. If the local oscillator frequency is 80.7315 MHz, the upper sideband USB is selected. If the local
oscillator frequency is 59.2685 MHz, the lower sideband LSB is selected.
70 MHZ IF, 2nd MIXER AND LO BUFFER
The output from the mixer U10 is led to the tuned amplifier Q13. The 70 MHz IF filter is consisting of two
monolithic crystal filters FL02 and FL03. From the filter the signal is led to the amplifier Q17 and to the
2nd mixer U11.
Because the 2nd mixer U11 is a high level mixer, it is necessary to amplify the 2nd local oscillator signal.
This is done in the LO buffer amplifier Q24
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