Oscillators – Relaxation oscillators
Reexamination Certificate
2000-10-03
2002-11-26
Kinkead, Arnold (Department: 2817)
Oscillators
Relaxation oscillators
C331S111000, C331S175000, C331S185000, C331S186000
Reexamination Certificate
active
06486745
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to a voltage controlled oscillator circuit and, more particularly, to an adjustable voltage controlled oscillator circuit, such as is employed in a mass air flow meter, for converting a voltage input to a frequency output.
BACKGROUND OF THE INVENTION
Referring to
FIG. 1
, an air flow meter
10
is shown positioned at the output of an air cleaner
12
and at the intake side of a throttle
14
for measuring the amount of air passing into an internal combustion engine
16
, such as for use in an automotive vehicle. The air flow meter generates a voltage signal proportional to the mass air flow. The internal combustion engine
16
is generally controlled by an engine control unit (ECU)
18
. The air flow meter
10
includes a voltage controlled oscillator (VCO) circuit
20
for converting the voltage generated internally by the air flow meter
12
to a frequency signal f for processing by the ECU
18
. Accordingly, the air flow meter
10
, with VCO circuit
20
, allows for measurement of the amount of air passing into the internal combustion engine
16
to allow the ECU
18
to control the amount of fuel being injected so as to regulate the air-to-fuel ratio in the combustion chamber.
A conventional voltage controlled oscillator circuit
20
for use in the air flow meter
10
is illustrated in FIG.
2
. The VCO circuit
20
includes an operational amplifier
28
having a negative terminal (−) coupled to a voltage input V
B
via a series of multiple external resistors, identified herein by resistor
24
. The positive terminal (+) of the operational amplifier
28
is coupled to reference voltage input V
REF
. A capacitor
30
is coupled between the output
26
and the negative terminal of amplifier
28
. The operational amplifier
28
, resistor
24
, and capacitor
30
form an integrator that generates a ramp voltage, identified as RAMP. In addition, the conventional VCO circuit
20
includes a comparator
32
that compares the output
26
of amplifier
28
to +1 volt, and provides an output to an inverter
34
. The inverter output is connected to the enable input of a oneshot monostable multivibrator circuit
36
which, in turn, generates a frequency output signal f at output
44
. The oneshot circuit
36
is coupled to capacitor
40
and resistor
38
, both of which are coupled to ground. The negative terminal of the amplifier
28
is further coupled to a current source
46
via switch
42
.
The current through the resistor
24
is referred to herein as I
IN
. When the ramp voltage is below one volt, the comparator output is low. When the ramp voltage is above one volt, the comparator output is high. The comparator output is applied through the inverter
34
to the oneshot circuit
30
. The oneshot circuit
36
generates a positive pulse when the input thereto asserts a positive signal. When the output frequency signal at output
44
is high, the current I
S
generated by current source
46
is applied to the negative terminal of the amplifier
28
, thereby causing the output
26
of the amplifier
28
to ramp with a positive slope. The negative slope of ramp signal increases with increased input voltage V
B
to change the frequency output.
One example of the ramp voltage (RAMP) at amplifier output
26
and output frequency signal
44
generated by the conventional VCO circuit
20
is illustrated in FIG.
3
. The ramp voltage decreases in amplitude on negative slope
52
until output signal
44
is pulsed high. This occurs when the ramp voltage crosses below one volt at which time the current source I
S
is applied to the negative terminal of the amplifier
28
, and ramp voltage begins to ramp up on positive slope
50
. The positive slope
50
of the ramp voltage is maintained until the output signal
44
asserts low. At that point, the ramp voltage slopes negative again and one period T of oscillation is complete.
The oscillation period (T=1/f) generally is inversely proportional to the input current I
IN
, and is proportional to the capacitor
40
in the oneshot circuit
36
. The VCO circuit
20
is generally stable, with two elements primarily factoring into the frequency equation: the external input resistor
24
; and the external oneshot capacitor
40
. However, the conventional VCO circuit
20
suffers from a number of drawbacks that limit overall accuracy and linearity. First, the operational amplifier
28
has a finite bandwidth and slew rate. There exists a rapid change in the ramp voltage when the oneshot pulse is asserted and de-asserted. This generally causes errors in the ramp voltage and is a source of overshoot
48
and rounding errors. These errors generally result in non-linearity in the voltage-to-frequency relationship. Secondly, the rapid changes in the amplifier output generally require significant current from a regulated voltage supply. This increases the complexity of the regulator design and increases the regulator output noise. Third, the oscillator frequency typically can only be modified by adjusting the value of the external resistor
24
or the value of reference voltage V
REF
which is typically generated by an external voltage source and a resistor divider network. The standard approach for adjusting an external resistor has been to perform laser trimming which is generally expensive and is implemented when the device is not under flow. This requires an independent measurement and trim process in the manufacturing flow which are costly and limit accuracy of the overall response. Finally, the VCO circuit output is not at a fixed duty cycle. This requires the response time of the engine control unit monitoring the frequency to be as fast as the narrowest pulse possible out of the VCO circuit, rather than half of the minimum. This can be a problem, particularly at high input voltages V
B
.
Accordingly, it is therefore desirable to provide for a voltage controlled oscillator circuit with reduced error signals. It is also desirable to provide for a voltage controlled oscillator circuit that can be easily adjusted without requiring laser trimming. It is further desired to provide for a voltage controlled oscillator circuit that reduces the amount of current supply required from the regulated supply. Yet, it is further desirable to provide for a voltage controlled oscillator circuit having a fifty percent duty cycle.
SUMMARY OF THE INVENTION
The present invention provides for an adjustable voltage controlled oscillator circuit for converting a voltage to a frequency. The voltage controlled oscillator comprises an input for receiving a voltage input signal, an integrator coupled to the input for receiving the voltage input signal and generating a ramp signal. According to one aspect of the present invention, the circuit also includes an adjustable current supply coupled to the integrator for supplying an adjustable amount of current. A comparator compares the ramp signal with a predetermined voltage. The circuit further includes an output for generating a frequency output signal as a function of the comparison, wherein the oscillator circuit is calibratible by adjusting current generated by the adjustable current supply. According to another aspect of the present invention, a first current source supplies current to the input of the integrator, and a second current source supplies current to the output of the integrator.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.
REFERENCES:
patent: 5394020 (1995-02-01), Nienaber
patent: 5811999 (1998-09-01), Hall et al.
Hazelton Lawrence D.
Kearney Mark B.
Manlove Gregory J.
Delphi Technologies Inc.
Funke Jimmy L.
Kinkead Arnold
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