Frequency to current converter circuit

• US 4,598,251 A
• Filed: 08/31/1984
• Issued: 07/01/1986
• Est. Priority Date: 06/16/1982
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Wehrs, David L.
• Primary Examiner(s): Miller, Stanley D.
• Assistant Examiner(s): Callahan, Timothy P.
• Application Number:
  US06/645,835
• Time in Patent Office: 669 Days
• Field of Search: 307/234, 307/261, 307/350, 307/351, 307/352, 307/519, 307/297, 307/264, 307/265, 307/518, 328/141, 328/58, 328/140, 340/347 AD, 340/347 M, 324/78 R, 324/78 S
• US Class Current: 327/102
• CPC Class Codes: G01R 23/06 by converting frequency into an amplitude of current or voltage
• CPC Class Codes: H03K 9/06 of frequency- or rate-modulated pulses

Abstract

A circuit (10) receives a cyclic input signal having a frequency determined by a source (11), which frequency may vary in accordance with a parameter or condition to be measured, for example. The circuit (10) very precisely provides an output current representative of the frequency of the input signal. In particular, the circuit uses a pair of regulated and different voltage (16)(17), one (16) of which is impressed upon an averaging circuit (30) for a preselected time during each cycle of the input signal, and the other voltage (17) is impressed upon the averaging circuit (30) during the time when the first signal is not connected to the averaging circuit. A control signal (V_o) is developed from the averaging circuit (30) which is a function of the average of the voltage provided to the averaging circuit, and the control signal (V_o) in turn is used for controlling a current control amplifier (60) and transistors (61)(62) which in turn control current flow through a load (40) connected to output terminals (36)(37) and through a feedback resistor (63). The use of regulated voltage sources (16)(17) for deriving the current control signal insures great stability, relatively low cost, and precise operation.

Claims

1. A circuit for converting a variable frequency cyclic input signal to a current output signal having a magnitude representative of the frequency of the input signal, the magnitude being a non-zero value for a frequency of zero Hertz, comprising:

regulated supply means for providing a first non-zero regulated electrical signal and a second non-zero regulated electrical signal, the first and second regulated signals having different magnitudes;

switching means having a switching means output responsive to the input signal and coupled to the regulated supply means for alternately providing the first regulated signal and the second regulated signal at the switching means output for lengths of time which vary relative to one another depending upon the frequency of the input signal;

averaging means coupled to the switching means output to receive the first and second regulated signals for providing an average signal from the regulated signals; and

output means for providing a current output signal having a magnitude controlled by the average signal.

2. The circuit of claim 1 wherein the regulated supply means comprises a voltage reference means for providing one of the regulated electrical signals.

3. The circuit of claim 2 wherein the regulated supply means further comprises an amplifier means for providing the first or the second regulated signal not provided for by the voltage reference means.

4. The circuit of claim 1 wherein the switching means provides one of the regulated signals for a predetermined length of time responsive to a predetermined transition of the cyclic input signal which occurs once each cycle and provides the other regulated signal for a remainder of such cycle.

5. Means to provide an analog output current which has a magnitude controlled by the frequency of a cyclical input signal having a variable cycle time period so that the magnitude is a non-zero value when the frequency is zero Hertz, the means comprising:

first means for providing a first non-zero regulated electrical signal;

second means for providing a second non-zero regulated electrical signal different from the first regulated signal;

third means for providing the first signal for a predetermined length of time less than the cycle time period of the input signal at an upper frequency limit once during each cycle of the input signal;

fourth means for providing the second regulated signal for a remainder of the cycle time period during each cycle of the input signal; and

output means for receiving the first and second regulated signals from the third and fourth means including means for providing a control signal having a value which is a function of an average magnitude of the first and second regulated signals received by the output means during a reference time period and including means responsive to the control signal for providing the output current having a magnitude controlled by the control signal.

6. The means to provide an analog output current of claim 5 wherein said means responsive to the control signal includes an amplifier, and current control means adapted to be coupled in series with a signal source and load, said control signal and a feedback signal being connected to control an output of said amplifier, the output of said amplifier controlling the output current through said current control means, and a feedback resistor sensitive to the current flowing through said current control means and providing the feedback signal to maintain the output current at a level which is a function of the control signal.

8. A signal frequency to current converter for providing an output current signal having a magnitude which is controlled by a frequency of a cyclical input signal having a cycle time period and which has a positive non-zero threshold value when the frequency of the input signal is zero, said converter comprising:

first means for providing a first non-zero voltage signal;

second means providing a second non-zero voltage signal different from the first voltage signal;

third means for providing a control signal;

means for coupling the first means to the third means for a predetermined length of time which is less than the cycle time period once during each cycle of the input signal;

means for coupling the second means to the third means for a portion of the cycle time period when the first means is not so coupled during each cycle of the input signal;

said third means providing a control signal with a value which is a function of an average voltage provided to the third means by the first and second means during each cycle time period; and

an output circuit coupled to the third means and including means responsive to the control signal to provide an output current having a magnitude controlled by the value of the control signal.

7. The means to provide an analog output current of claim 8 wherein the first and second regulated signals are both nonzero voltage signals and the analog output signal is a current having a positive value when the frequency of the input signal is zero.

9. The signal frequency to current converter of claim 8 wherein said output circuit includes an amplifier, and means establishing an output current path including terminals connectable to a load through which the output current will pass, said third means being connected so the control signal and a feedback signal control an output of said amplifier, the output of said amplifier controlling the output current through said current path, and a feedback resistor sensitive to the output current through said current path providing the feedback signal to the amplifier to control the output of the amplifier at a level which provides the output current through the current path which is substantially a linear function of the control signal.

10. The signal frequency to current converter of claim 8 wherein said means for coupling includes switch means and switch control means, said switch control means operating said switch means to connect the first voltage signal to the output for said predetermined time period, said switch control means including means triggered once during each cycle of said input signal to operate the switch means to couple the first means to the third means for the predetermined time only.

11. The signal frequency to current converter of claim 8 wherein said third means comprises a resistance-capacitance circuit having a preselected time constant.

12. The signal frequency to current converter of claim 8 wherein said first means and second means comprise sources of voltage including means comprising active circuit means to regulate a magnitude of each voltage signal at a nonzero value.

13. The signal frequency to current converter of claim 12 wherein the first means provides the first voltage signal with a higher magnitude than the magnitude of the second voltage signal from the second means.

14. The signal frequency to current converter of claim 12 wherein said first means and second means comprise regulated voltage supply means, said regulated voltage supply means including voltage reference means comprising the second means, said first means including a resistor and a current control amplifier controlling a current through said resistor, and said first means being coupled to provide the first voltage signal as a voltage which is a function of the current through the resistor.

15. The signal frequency to current converter of claim 14 wherein the second means further comprises a potentiometer coupled to receive a reference voltage signal from the voltage reference means and being adjustable to permit adjusting the magnitude of the second voltage signal.

16. The signal frequency to current converter of claim 15 wherein said amplifier is a differential input amplifier having a first input coupled to receive the reference voltage signal to provide a reference input, a second input of the amplifier being coupled to be sensitive to a control voltage dependent upon the current controlled by the amplifier, and means to permit adjustment of the control voltage provided when a preselected current is controlled by the amplifier.

17. The signal frequency to current converter of claim 8 wherein said output circuit includes a pair of terminals connectable to a series connected source and load, a power supply connected to one terminal and providing power for the first, second and third means, means in the output circuit for providing a current path from the power supply to the second terminal, the means responsive including means in the current path for controlling the output current through the terminals.

18. A frequency-to-current converter circuit for providing an output current with a magnitude which has a predetermined relationship to a frequency of a cyclic input signal, the circuit comprising:

means for providing a non-zero regulated first voltage;

means for providing a non-zero regulated second voltage;

means for alternately supplying the first and second voltages for first and second time periods, respectively, during each cycle, the first and second time periods having relative durations which are a function of the frequency of the input signal;

means for averaging the first and second voltages supplied during the first and second time periods to produce an average voltage;

means for providing a feedback voltage which is a function of the magnitude of the output current; and

means for controlling the magnitude of the output current as a function of the average voltage and the feedback voltage so that the output current maintains the predetermined relationship to the frequency of the input signal.

19. A circuit for converting a cyclic input signal to an analog output signal having a magnitude representative of frequency of the input signal, the magnitude being a non-zero value when the frequency is zero Hertz, the circuit comprising:

regulated supply means for providing a first regulated electrical signal and a second regulated electrical signal, the first and second regulated signals having different non-zero magnitudes, wherein the regulated supply means comprises first and second adjusting means coupled to permit adjusting the magnitudes of the

first and second regulated electrical signals;

switching means having a switching means output responsive to the input signal and coupled to the regulated supply means for alternately providing the first regulated signal and the second regulated signal at the switching means output;

averaging means coupled to the switching means output to receive the first and second regulated signals for providing an average signal from the regulated signals; and

output means for providing a current output signal having a magnitude controlled by the average signal.

20. The circuit of claim 19 wherein the first and second adjusting means are arranged such that the magnitude of either regulated signal can be adjusted without affecting the magnitude of the other regulated signal.