Circuit Description
85
CG635 Synthesized Clock Generator
transition times, but terminating the output will reduce the output levels by a factor of
two.
The CMOS output offset and amplitude are controlled by the analog signals
CMOS_OFFS and CMOS_AMPL. The output is biased at the midpoint of the logic
transition by U200. The output may be offset from the midpoint by the CMOS_OFFS
signal, and the output is pulled above the midpoint (or pulled below it) by the outputs of
the fast differential current sink, U203. The ±CMOS_DRV PECL level signals are
terminated by R205 and R206 and drive the inputs to the fast current sink via the
cascode transistors Q200 and Q201.
The amplitude of the current sink is controlled by CMOS_AMPL. U205A servos U203
to provide the correct current by comparing the output current to CMOS_AMPL. When
the clock level is “0”, the OUT- output of U203 sinks current, drawing it from the
cascode transistor U206, pulling the output low. When the clock level is “1”, the OUT+
output of U203 sinks current, drawing it from the cascode transistor U205. The collector
current of Q205 is mirrored by the transistors Q203 and Q204, pulling the output high.
The CMOS output has a source impedance of 50
ȍ
, consisting of the parallel
combination of R250-R252 in parallel with the series combination of R247-R249. The
parallel combination of L200 and R253 match return pulses into the collector
capacitance of Q204 and Q206, providing a high return loss at all frequencies.
Front-Panel Display and Keypad
Schematic sheet “CG_FP1B”
The front panel time-multiplexed display PCB has 13 digits, 34 lamps and 33 keys.
There are seven strobe lines, each of which allows up to two digits and six lamps to be
refreshed and up to six keys to be read during 1 ms intervals. An eighth time interval is
use to intensify/blink one of the 13 digits to indicate which digit would be affected by a
“modify up/down” key press. Details of the refresh operation are provided in the section
“Microcontroller Display and Keypad Scanning.”
Power Supply
Schematic sheet “CG_PS1B”
The modular power supply for the CG635 is in a separate shielded case inside the
instrument. The power supply consists of universal input, 60 W, +24 V, OEM switching
power supply which is “on” whenever power is applied to the unit (whether or not the
power switch is in the “on” position). This supply provides power to the optional
timebase whenever the unit is connected to the mains and provides power to two DC-DC
converters when the front panel power switch is pressed “on”.
One of the DC-DC converters is a switching power supply (U1) that operates at 260 kHz
to provide up to 3 A at +3.3 VDC with an efficiency of about 90 %. The input to this
