Device with automatic compensation of an ac attenuator
First Claim
1. Voltage measure-apparatus having an attenuator automatically compensated for frequency response comprising:
- a first attenuator having an input and an output referenced to a signal ground, the first attenduator itself comprising;
an input section connected between the input and the output of the first attenuator;
(ii) a grounded section connected between the output of the first attenuator and the signal ground; and
(iii) a virtual trimmer impedance connected between the output of the first attenuator and the signal output of the variable gain amplifier means recited below;
means coupled to the output of the first attenuator for producing a measurement of the value of an AC voltage applied to the input of the first attenuator;
variable gain amplification means having a signal input coupled to the outptu of the first attenuator, having a gain control input coupled to the processing means recited below and also having a signal output;
means for applying to the input of the first attenuator a step voltage having a transition in the direction of a selected polarity;
sampling means coupled to the output of the first attenuator for making a plurality of samples subsequent to the transition in and during the ensuing duration of the step voltage applied to the first attenuator andprocessing means, having an input coupled to receive the samples made by the sampling means and having an output coupled to the gain control input of the variable gain amplification menas, for varying the effective value of the virtual trimmer impedance by specifying the variable gain in accordance with the values of the individual samples within the plurality thereof, thereby compensating the frequency response of the first attenuator.
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Accused Products
Abstract
An ac measuring instrument automatically compensates an input attenuator to achieve precision frequency response. The output of the input attenuator is applied to both the conventional measurement portion of the instrument and to an amplifier of variable gain. The output of the variable gain amplifier drives either a resistive or capacitive impedance coupled to the output of the input attenuator. The impedance is thus driven by a voltage that is a scaled replica of the output of the input attenuator. That scale factor may vary from near zero to greater than one, and the impedance acts as a virtual trimmer whose apparent value varies as the scale factor of its actual value according to the gain established for the variable gain amplifier. The proper gain therefor is determined by a servo loop that applies a test pulse to the input attenuator and subsequently samples the output thereof at least twice. An amplitude difference in the samples indicates an exponential component that arise from an uncompensated attenuator. The servo loop is responsive to the signed difference between the samples to adjust the value of the virtual trimmer until the signed difference is sufficiently near zero. Proper choice of the time interval between samples will adjust the servo loop to be most sensitive to exponential components of certain time constants rather than others. This allows the servo loop to ignore the effects of dielectric absorbtion while compensating the attenuator.
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Citations
20 Claims
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1. Voltage measure-apparatus having an attenuator automatically compensated for frequency response comprising:
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a first attenuator having an input and an output referenced to a signal ground, the first attenduator itself comprising; an input section connected between the input and the output of the first attenuator; (ii) a grounded section connected between the output of the first attenuator and the signal ground; and (iii) a virtual trimmer impedance connected between the output of the first attenuator and the signal output of the variable gain amplifier means recited below; means coupled to the output of the first attenuator for producing a measurement of the value of an AC voltage applied to the input of the first attenuator; variable gain amplification means having a signal input coupled to the outptu of the first attenuator, having a gain control input coupled to the processing means recited below and also having a signal output; means for applying to the input of the first attenuator a step voltage having a transition in the direction of a selected polarity; sampling means coupled to the output of the first attenuator for making a plurality of samples subsequent to the transition in and during the ensuing duration of the step voltage applied to the first attenuator and processing means, having an input coupled to receive the samples made by the sampling means and having an output coupled to the gain control input of the variable gain amplification menas, for varying the effective value of the virtual trimmer impedance by specifying the variable gain in accordance with the values of the individual samples within the plurality thereof, thereby compensating the frequency response of the first attenuator. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
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11. A method of compensating the frequency response of an attenuator comprising the steps of:
first, a. applying an abrupt step in voltage to the input of an attenuator to produce an attenuated voltage step; b. sampling, subsequent to step a, the attenuated voltage step a plurality of times; c. determining for the plurality of samples a difference therebetween; d. amplifying the attenuated step by a gain selected in accordance with the determined difference; e. coupling the amplified attenuated voltage step through an impedance to the output of the attenuator; f. if the different of step c exceeds a selected limit then changing the selected gain used in step d to produce a smaller difference in step c; and g. repeating steps a-f until the difference of step c no longer exceeds the selected limit in step f;
then,h. applying a work voltage to be measured to the input of the attenuator; i. amplifying the attenuated work voltage by the changed selected gain determined in step f; and j. coupling the amplified attenuated work voltage through the impedance to the output of the attenuator. - View Dependent Claims (12, 13, 14, 15, 16, 17)
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18. A method of compensating the frequency response of an attenuator comprising the steps of:
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creating a scaled replica of the voltage appearing at the output of the attenuator; driving the output of the attenuator with the scaled replica through a virtual trimmer impedance; and varying the effective value of the virtual trimmer impedance be changing the scale of the replica.
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19. A method of compensating he frequency response of an attenuator comprising the steps of:
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creating a scaled replica of the AC components above a selected frequency in the voltage appearing at the output of the attenuator; driving the output of the attenuator with the scaled replica through a virtual trimmer capacitance; and varying the effective value of the virtual trimmer capacitance by changing the scale of the replica.
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20. A method of compensating the frequency response of an attenuator comprising the steps of:
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creating a scaled replica of the DC and AC components below a selected requency in the voltage appearing at the output of the attenuator; driving the output of the attenuator with the scaled replica through a virtual trimmer resistance; and varying the effective value of the virtual trimmer resistance by changing the scale of the replica.
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Specification