In a time-multiplexed system, the problem of folded data"is present whenever the rate of data change is faster than one-half the sampling rate. When this occurs, it is not known whether the measured quantity ha reversed itself several times between samples or if there has been no reversal at all. It is considered desirable limit the bandwidth of the data so that this ambiguity is not present; however, with refined techniques of analysis, this is not a rigid requirement. The form in which the data is displayed or recorded is also a limitation on measurement. In general, time-history plots of the measured quantity are desired. In this case, the speed at which the recording medium moves is often a severe limitation. If sampling is not regular, demultiplexing difficulties are magnified. 77.5 Transmitters and Batteries The transmitter is made up of two components: the subcarrier oscillator and the radio frequency oscillator. he subcarrier can be bridge controlled(BCO) or voltage controlled(VCO). The subcarrier center frequency is 4,000 Hz, which can be modulated +400 Hz by the strain or voltage being measured. Using BCOs, a strain as large as 2, 500 microinches per inch(uin. /in ) and as small as 2 uin /in. can be measured and transmitted The temperature measurement range of the vCO is from -200 to 4,000oF. With a copper-constantan thermo- ouple, a temperature change as small as 2@F can be sensed and transmitted The single-resistance strain gauge transmitter does not have a subcarrier oscillator and can be used from -40 to 212.E. It has only a radio frequency oscillator, which is modulated by the sensor signal. For this reason it is not suitable for static strain measurements and must be used for dynamic strain measurements only. It has a frequency response to 25,000 Hz or greater. A static strain signal transmitted by this device will drift. It provided with self-contained rechargeable nickel-cadmium batteries. Pins protruding through the epoxy case are used for all electrical connections. Only one screw adjustment is provided, and this is used to set the radio Rechargeable nickel-cadmium batteries are used with the BCO and the VCO. The BCo batteries have useful lives of 4 and 9 hr. A VCo battery has 40 hr useful life The single-resistance strain gauge transmitter has a built-in nickel-cadmium battery with a life of 4 hr. 77.6 Receivers and discriminators A typical industrial receiver has a tuning range of 88 to 108 MHz. When the transmitter i In its greatest sensitivity mode, the output of the discriminator is approximately 1 V for a 25-uin with a single active gauge in the bridge. At the most insensitive mode 1 V is obtained for an approximately 500-uin /in strain. The discriminator can withstand a 500%overload, which means that a 5-V signal btained from a strain of 125-uin /in. at the maximum sensitivity and from 2, 500-pin /in. at the minimum sensitivity. 77.7 Antennas and Total System Operation A nickel-cadmium battery supplies the power to the transmitter. For the BCO, the resistance change of the strain gauge changes the frequency of the subcarrier. In the case of the VCO, the millivolt output of the thermocouple changes the frequency of the subcarrier. This change modulates the radio frequency transmitted by the antenna. The receiving antenna picks up the signal and conducts it by wire link to the radio receiver, which is tuned to the transmitting frequency. The radio receiver demodulates the FM carrier to reproduce the subcarrier signal. The subcarrier signal is then fed to the discriminator, which demodulates this signal to obtain dc voltage, which is then amplified by the dc amplifier and recorded on the oscillograph. The oscillograph record, properly calibrated, is then a display of the strain in microinches per inch for the BCOs, or the temperature in degrees for the VCO. At the same time the dc signal can be read on a vtvm and can be used as a check on the oscillograph The transmitter subcarrier oscillators are factory set to operate at a center frequency of about 4, 000 Hz. They have a frequency range of +400 Hz about the 4,000-Hz center frequency. The center frequency is set with a e 2000 by CRC Press LLC© 2000 by CRC Press LLC In a time-multiplexed system, the problem of “folded data” is present whenever the rate of data change is faster than one-half the sampling rate. When this occurs, it is not known whether the measured quantity has reversed itself several times between samples or if there has been no reversal at all. It is considered desirable to limit the bandwidth of the data so that this ambiguity is not present; however, with refined techniques of analysis, this is not a rigid requirement. The form in which the data is displayed or recorded is also a limitation on measurement. In general, time-history plots of the measured quantity are desired. In this case, the speed at which the recording medium moves is often a severe limitation. If sampling is not regular, demultiplexing difficulties are magnified. 77.5 Transmitters and Batteries The transmitter is made up of two components: the subcarrier oscillator and the radio frequency oscillator. The subcarrier can be bridge controlled (BCO) or voltage controlled (VCO). The subcarrier center frequency is 4,000 Hz, which can be modulated ±400 Hz by the strain or voltage being measured. Using BCOs, a strain as large as 2,500 microinches per inch (min./in.) and as small as 2 min./in. can be measured and transmitted. The temperature measurement range of the VCO is from –200 to 4,000°F. With a copper-constantan thermo￾couple, a temperature change as small as 2°F can be sensed and transmitted. The single-resistance strain gauge transmitter does not have a subcarrier oscillator and can be used from –40 to 212°F. It has only a radio frequency oscillator, which is modulated by the sensor signal. For this reason, it is not suitable for static strain measurements and must be used for dynamic strain measurements only. It has a frequency response to 25,000 Hz or greater. A static strain signal transmitted by this device will drift. It is provided with self-contained rechargeable nickel-cadmium batteries. Pins protruding through the epoxy case are used for all electrical connections. Only one screw adjustment is provided, and this is used to set the radio frequency. Rechargeable nickel-cadmium batteries are used with the BCO and the VCO. The BCO batteries have useful lives of 4 and 9 hr. A VCO battery has 40 hr useful life. The single-resistance strain gauge transmitter has a built-in nickel-cadmium battery with a life of 4 hr. 77.6 Receivers and Discriminators A typical industrial receiver has a tuning range of 88 to 108 MHz. When the transmitter is used in its greatest sensitivity mode, the output of the discriminator is approximately 1 V for a 25-min./in. strain with a single active gauge in the bridge. At the most insensitive mode 1 V is obtained for an approximately 500-min./in. strain. The discriminator can withstand a 500% overload, which means that a 5-V signal will be obtained from a strain of 125-min./in. at the maximum sensitivity and from 2,500-min./in. at the minimum sensitivity. 77.7 Antennas and Total System Operation A nickel-cadmium battery supplies the power to the transmitter. For the BCO, the resistance change of the strain gauge changes the frequency of the subcarrier. In the case of the VCO, the millivolt output of the thermocouple changes the frequency of the subcarrier. This change modulates the radio frequency transmitted by the antenna. The receiving antenna picks up the signal and conducts it by wire link to the radio receiver, which is tuned to the transmitting frequency. The radio receiver demodulates the FM carrier to reproduce the subcarrier signal. The subcarrier signal is then fed to the discriminator, which demodulates this signal to obtain a dc voltage, which is then amplified by the dc amplifier and recorded on the oscillograph. The oscillograph record, properly calibrated, is then a display of the strain in microinches per inch for the BCOs, or the temperature in degrees for the VCO. At the same time the dc signal can be read on a VTVM and can be used as a check on the oscillograph. The transmitter subcarrier oscillators are factory set to operate at a center frequency of about 4,000 Hz. They have a frequency range of ±400 Hz about the 4,000-Hz center frequency. The center frequency is set with a