Transmission line for high-frequency clock

Details Transmission line for high-frequency clock Transmission line for high-frequency clock

1997-09-22

2002-07-16

Bocure, Tesfaldet (Department: 2631)

Pulse or digital communications

Cable systems and components

Reexamination Certificate

active

06421391

ABSTRACT:

The invention concerns a transmission line for delivery of high-frequency clock signals.
BACKGROUND OF THE INVENTION
FIG. 1
illustrates a clock
3
which feeds a clock signal to a transmission line
6
, which may take the form of a trace on a printed circuit board (not shown). The trace
6
leads to a clocked device
9
, which uses the clock signal as an input.
At low clock frequencies, the apparatus shown encounters no significant problems. For example, when the line
6
is 15.12 inches long, and the clock runs at 33 MHz, clock signal
12
delivered to the clocked device
9
was found to be acceptable, as indicated.
However, if the clock frequency is increased, problems can be encountered. For example, if the clock signal is increased to 66 MHz, as in
FIG. 2
, then the clock signal
16
, measured at the input
17
of device
9
A, was found to be unacceptable, as indicated.
FIG. 3
illustrates specific defects found in the clock signal.
When the clock signal was supposed to be LOW, as in region
18
in
FIG. 3
, it failed to remain below the upper limit
20
of the LOW region, as excursions
23
and
26
indicate. When the clock signal was supposed to be HIGH, as in region
29
, it failed to remain above the lower limit
33
of the HIGH region, as excursions
36
and
39
indicate.
These defects render the clock signal
16
unacceptable for many purposes.
OBJECTS OF THE INVENTION
An object of the invention is to provide an improved clock signal in a digital system.
SUMMARY OF THE INVENTION
In one form of the invention, the transmission line carrying the clock signals is lengthened, in order to cause the reflected impedance of the clocked device, seen by the clock, to be both high and capacitive. For a clocked device having an input capacitance of about 5 to 20 pico-Farads, a line length of about ½ wavelength of the clock pulses provides satisfactory results.


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