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VOLTAGE RATIO DETECTOR FOR MILLIVOLT SIGNALS

J. R. HOUGHTON

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS

Technical Note 266

ISSUED DECEMBER 13, 1965

VOLTAGE RATIO DETECTOR FOR MILLIVOLT SIGNALS

J. R. Houghton

Institute for Basic Standards

National Bureau of Standards

Washington, D.C.

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, Government Printing Office

Washington, D.C, 20402 - Price 15 cents
VOLTAGE RATIO DETECTOR FOR

MILLIVOLT SIGNALS

by J. R. Houghton

ABSTRACT

A voltage ratio detector circuit for measuring

ratios of a-c and d-c signals 5 millivolts or larger

is described. The ratio is determined with a precision voltage divider which is accurate to within

0.001 percent of the indicated ratio when the ratio

is near one. The detector has sufficient sensitivity and stability to indicate differences between

two signals of 0.01 percent. Experimental results

are presented to show the relative improvement in

sensitivity of this voltage ratio detector over the

previously used transfer admittance method for the

calibration of vibration pickups.

Key words: Voltage ratio, vibration, calibration,

thermal converter, electronic circuit, test method.

1. INTRODUCTION

In the calibration of vibration standards and vibration transducers

at NBS, the ratio of two a-c voltage signals is the required measurement.

These voltage signals range from 5 millivolts to 5 volts in amplitude,

differ in phase or change phase relative to each other as the frequency

varies from 5 to 2500 Hz, and have source impedances between 100 and

1000 ohms.

Available ratio measuring devices were found to be dependent on the

phase difference of the two a-c signals and required input signal amplitudes greater than the signals generated by many vibration transducers.

To offset these deficiencies a ratio detector was designed and constructed

to meet the particular requirements for measurements of voltage ratios in

vibration measurements. These requirements are:

1. To handle low signal levels to 5 mV,

2. To be independent of phase differences between the two

signals,

3. To permit direct comparison of two signals in one measurement,

4. To compare signals ratios up to 20:1 which is accurate to

within 0.1 percent, and
5. To compare signals from piezoelectric or velocity coil

type of vibration sensing devices.

Since available d-c instrumentation provides a very sensitive means

for determining ratios of d-c signals, the possibility of converting two

separate a-c signals to independent d-c signals was investigated. This

would permit the signals to be balanced with a high sensitivity galvanometer and would eliminate the problems caused by phase differences between

the two a-c signals.

Of the various devices available for converting a-c signals to d-c

signals, thermal converters-*- were chosen since the d-c output is the true

rms equivalent (heating value) of the a-c input, i.e. the conversion con- tains the true effects of the harmonics in the input signal's wave form.

Development of thermal converters has been in progress at NBS for several

years. The converter is used as a transfer device in the calibration of

current and voltage instruments!- 1>2J .

2. RATIO DETECTOR DESIGN

2.1 Description of Circuit and Components

A block diagram of the circuit designed to determine the ratio of

two voltages is shown in figure 1. Several experimental ratio detectors

were built following the basic design of figure 1.

The final circuit is shown in figure 2. A description and discussion of the circuit components, starting from the left in figures 1 and 2,

is given below.

Switch 1 permits equalization of the transfer characteristics of

channels A and B before measuring the ratio. The CHECK position of

switch 1 causes signal B to be a common input to both channels. The trim

variable resistors are adjusted to indicate a null on the galvanometer.

The RATIO position of switch 1 connects signal A to channel A and through

the voltage divider. The divider is adjusted until the galvanometer

returns to the null position. The ratio of signal B to signal A is then

indicated on the divider. Signal A must be equal to or greater than

Commercially available thermal converters have a heater current range

from 2 mA to 1000 mA for a corresponding heater resistance range of

1100 ohms to 0.2 ohms. Amplifiers are required to provide such currents

from the output signals of vibration transducers.

2Numbers in brackets designate references at the end of the paper.
signal B. The switching impedance of switch 1 should be considered in

relation to the output impedance of the transducer being tested. Switch 1 was chosen to have as high a capacitive reactance as possible between

channel A and B because the internal impedance of most vibration pickups

is capacitive. For signals from piezoelectric transducers, the capacitance from the high side of input terminal A or input terminal B to

ground, or from the high side of terminal A to the high side of terminal

B can act like a partial short causing a loss of accuracy in the final

voltage ratio.

Cathode follower amplifiers are provided to reduce loading errors between the signal and voltage divider or between the signal and the

voltage amplifiers. If loading errors can be neglected or if either

A or B is a d-c signal, the cathode follower amplifiers should be removed

from the circuit. The gain linearity of these amplifiers is important

because signal A may be up to twenty times the magnitude of signal B

which is used for balancing the two channels.

Voltage dividers are available for a-c and d-c signals with indicated ratios accurate to within 0.001 percent at a ratio of one. The

divider may be either an a-c or d-c divider, whichever is compatible with

signal A.

The voltage amplifiers are necessary to provide high input impedance

and to amplify the signal voltage for operation of the thermal converters.

Gain and trim variable resistors are provided to adjust the output

current from both voltage amplif iers-*.

Thermal converters provide a root-mean-square rectification of a-c signals for frequencies of approximately 5 Hz and higher. Below 5 Hz

the thermal converter output begins to follow the wave shape of the input

current. Briefly, a thermal converter is a thermo- junction welded to a heater wire and mounted in a small highly-evacuated glass bulb. Current

passes through the heater, causing a rise in temperature of the heater

wire and generating a d-c voltage in the thermo- junction proportional to

the temperature rise. Since temperature of the thermo- junction is a critical characteristic of the converter, thermal insulation is provided

by enclosing the two thermal converters in a copper box and surrounding

them with insulating material. The thermal converters were electrically

insulated between the heater wire and the thermo- junction.

3

A heater current of 50 mA to the thermal converter was used for the

design discussed in this paper. This is well below the rated current

of the converters chosen (80 mA with a safety factor of 2) and still

high enough to produce ample output from the thermocouple.
The galvanometer was of the conventional electronic null indicator

type.

The voltmeter section was provided to obtain an approximate balance

between the two signals before they are switched to the thermal converters

by switch 2. This was desirable to prevent inadvertent burn out of a thermal converter heater while attempting to balance the galvanometer^.

Initially, an approximate determination of the ratio should be made from

indications of the input signals on the panel voltmeters (VM) . This is

done through adjustments of the gain settings of the amplifiers and gain

variable resistors. Final balance of the currents through the thermal

converters and final determination of the ratio are made when switch 2

is set to the thermal converter (TC) position.

2.2 Procedure for Determining Voltage Ratio

A procedure has been developed for using the voltage ratio detector

shown in figures 1 and 2. This procedure assumes prior knowledge of the

type of voltage signal, a-c or d-c, and some information about the relative magnitudes of the signals. The ratio of the magnitude of signal A

to signal B can be determined using the ratio detector shown in figure 2 as follows:

1. Set switch 1 to C (CHECK).

2. Set switch 2 to VM.

3. Turn power switches ON and allow about 15 minutes for

thermal equilibrium of the electronic equipment.

4. Connect signal sources to input terminals A and B with the

larger signal connected to terminal A.

5. Adjust the gain of the amplifiers and the gain variable

resistors to read 0.5 volts on the VM's.

4When a new circuit is put into operation, it is recommended that a check

be made to determine that the current through the thermal converters and

voltmeters are equal. In this circuit (Fig. 2), the current through the

voltmeter was 50 mA when the voltmeter indicated 0.5 volts. The current

to each thermal converter was checked with a milliammeter inserted in

place of the short. With switch 1 in the CHECK position and switch 2 in

the TC position, the input signal level at B was increased until 50 mA

was indicated on the' milliammeter. Then switch 2 was set to the VM position. The resistance of each voltmeter circuit was adjusted with variable resistors until the voltmeter read 0.5 volts.
6. Set switch 1 to R (RATIO).

7. Adjust the divider until VM A reads 0.5 volts.

8. Set switch 1 to C.

9. Set switch 2 to TC.

10. Tune the trim variable resistors to balance the galvanometer.

11. Set switch 1 to R.

12. Adjust the divider to balance the galvanometer.

13. Return switch 1 to C as a check on the balance of the

detector circuit.

14. Retune the trim variable resistors if necessary to balance

the galvanometer.

15. Set switch 1 to R.

16. If necessary, adjust the divider to balance the galvanometer.

17. Set switch 2 to VM before changing the signal level or turning off the equipment.

If the agreement between readings of the galvanometer with switch 1 in

the C or R position (steps 13 to 16) is within acceptable limits, the

voltage ratio is read from the divider.

3. RESULTS AND DISCUSSION

3.1 Voltage Ratio Detector Applications

The applications of an a-c voltage ratio detector are numerous in

the electrical measurements field. Three applications are of interest

in the calibration of vibration transducers.

[3] The reciprocity calibrations of vibration standards require measure- ments of voltage ratio and transfer admittance. The need for increased

accuracies in these measurements was the primary reason for developing

this circuit. In using a vibration standard calibrated by the reciprocity

method to calibrate transfer standards (vibration pickups), it has been

necessary to make transfer admittance measurements. The ratio detector
has been used in a similar way to compare transfer standards with the

primary standard.

A second application for this voltage ratio detector in a vibration

standards laboratory was to use it with a compressor oscillator to survey

the characteristics of a. device continuously over a wide range of

frequencies —of ten' called a sweep frequency test. A compressor oscillator

is one which keeps a signal constant by using feedback. This test is

useful to "weed-out" unsuitable transfer standards before expending any

significant time making a calibration. This ratio detector adapted for

sweep frequency tests is shown in figure 3. It is necessary that signals

A and B come from similar sources such as two acceleration sensitive

pickups or two velocity sensitive pickups. A velocity signal could be

differentiated electrically to be compared with an acceleration signal.

In sweep frequency testing the RATIO position of switch 1 of the ratio

detector is used, and as the oscillator changes frequencies an autographic

record of the ratio of two signals versus frequency is made. Since the

CHECK position of switch 1 cannot be used to balance the electrical com- ponents during a sweep frequency test, it is necessary to make one run through the frequency range of interest with switch 1 in the CHECK position. The record made with the switch in the RATIO position can be

compared with the record for the CHECK position. The differences between

these records provides information on the performance characteristics of

the vibration pickup.

A third and perhaps less significant application of the ratio detector

is for measuring the magnitudes of a-c signals. By using a d-c standard

reference cell as one of the input voltages, the true root-mean- square

magnitude of an a-c voltage can be determined within 0.1 percent using

the circuit shown in figure 2. The cathode followers and the capacitors

following the voltage amplifiers are by-passed for such measurements.

This technique for measuring the magnitude of an a-c voltage signal can

be used for checking the performance of vacuum tube voltmeters used in

vibration pickup calibrations.

3.2 Transfer Admittance Versus Voltage Ratio Detector

Data from the calibrations of a limited number of vibration pickups

show that the voltage ratio detector has significant advantages over the

transfer admittance method. A basic advantage is that two signals can be

compared simultaneously without the delays between the four measurements

required to obtain the transfer admittance. For the operator, there are advantages in the greater sensitivity and reduced time (approximately

one-half) to complete a calibration with the voltage ratio detector. The

ratio detector built as shown in figure 2 has consistently given results

that are repeatable within 0.02 percent and with a sensitivity of about

1 part per 10,000 when comparing two a-c signals and the ratio is near

one.
A piezoelectric accelerometer suitable for use as a transfer standard

was calibrated using the transfer admittance method and the voltage ratio

detector. The results given in table 1 were obtained on two different

exciters at different times.

Table 1 - Calibration Factors for a Piezoelectric Accelerometer

Applied

Calibratiori factor

Transfer admit- Voltage ratio

Frequency acceleration tance measurement measurement

Hz g mV/g mV/g

2000 10 13.18 13.260

5 13.15 13.262

2 13.13 13.266

1000 10 _ 13.085

5

- 13.084

2 - 13.086

900 10 13.13 _ 5 13.11 - 2 13.11 -

500 10 13.09 13.048

5 13.04 13.040

2 13.00 13.043

100 10 12.96 12.993

5 12.97 12.996

2 12.97 13.002

10 2 12.95 13.19

1 12.89 13.15

0.5 12.98 13.16

Of particular note is the reduced scatter in the results obtained

with the ratio detector as compared with the transfer admittance method.

Comparisons of the sets of three factors measured for each frequency

(values at 10 Hz excluded) show repeatabilities of 0.02 to 0.07 percent

for the results obtained with the voltage ratio detector and of 0.08 to

0.7 percent for the transfer admittance method.
The number of significant figures given in table 1 are more than can

be justified by the overall accuracies of the vibration standards. They

are shown here for comparisons of the relative sensitivity and repeatability of the transfer admittance and ratio detector methods.

4. CONCLUSION

A voltage ratio detector for comparing a-c signals of 5 mV and

larger was designed and put into operation. The principle of operation

for this ratio detector is to divide the larger of two signals with a precision divider such that the output of the divider equals the smaller

signal. Then the signals are amplified, converted to d-c signals with

thermal converters and compared on a galvanometer. The transfer characteristics of the amplifiers and thermal converters are equalized by using

the smaller of the two signals as a common input to the two amplifiers.

Based on the experimental measurements, some of which are presented

in this paper, and observations of improvements in speed, accuracy and

convenience during calibrations of vibrations and vibration transducers,

it is believed that the current requirements for measurements of voltage

ratio were met.

ACKNOWLEDGMENT

Messrs. James A. Miller and Douglas R. Bryant assisted in the design

and construction of the voltage ratio detector described in this report.

REFERENCES

1. AC-DC Transfer Instruments for Current and Voltage Measurements,

F. L. Hermach, IRE Transactions Vol. 1-8, 1958.

2. A Differential Thermocouple Voltmeter, J. E. Griffin, F. L.

Hermach, AIEE Transactions Paper No. 62-819, 1962.

3. Calibration of Shock and Vibration Pickups, ASA Publication

S2. 2-1959, Nov. 27, 1959.
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