Operation 16
CG635 Synthesized Clock Generator
example, to initialize the instrument to its default settings, you would press ‘SHIFT’,
‘INIT’, ‘Hz’ sequentially.
Detailed descriptions of each of the secondary functions can be found later in this
chapter.
Q and Q
¯ Outputs
The Q and Q
¯ outputs on the front panel are high-speed, differential, ECL-compatible
drivers that operate from DC to 2.05 GHz with a nominal 50 % duty cycle. The rise and
fall times of these outputs are <100 ps. The outputs provide the user with fast,
complementary voltages at the selected frequency, amplitude, and offset. To operate at
specification, BOTH outputs should be terminated into 50
ȍ
, even if only one output is
used.
The user can easily switch between five standard output voltage levels by pressing the Q /
Q
¯
Ÿ
and
ź
keys in the OUTPUT LEVELS section of the front panel. When the Q / Q
¯
outputs are at a standard level, the appropriate standard level LED will be highlighted.
The meaning of the five standard levels is summarized in Table 7.
Table 7: Q / Q
¯ Standard Output Levels
Label Description
V
HIGH
(V)
V
LOW
(V)
PECL5V
ECL run on +5 VDC supply
4.00
3.20
PECL3.3V
ECL run on +3.3 VDC supply
2.30
1.50
LVDS
Low voltage differential signaling 1.43
1.07
+7 dBm
1 V
pp
with 0.0 VDC offset
0.50
–0.50
ECL
ECL run on negative supply
–1.00
–1.80
V
HIGH
and V
LOW
indicate the voltage driven by the Q and Q
¯ outputs for the high and low
logic levels, respectively.
Various ECL logic families have different logic thresholds that may vary with
temperature. The ECL levels in the table above were chosen to lie between the levels for
the 10 k and 100 k ECL logic families when operated at 25 ºC. The differences are small:
at 25 ºC the typical V
HIGH
for an ECL part run off a negative supply is -0.945 V (for the
10 k series) and –1.020 V (for the 100 k series), while V
LOW
is –1.745 V (for the 10 k
series) and –1.820 V (for the 100 k series). As seen in Table 7, the CG635 will provide a
V
HIGH
of –1.00 V and a V
LOW
of –1.80 V.
The user also has the ability to set the Q / Q
¯ outputs to nonstandard levels. When the
outputs differ from the standard levels, the Q / Q
¯ VAR LED in the OUTPUT LEVELS
section will turn on. In this case, the highlighted standard LED indicates the standard
level nearest the current output levels. Pressing the Q / Q
¯
Ÿ
and
ź
keys when the VAR
LED is on will force the outputs to the nearest standard level in the direction indicated by
the key.
The voltages for Q / Q
¯ high and low states can be viewed in the main display by pressing
‘Q/Q
¯ HIGH’ and ‘Q/Q
¯ LOW’ keys in the DISPLAY section of the front panel.
