Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
1999-08-27
2001-01-09
Mis, David (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S143000, C331S175000, C331S17700V
Reexamination Certificate
active
06172573
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an oscillator having a Schmitt trigger. More specifically, the invention relates to an oscillator having compensation for a response delay of the Schmitt trigger.
2. Description of Related Technology
FIG. 1
illustrates a schematic diagram of a conventional Schmitt trigger oscillator
10
. The conventional oscillator
10
includes a charging current source I
1
, a discharging current source I
2
, a Schmitt trigger
12
, and a capacitor C
1
, all coupled together as shown. The oscillator
10
provides a square wave output signal Vo
2
having a frequency based on the currents supplied by the current sources I
1
, I
2
, the value of the capacitor C
1
, and the logic thresholds of the Schmitt trigger
12
.
In general, the charging current source I
1
is always on and draws power from a supply Vcc to charge the capacitor C
1
with a charging current. When the discharging current source I
2
is off, the charging current flows into the capacitor C
1
and a charge/discharge voltage Vo
1
increases linearly. When the charge/discharge voltage Vo
1
crosses a high logic or charge threshold voltage of the Schmitt trigger
12
, the oscillator output signal Vo
2
transitions to a high level, which turns on the discharging current source I
2
. Because the discharging current source I
2
draws more current than supplied by the charging current source I
1
, a net charge is removed from the capacitor C
1
and the charge/discharge voltage Vo
1
across the capacitor C
1
decreases. When the charge/discharge voltage Vo
1
crosses a low logic or discharge threshold voltage, which is lower than the charge threshold voltage, the output of the Schmitt trigger
12
transitions to a low level, thereby deactivating the discharging current source I
2
to begin the charge/discharge cycle again.
FIG. 2
illustrates idealized graphical representations of the input and output signals associated with the Schmitt trigger
12
of FIG.
1
. In particular, detail (a) shows the charge/discharge voltage Vo
1
, which is coupled to the input of the Schmitt trigger
12
. As shown by detail (a), the charging period T
1
is a function of the charging current and the voltage difference between the charge threshold voltage VB and the discharge threshold voltage VA. Similarly, the discharge period T
2
is a function of the net discharge current (I
2
−I
1
) divided by the value of the capacitor C
1
and the voltage difference between the charge threshold voltage VB and the discharge threshold voltage VA.
Detail (b) of
FIG. 2
shows the continuous square wave output signal Vo
2
of the Schmitt trigger
12
. As shown by detail (b), while the oscillator output signal Vo
2
is at a high level, the discharging current source I
2
is on and the capacitor C
1
is discharging. Additionally, while the oscillator output signal Vo
2
is at a low level, the discharging current source I
2
is off and the capacitor C
1
is charged via the charging current source I
1
.
FIG. 3
illustrates graphical representations of the charge/discharge voltage Vo
1
and the oscillator output signal Vo
2
as affected by a charge response delay Td of the Schmitt trigger
12
used in the oscillator
10
of FIG.
1
. The charge response delay Td of the Schmitt trigger
12
occurs as the output of the Schmitt trigger
12
transitions between charge and discharge modes of operation. As a result of the response delay Td, the minimum charge voltage of the capacitor C
1
undershoots the discharge threshold voltage VA of the Schmitt trigger
12
, and the maximum charge voltage of the capacitor C
1
overshoots the charge threshold VB of the Schmitt trigger
12
. Because the effective charge and discharge threshold voltages have moved apart to VY and VX, respectively, the period of the charge/discharge voltage Vo
1
increases and the frequency decreases.
Detail (b) of
FIG. 3
shows the oscillator output signal Vo
2
that results from the non-ideal charge/discharge voltage Vo
1
shown in detail (a). As shown by detail (b) of
FIG. 3
, the interval during which the oscillator output signal Vo
2
is at a low level increases to T
1
′ and the interval during which the oscillator output signal Vo
2
is at a high level increases to T
2
′. The charge response delay Td causes the charging of the capacitor C
1
to terminate at the higher voltage VY, which exceeds the ideal charge threshold voltage VB by an amount equal to the charging rate I
1
/C
1
multiplied by the response delay time Td. Similarly, the charge response delay Td causes the discharging of the capacitor C
1
to terminate at the lower voltage VX, which is less than the ideal discharge threshold voltage VA by an amount equal to the discharge rate multiplied by the response delay time Td.
The charging interval T
1
′, during which the discharging current source I
2
is off, and the discharge interval T
2
′, during which the discharging current source I
2
is on, can be expressed as a function of the response delay time Td, the charging current I
1
, and the discharging current I
2
, as shown in Equations 1 and 2 below.
T1
′
=
T1
+
Td
(
1
+
I2
-
I1
I1
)
Equation
1
T2
′
=
T2
+
Td
(
1
+
I1
I2
-
I1
)
Equation
2
For Equations 1 and 2 above, the value of I
2
is assumed to be greater than I
1
so that the discharging current source I
2
can draw a net charge away from the capacitor C
1
while the charging current source I
1
is on. As can be seen from Equations 1 and 1, as the value of I
2
increases for a given value of I
1
the value of T
1
′ increases rapidly to exceed T
1
+Td and the value of T
2
′ approaches T
2
+Td. As Equations 1 and 2 demonstrate, even small variations in the response delay time Td of the Schmitt trigger
12
can result in large variations in the charging interval T
1
′, particularly where the discharge rate is relatively high compared to the charge rate. These variations in the charging interval become problematic when using the above-described Schmitt trigger oscillator
10
as a radio frequency oscillator. Furthermore, these problems are compounded significantly in radio frequency oscillator applications requiring a relatively large duty cycle because the response delay time Td has a proportionally larger impact on the control of the charging interval as the desired charging interval time decreases.
SUMMARY OF THE INVENTION
Generally, the invention provides an oscillator having compensation for the response delays of a Schmitt trigger so that output signal of the Schmitt trigger oscillator is not substantially affected by the response delays. The oscillator may include a first current source coupled to a supply voltage and adapted to produce a charging current, a charge current control unit coupled to the first current source and a ground potential, and a charge and discharge unit coupled to the charge current control unit. The oscillator may further include a discharge current control unit coupled to the charge and discharge unit, a second current source unit coupled to the discharge current control unit and the ground potential and adapted to produce a discharge current, and a Schmitt trigger circuit coupled to the supply voltage, the charge current control unit, the discharge unit control unit, the second current source, and the charge and discharge unit.
The Schmitt trigger circuit may be adapted to receive a charge/discharge voltage from the charge and discharge unit and to generate an output signal therefrom. The Schmitt trigger circuit may additionally provide a charge threshold voltage to the charge control unit. The charge current control unit may be adapted to compare the charge/discharge voltage to the charge threshold voltage and, based on the comparison, to divert the charging current from the charge and discharge unit. The Schmitt trigger circuit may additionally provide a discharge threshold voltage to the discharge control unit and the discharge current control may be adapted to comp
Fairchild Korea Semiconductor Ltd.
Marshall O'Toole Gerstein Murray & Borun
Mis David
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