COMMUNICATIONS

203

[3] C. M. Weil, "Progagation of plane waves through two parallel di-

TABLE II E-FIELD MEASUREMENTS WITH AND WITHOUT CHAIR

Plane of Measurement Head

Thorax

Abdomen

tEquivalent

Location of Measurement Point

Field Intensity nanoJoul es/metre3 (microWatts/centimetre2t) With Chair Without Chair

Center

4.8 (288)

4.3

(258)

Front

4.6 (276)

4.6

(276)

Back

3. 9

(234)

3.9

(234)

Right

3.7

(222)

3.7

(222)

Left

5.1

(306)

5.2

(312)

Center

1.8 (108)

1.7

(102)

Front

2. 1

(126)

2.1

(126)

Back

1.8 (108)

1.9

(114)

Right

2.2

(132)

2.1

(126)

Left

2.5 (150)

Center

2.9 (174)

electric sheets," IEEE Trans. Biomed. Eng., vol. BME-21, pp. 165168, Mar. 1974. [4] V. R. Reno, "Microwave reflection, diffraction and transmission studies of man," Naval Aerospace Medical Research Laboratory, Pensacola, FL, Rep. NAMRL-1199, 1973. [5] P. F. Wacker and R. R. Bowman, "Quantifying hazardous electromagnetic fields: Scientific basis and practical considerations," IEEE Trans. Microwave Theory Techniques, vol. MTT-19, pp. 178-187, 1971.

A Signal Simulator for Strain Gauge Type Pressure Transducer Amplifiers ROBERT MAURO

Abstract-A simulator circuit is presented which permits the introducdon of external electrical signals mto both dc and carrier type strain gauge amplifiers.

plane wave power density units,

without the chair in position. The isotropic probe used in the measurements has been described previously [S5 One series of measurements was taken within the chair cavity normally occupied by the animal. Measurement points were located on three planes selected so that they would intersect the head, thorax, and abdomen of a monkey contained in the chair. Points on the head and thoracic planes were located at the center of the cavity and 2 cm from the center at each of the cardinal points. A single measurement was taken at the center on the abdominal plane. The results of the measurements are shown in Table II. The agreement between field intensities in the chair cavity and those at the same field location without the chair indicate minimal disturbance of the field by the chair. The chair has been in continuous use over a period of nine months with three to five animals per day restrained for two hours each. Another chair of slightly different design was used for several weeks to restrain monkeys for 24 hours/day for six days/week. At no time did an animal escape nor was it necessary to replace any of the components of the chairs. Several advantages accrue from this design for primate restraint devices to be used in microwave bioeffects research. Accuracy of power density estimations of the incident field affecting the animal is considerably improved by reduction of field variations due to multiple reflections and standing waves. Experimenter safety and ease of animal handling is not compromised to gain better electrical performance and the animal is subjected to the minimum practicable restraint. A sufficient number of foam plastics are available in different densities and thickness to permit restraint designs to be optimized for greater mechanical strength or minimum perturbation of the field as required by the experiment. The basic design of the chair is simple and adaptable for use in a wide range of biological investigations. .

REFERENCES [1] P. E. Tyler, "Overview of electromagnetic radiation research: Past, present, and future," Ann. N.Y. Acad. Sci., vol. 247, pp. 6-14, 1975. [2] S. F. Cleary, "Uncertainties in the evaluation of the biological effects of microwave and radiofrequency radiation," Health Phys., vol. 25, pp. 387-404, 1973.

When working with pressure transducer systems it is difficult to be certain that the data being recorded are actually faithful reproductions of the physiologic events being measured. The circuit shown in Fig. 1 can help minimize this problem by eliminating the amplifier as a source of error; it is particularly useful for determining the frequency response and dynamic characteristics of the amplifier as well as for verifying the proper operation of its systole (peak detection), diastole (valley detection), and averaging circuits. The simulator works equally well with both dc and carrier type amplifiers, and when connected to the gauge input does not disturb the normal functioning of the amplifier, so that the balance and calibrate circuits are still operative. When a dc gauge voltage is employed pin I of the gauge connector in Fig. 1 is high, and the output of the comparator (IC 1) is always low. For this case Qi is cut off, and the amplifier (IC2) has a gain of plus one. The four 30042 resistors shown in the figure form a substitute bridge that replaces the normally used pressure transducer bridge. Therefore, the input signal el (t) is effectively connected directly to the bridge through the 33-kE2 resistor Rs; this forms a current source is(t) _ el (t)/R8. Thus, if the bridge is balanced prior to connecting is(t), the output is then given by is(t) * (R/2) or el (t) (R/2Rs). With the given value of R. and an input signal el (t) of 1 V, the resulting output is approximately 4.5 mV. Assuming that we are simulating a pressure transducer having a nominal sensitivity of 37.5 MV/V/mm Hg with a gauge excitation of 7.5 V, this output corresponds to an equivalent input pressure of about 120 mm Hg. For the case of ac gauge excitation the pressure transducer output produced by an actual physiologic signal is an amplitude-modulated version of the gauge voltage. Therefore, when attempting to simulate an input signal on a carrier type pressure amplifier, the current source is(t) previously discussed must now be modulated at the excitation frequency. In the circuit shown this is taken care of automatically when the simulator is connected to an ac pressure amplifier. When ac excitation is applied to the bridge, a square wave is produced at the comparator output, and Qi is switched on and off at the modulation rate. With Qi shorted the amplifier -

Manuscript received November 10, 1975; revised February 4, 1976. The author is with the Department of Electrical Engineering, Manhattan College, Riverdale, NY 10471.

IEEE TRANSACTIONS ON BIOMEDICAL

204

ENGINEERING, MARCH 1977

TABLE I GAUGE PLUG MODIFICATIONS FOR SIMULATOR CONNECTION TO OTHER PRESSURE AMPLIFIERS ._____________

1

~4

B D

D C

Grass 1 2

2 3

C A

A B

3 4

3 4

5

E

E

6

-

Gauge Output Voltage Shield

6

Cambridge,

Sanborn

Gauge Excitation Voltage

Other

Offner

EFM

-

jumper

5

-

resistor

to pin 1*

to pin 2

Epsco 2

5

-

resistor

to pin 1*

*Note: The resistor value is chosen to achieve the correct output signal amplitude.

Fig. 1. Pressure amplifier signal simulator.

gain is minus one and with QI open it is plus one. In this way the incoming electrical signal el (t) is modulated at the gauge excitation frequency before being applied to the bridge, and the resulting bridge output can be processed as though it were a conventional strain gauge signal. In Fig. 1, the signal generator shown within the dotted lines produces a dc offset 1-Hz square wave with HI and LO output

levels corresponding to 120 and 80 mm Hg, respectively [ 1]. This signal was found to be useful for examining the amplifier dynamic response as well as for checking the systole and diastole meter circuits. Of course this oscillator can be replaced by any other convenient electrical signal. The circuit as shown is wired for connection to pressure amplifiers manufactured by Electronics for Medicine, Inc. However, by making use of Table I this simulator may be readily connected to most other popular strain gauge amplifiers. REFERENCES

11] National Semiconductor Linear DataHandbook, p. 4-9, Jan. 1974.