YOKOGAWA TA120E, Руководство пользователя

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1-4
IM 704420-01E
1.2 Measurement Principle
Pulse Width of the 3T Data Signal of a CD
The time shorter than the period of the reference clock is called the fractional time. In
general, since the 3T data signal and the reference clock are not synchronized, fractional
time exists both at the beginning and at the end of measurements. This instrument
generates a “fractional pulse” which is a pulse signal with a period equal to the sum of
the fractional time and a given time period.
If the period of the reference clock and the pulse width of the fractional pulses are taken
to be t0, T
a
, and T
b
, respectively, 3T can be broken into the following terms: integer
multiple of the reference clock, N
×
t
0
, and the pulse width of the fractional pulses, T
a
, T
b
.
3T = N
×
t
0
+ (T
a
– T
b
)
This instrument converts the pulse width (T
a
, T
b
) of the fractional pulse that it generated
at the beginning and end of the measurement to voltage values, which are then
converted to digital values using an 8-bit A/D converter.
3T is determined by substituting the pulse width of the fractional pulses that were
measured into the variables T
a
and T
b
of the above equation.
3T
T
a
T
b
Fractional time
3T data signal of a CD
Reference clock 1
Reference clock 2
Fractional pulse
Time-voltage conversion
3T = N
×
t
0
+ (T
a
– T
b
)
k: Coefficient used in the A/D conversion
A/D conversion A/D conversion
∆
V
a
= k
×
T
a ∆
V
b
= k
×
T
b
1 2
t
0
N
Fractional time
Time Difference between the Data Signal and Clock Signal of a DVD.
Measurement is made using the same principle that is used in “Pulse Width of the 3T
Data Signal of a CD” above. The following points differ:
• The reference clock is either the clock signal that is applied to the clock input
connector or a clock signal that is regenerated by the PLL circuit.
• Tc, the pulse width of the fractional pulse at the beginning of the measurement, is the
D-to-C time difference (that is to be determined) between the data signal and the
clock signal.
T
c
DVD data signal
DVD clock signal
Fractional pulse
Time-voltage conversion
A/D conversion
D-to-C
∆
V
c
= k
×
T
c
D-to-C = Tc
k: Coefficient used in the A/D conversion