Circuit Description
83
CG635 Synthesized Clock Generator
A P-channel MOSFET (U613) provides –5 VDC on the RJ-45 connector. The MOSFET
is turned “off” by U611B if the current seen in the shunt resistors (R637, R638 & R642)
exceeds 375 mA. A co-packaged Schottky diode in U613 prevents the –5 VDC from
being pulled above ground.
RS-232 and GPIB Interfaces
Main Board, Schematic sheet “CG_MB6D”
The TXD and CTS outputs from the microcontroller are converted to RS-232 levels by
U604. The received RS-232 signals, RXD and RTS, are converted to TTL levels for the
microcontroller by U604. U604 also provides an RS-232 high level for the CD and DSR
(carrier-detect and data-set-ready) lines on the RS-232 DB-9 connector (J603).
The GPIB interface (U601) is connected to the rear panel GPIB connector (J602) via the
buffers U602 and U603. Data is read and written to U601 via the bi-directional Port-B.
Other control lines for U601 come from U501. U601 indicates a need for service by
asserting –GPIB (the –IRQ maskable interrupt request) to the microcontroller.
Power Supply Interface
Main Board, Schematic sheet “CG_MB6D”
The CG635 uses a universal input switching power supply in a separate enclosure to
power the system. The power supply has a +24 V output which is “on” whenever the
instrument has line voltage (whether or not the power switch is “on”). This power supply
powers the optional timebase (an OCXO or rubidium) even while the instrument is
“off”. LED lamps on the main circuit board and in the modular power supply indicate
the presence of the +24 V supply. DC to DC inverters, which provide ±15 V, ±5 V, and
+3.3 V to power the instrument, are enabled when the –ENABLE pin on the modular
power supply is pulled to ground by the front panel power button.
A dual comparator (U600) is wire-or’d to alert the microcontroller via –DROPOUT that
the power switch has been turned “off” or that the +24 VDC main power supply is
coming out of regulation. The –DROPOUT asserts an –XIRQ to the microcontroller
which suspends operations until the +24 V recovers.
Front-Panel Output Drivers
To reduce the effect of circuit parasitics, components for the front-panel output drivers
(Q / Q
¯ and CMOS) are located on a small circuit board directly behind the output BNCs.
The PCB and BNC block are secured to the front panel by two screws which hold the
connector block to the front panel. To remove the output drivers you must remove these
two screws, and the four side-panel screws, and tilt the front panel forward. The output
driver block may then be removed vertically.
A twenty-pin connector (J604 on the main PCB and J100 on the driver PCB) passes the
clock signals, amplitude and offset control voltages and the power supplies from the
main PCB to the driver PCB. The connectors are positioned so as to reduce the path
length for the fast signals.
