Elecraft XG3 RF - Manual (Page 27)

27
Circuit Description
Refer to the schematic diagram on page 28. The RF signal is generated by U1, a CS2100 fractional N clock
synthesizer and multiplier that synthesizes the desired output frequency from a stable, crystal-controlled 10
MHz clock.
The four selectable signal output levels from the XG3 are provided by six pHEMT,GaAs FET IC switches
(U3, U5, U7, U8, U10 and U11). Each switch passes the signal between pin 5 and pin 1 or between pin 5
and pin 3. The U3 and U5 switch pair directs the RF signal through either a -13 dB attenuator formed by
R10, R11 and R12 or through the MAR6 wideband amplifier that provides +20 dB gain. The U7 and U9
switch pair direct the signal through C21 without attenuation or through a -40 dB attenuator formed by
R15, R16, R17, R18 and R19. In the same manner, the U10 and U11 switch pair either pass the signal
through C26 without attenuation or through a -34 dB attenuator formed by R20, R21, R22, R23 and R31.
The MAR6 amplifier and attenuators are switched into the circuit as needed to produce output levels of 0
dBm, -33 dBm, -73 dBm and -107 dBm. The RF output is protected from moderately excessive levels by
D4 and D18.
Power is applied to the MAR6 amplifier only when it is needed to conserve the battery. Q1 is switched on
by Q4 only when the signal is routed to the amplifier.
U2 is a flash microcontroller that provides frequency data to U1 and operates the IC switches to provide the
requested output level. U13, U12 and U9 are inverters to provide the correct control polarity to the V1
inputs of the IC switches.
Connector P1 is used only by the factory for initial programming of the microcontroller.
Firmware is input from an RS232 line via J1. Q2 and Q3 convert the RS232 signal levels at J1 to the proper
levels for the microcontroller.
The LEDs and supply power inputs are shown on page 29. The Band and Level LEDs are driven directly
from the microcontroller outputs shown.
The XG3 may be powered from an external 12V supply or its internal 9V battery. D5 is a 3.6 V Zener
diode that drops the nominal 12V supply to about 8.4 VDC. D3 provides reverse voltage protection in case
the battery or external supply is connected incorrectly. Q5 and Q6 and SW1 control power the XG3. U6 is a
voltage regulator that provides the 3VDC needed by the microcontroller.