Oscillator having compensation for a schmitt trigger response delay

An oscillator having compensation for the response delays of a Schmitt trigger has a first current source that provides a charging current to a capacitor and a second current source for providing a discharging current to the capacitor. The oscillator includes a Schmitt trigger circuit that receives a charge/discharge voltage from the capacitor and generates a square wave oscillator output signal therefrom. The oscillator further includes charge and discharge current control units that compare the charge/discharge voltage from the capacitor to respective charge and discharge threshold voltages. Based on the comparison, the control units divert the flow of charging and discharging current from the capacitor during the response delays of the Schmitt trigger circuit.

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 I1, a discharging current source I2, a Schmitt trigger 12, and a
 capacitor C1, all coupled together as shown. The oscillator 10 provides a
 square wave output signal Vo2 having a frequency based on the currents
 supplied by the current sources I1, I2, the value of the capacitor C1, and
 the logic thresholds of the Schmitt trigger 12.
 In general, the charging current source I1 is always on and draws power
 from a supply Vcc to charge the capacitor C1 with a charging current. When
 the discharging current source I2 is off, the charging current flows into
 the capacitor C1 and a charge/discharge voltage Vo1 increases linearly.
 When the charge/discharge voltage Vo1 crosses a high logic or charge
 threshold voltage of the Schmitt trigger 12, the oscillator output signal
 Vo2 transitions to a high level, which turns on the discharging current
 source I2. Because the discharging current source I2 draws more current
 than supplied by the charging current source I1, a net charge is removed
 from the capacitor C1 and the charge/discharge voltage Vo1 across the
 capacitor C1 decreases. When the charge/discharge voltage Vo1 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 I2 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 Vo1, which is
 coupled to the input of the Schmitt trigger 12. As shown by detail (a),
 the charging period T1 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 T2 is a
 function of the net discharge current (I2-I1) divided by the value of the
 capacitor C1 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 Vo2 of
 the Schmitt trigger 12. As shown by detail (b), while the oscillator
 output signal Vo2 is at a high level, the discharging current source I2 is
 on and the capacitor C1 is discharging. Additionally, while the oscillator
 output signal Vo2 is at a low level, the discharging current source I2 is
 off and the capacitor C1 is charged via the charging current source I1.
 FIG. 3 illustrates graphical representations of the charge/discharge
 voltage Vo1 and the oscillator output signal Vo2 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 C1 undershoots the discharge
 threshold voltage VA of the Schmitt trigger 12, and the maximum charge
 voltage of the capacitor C1 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 Vo1 increases and the frequency decreases.
 Detail (b) of FIG. 3 shows the oscillator output signal Vo2 that results
 from the non-ideal charge/discharge voltage Vo1 shown in detail (a). As
 shown by detail (b) of FIG. 3, the interval during which the oscillator
 output signal Vo2 is at a low level increases to T1' and the interval
 during which the oscillator output signal Vo2 is at a high level increases
 to T2'. The charge response delay Td causes the charging of the capacitor
 C1 to terminate at the higher voltage VY, which exceeds the ideal charge
 threshold voltage VB by an amount equal to the charging rate I1/C1
 multiplied by the response delay time Td. Similarly, the charge response
 delay Td causes the discharging of the capacitor C1 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 T1', during which the discharging current source I2
 is off, and the discharge interval T2', during which the discharging
 current source I2 is on, can be expressed as a function of the response
 delay time Td, the charging current I1, and the discharging current I2, as
 shown in Equations 1 and 2 below.
 ##EQU1##
 For Equations 1 and 2 above, the value of I2 is assumed to be greater than
 I1 so that the discharging current source I2 can draw a net charge away
 from the capacitor C1 while the charging current source I1 is on. As can
 be seen from Equations 1 and 1, as the value of I2 increases for a given
 value of I1 the value of T1' increases rapidly to exceed T1+Td and the
 value of T2' approaches T2+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 T1', 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
 compare the charge/discharge voltage to the discharge threshold and, based
 on the comparison, to divert the discharge current from the charge and
 discharge unit.
 The invention itself, together with further objects and attendant
 advantages, will best be understood by reference to the following detailed
 description, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 4 is a block diagram of an oscillator 90 according to the invention
 having compensation for the response delays of a Schmitt trigger. The
 oscillator 90 includes a charge current source unit 100, a charge current
 control unit 200, a charge and discharge unit 300, a discharge current
 source unit 400, a discharge current control unit 500, and a Schmitt
 trigger circuit unit 600.
 The charge current source unit 100 is coupled to a supply voltage Vcc and
 supplies a continuous current I1 to the charge current control unit 200
 when the supply voltage Vcc is present. The charge current control unit
 200 receives the current I1 from the charge current source unit 100 and,
 as described in more detail below, routes the current I1 to the charge and
 discharge unit 300 or shunts it to a ground potential 202. The charge and
 discharge unit 300 generates a charge/discharge voltage Vo1, which is
 provided to the charge current control unit 200 and the Schmitt trigger
 circuit unit 600. The discharge current source unit 400, as described in
 more detail below, periodically generates a current I2 in response to
 signals received from the Schmitt trigger circuit unit 600. The discharge
 current control unit 500 draws the current I2 through the charge and
 discharge unit 300 or from the supply Vcc. The Schmitt trigger circuit
 unit 600 receives power from the supply Vcc and generates an oscillator
 output signal Vo2 in response to the charge/discharge voltage Vo1. The
 Schmitt trigger circuit unit 600 provides a charge threshold voltage VB to
 the charge current control unit 300, a discharge threshold voltage VA to
 the discharge current control unit 500, and a control voltage Vcon to the
 discharge current source unit 400.
 In operation, the charge and discharge unit 300 is initially discharged so
 that the charge/discharge voltage Vo1 is at a level associated with the
 discharged condition (e.g., zero volts). Also, initially, the oscillator
 output signal Vo2 of the Schmitt trigger circuit unit 600 and the control
 signal Vcon are at a low level so that the discharge current source unit
 400 is off. While the level of the charge/discharge voltage Vo1 is less
 than the charge threshold voltage VB, the charge control unit 300 routes
 the current I1 from the charge current source unit 100 to the charge and
 discharge unit 300, and the charge/discharge voltage Vo1 increases
 linearly.
 When the charge/discharge voltage Vo1 crosses the charge threshold voltage
 VB, the charge current control unit 200 diverts the current I1 away from
 the charge and discharge unit 300 and shunts it to the ground potential
 202 so that the charge/discharge voltage Vo1 stops increasing.
 Additionally, following a charge response delay Td, the output of the
 Schmitt trigger circuit unit 600 and the control signal Vcon transition to
 a high level. The discharge current source unit 400 is turned on by the
 high level control signal Vcon and draws the current I2 through the
 discharge current control unit 500.
 While the charge/discharge voltage Vo1 is greater than the discharge
 threshold voltage VA, the discharge current control unit 500 draws current
 I2 from the charge and discharge unit 300, thereby decreasing the
 charge/discharge voltage Vo1. When the charge/discharge voltage Vo1 falls
 below the charge threshold voltage VB, the charge current control unit 200
 routes the current I1 to the charge and discharge unit 300; however,
 because the current I2 is greater than the current I1, a net current I2-I1
 is removed from the charge and discharge unit 300, which causes the
 charge/discharge voltage Vo1 to decrease linearly.
 When the charge/discharge voltage Vo1 falls below the discharge threshold
 voltage VA, the discharge current control unit 500 stops drawing current
 I2 from the charge and discharge unit 300 and begins to draw the current
 I2 from the supply Vcc. After the response delay Td, the oscillator output
 signal Vo2 of the Schmitt trigger circuit unit 600 and the control signal
 Vcon both transition to a low level. The low level control signal Vcon
 turns off the discharge current source unit 400 so that the current I1
 flowing into the charge and discharge unit 300 causes the charge/discharge
 voltage Vo1 to increase linearly, thereby re-starting the above-described
 charge/discharge cycle. The charge/discharge cycle repeats continuously so
 that the oscillator output signal Vo2 provides a continuous square wave
 signal having a low-level portion consistent with the charge interval and
 a high level portion consistent with the discharge interval.
 FIG. 5 is an exemplary schematic diagram of the oscillator 90 of FIG. 4.
 The charge current control unit 200 includes PNP transistors Q1 and Q2
 connected in a differential (i.e., emitter coupled) configuration. The
 emitters of Q1 and Q2 receive the current I1 from the charge current
 source unit 100, which is represented as a constant current source IS1
 that draws power from the supply Vcc. The collector of Q1 is connected to
 the charge and discharge unit 300, which is a capacitor C1, and the
 collector of Q2 is connected to the ground potential 202. The base
 terminal of Q2 receives the charge threshold voltage VB from the Schmitt
 trigger circuit unit 600.
 The discharge current control unit 500 includes NPN transistors Q3 and Q4
 connected in a differential configuration. The collector and base
 terminals of Q3 are connected to the capacitor C1 of the charge and
 discharge unit 300 and the collector terminal of Q4 is connected to the
 supply voltage Vcc. The emitters of Q3 and Q4 are both connected to the
 discharge current source unit 400, and the base of Q4 is connected to the
 discharge threshold voltage VA from the Schmitt trigger circuit unit 600.
 The discharge current source unit 400 includes a current source IS2 that
 generates a current I2 and further includes a switch SW that is turned
 on/off (i.e., closed/open) based on the control signal Vcon from the
 Schmitt trigger circuit unit 600.
 The Schmitt trigger circuit unit 600 includes resistors R1-R3, first and
 second comparators Comp1, Comp2, and an RS flip-flop FF, coupled together
 as shown. The resistors R1-R3 form a voltage divider network between the
 supply voltage Vcc and the ground potential 202, thereby generating the
 charge threshold voltage VB and the discharge threshold voltage VA at the
 nodes formed by R1/R2 and R2/R3, respectively. The first comparator Comp1
 receives the charge threshold voltage VB at an inverting input, receives
 the charge/discharge voltage Vo1 at a non-inverting input, and provides an
 output based on the comparison to a set input S of the flip-flop FF. The
 second comparator Comp2 receives the discharge threshold voltage VA at a
 non-inverting terminal, receives the charge/discharge voltage Vo1 at an
 inverting input, and provides an output based on the comparison to a reset
 input R of the flip-flop FF. The output Q of the flip-flop FF is used as
 the control signal Vcon and the oscillator output signal Vo2.
 In operation, the capacitor C1 is initially discharged so that the
 charge/discharge voltage Vo1 is substantially near zero volts, the output
 of the second comparator Comp2 is at a high level, the output Q of the
 flip-flop is at a low level, which causes the control voltage Vcon and the
 oscillator output signal Vo2 to both be at a low level, and the switch SW
 is off, which causes transistors Q3 and Q4 to both be off. Also,
 initially, because the charge/discharge voltage Vo1 is less than the
 charge threshold voltage VB, transistor Q1 is on and transistor Q2 is off
 so that the current I1 provided by the current source IS1 flows into the
 capacitor C1.
 As the current I1 flows into C1, the voltage on C1 increases linearly. When
 the charge/discharge voltage Vo1 exceeds the discharge voltage threshold
 VA the output of the second comparator Comp2 transitions to a low level,
 thereby providing a low level input to the R input of the flip-flop FF. As
 the voltage Vo1 approaches the charge threshold voltage VB, transistor Q1
 begins to turn off and transistor Q2 begins to turn on. When the
 charge/discharge voltage Vo1 exceeds the charge threshold voltage VB, Q1
 turns off and Q2 turns on to shunt the current I1 to the ground potential
 202, thereby preventing further current from flowing into capacitor C1 so
 that the charge/discharge voltage Vo1 stops increasing. Additionally, when
 the charge/discharge voltage Vo1 exceeds the charge threshold VB, after a
 response delay, the output of the first comparator Comp1 transitions to a
 high level. The high level output of the first comparator Comp1 is
 provided to the S input of the flip-flop FF, which causes the output Q of
 the flip-flop FF, the control voltage Vcon, and the oscillator output Vo2
 to all transition to a high level.
 The high level control signal Vcon causes the switch SW to close, which
 provides a current path for the second current source IS2 to turn on
 transistor Q3 and to draw the current I2 through Q3. Thus, Q3 begins to
 draw current from C1, which causes the charge/discharge voltage Vo1 to
 decrease. Although Q1 is momentarily off when Q3 begins to draw current
 from C1, as the charge/discharge voltage Vo1 falls below the charge
 threshold VB, Q1 turns on to supply the current I1. However, the current
 I2 drawn by the second current source IS2 is greater than the current I1
 so that a net current I2-I1 is removed from C1, thereby causing the
 charge/discharge voltage Vo1 across C1 to decrease linearly.
 When the charge/discharge voltage Vo1 approaches the discharge threshold
 VA, the transistor Q3 begins to turn off and Q4 begins to turn on so that
 current is no longer removed from C1 and the charge/discharge voltage Vo1
 stops decreasing. Additionally, when Vo1 falls below VA, after a response
 delay, the second comparator Comp2 transitions to a high level output,
 which drives the reset input R of the flip-flop FF high so that the
 flip-flop output Q and the oscillator output signal Vo2 transition to a
 low level. The low level control signal Vcon causes the switch SW to open,
 which disables Q3 and Q4 to prevent additional current from being drawn
 from C1. The above described charge/discharge cycle repeats continuously
 so that the oscillator output signal Vo2 is a square wave type waveform.
 In contrast to conventional Schmitt trigger oscillators, the oscillator 90
 uses transistors Q1-Q4 to divert the flow of charging and discharging
 currents so that they do not flow into or out of C1 during the response
 delay time of the Schmitt trigger circuit 600. Thus, the circuit shown in
 FIG. 5 prevents the charge/discharge voltage Vo1 from substantially
 exceeding the charge threshold voltage VB and from falling substantially
 below the discharge threshold voltage VA.
 FIG. 6 illustrates the input and output waveforms associated with the
 oscillator 90 of FIGS. 3 and 4. Detail (a) shows the charge/discharge
 voltage Vo1 and detail (b) shows the oscillator output signal Vo2. Details
 (c) and (d) highlight the shape of the charge/discharge voltage Vo1 at the
 point in time where transistors Q2 and Q4 are being turned on to shunt
 current away from C1 to hold the voltage on C1 constant during the
 response delay intervals Td1 and Td2.
 As shown in FIG. 6, the charge and discharge periods are increased by an
 amount equal to the response delay times Td1 and Td2 of the Schmitt
 trigger circuit 600 and are not affected by the relative sizes of the
 charge and discharge currents. Thus, the above-described invention
 provides a predictable error in the form of a constant delay for the
 charge and discharge intervals, which facilitates radio frequency
 oscillator applications that requiring accurate frequency characteristics.
 A range of changes and modifications can be made to the preferred
 embodiment described above. The foregoing detailed description should be
 regarded as illustrative rather than limiting and the following claims,
 including all equivalents, are intended to define the scope of the
 invention.