Meggitt P702B/P702BF-1 Power unit/amplifier

Introduction

The P702B power unit/amplifier is a portable power source with integral signal conditioning for use with

internally amplified accelerometers. Signal conditioning includes amplification, integration and fault

indication.

Description

The P702B power unit/amplifier is a portable unit specifically designed to furnish 2.4 mA constant DC

current (+27 VDC) necessary to operate the internal charge amplifiers of Meggitt Sensing Systems and

similar piezoelectric transducers (accelerometers and velocity sensors.) In addition the P702B provides

signal conditioning, includes features such as selectable gain levels of 1. 10. 100. extremely low-noise,

integration to yield velocity output, battery condition indication, and visible overload or open/shorted cable

indication. This gives the test engineer broad capabilities for the accurate measurement and recording of

vibration signals over a wide range of test situations. Also virtually any high impedance charge-type

accelerometer can be operated with the P702B with the addition optional charge converter accessory,

CC701. The P702B also has built-in ESD protection to guard against damage of the internal electronics.

All switches and controls are on the front panel. Transducer input and output BNC connectors are

located on the rear panel, as is a connector for the optional line adapter/charger, See Figure 1 in the

appendix.

The P702BF-1 is identical to P702B, except that the frequency response of the P702BF-1 is specified as

–3 dB from 0.5 Hz to 20 kHz (versus 0.5 Hz to 50 kHz for the P702B) in acceleration mode. Other than

the high end frequency response, the P702BF-1 is identical to the P702B in terms of operation,

maintenance and troubleshooting.

Operation

Theory of operation

The P702B power unit/amplifier consists of the major circuits and components shown in Figure 2 in the

appendix. Panel functions are shown in Figure 1. The power module provides + 27 VDC through an

internal 2.4 mA (other values between 2.0 and 10 mA can be furnished on request) constant current

diode to the transducer, as well as the DC excitation for the selectable gain amplifier, the fault indicator

circuit, and the integrating circuit.

The power is normally supplied from the three internal 9 volt batteries. When the optional LA704 Line

Adaptor is used, the internal batteries are automatically disconnected when the adaptor's output cable is

plugged into the power jack on the rear of the P702B unit. See Figure 3 in the appendix for correct

jumper placement.

The vibration signal fed into the P702B Transducer input connector passes through a low pass filter

(optional) and to the first gain stage which amplifies the signal by X1 or X10 according to the gain switch

setting. The signal is split and one path is routed to an integrating amplifier to become an output signal

proportional to velocity (mV/in/sec). It then rejoins the non-integrated signal at the ACCeleration/VELocity

selector switch. When the switch is in the ACCeleration position, the signal goes directly to the second

stage gain amplifier which can introduce an additional X1 or X10 gain. By using different combinations of

first and second stage gains, the output can be set for gains of X1. X10 or X100.

The fault monitoring circuit is built around a dual comparator which senses the DC voltage supplied to the

transducer's internal amplifier. The comparator circuit is of the window type and when the transducer's

bias output voltage exceeds the normal range, 6-16 VDC, depending on the transducer type, it illuminates

the FAULT indicator. Abnormal voltages which can be caused by extended signal overloads within the

transducer's internal charge amplifier may trip the circuit. Normally, small transients will not cause the

fault indicator to light as it has a rather long time constant to prevent low frequency, high amplitude

signals from causing the indicator to illuminate erroneously.

The P702B can be supplied by the factory with an optional 6 dB per octave roll-off RC low-pass filter

incorporated into the amplifier.