The dataTaker is able to measure both the frequency, and the period of frequency. The period of the frequency is the inverse of the frequency.

The frequency signals are connected to the analog input channels of the dataTaker in the same manner as low level voltage signals.

The dataTaker 50 has 5 differential analog input channels, which can also be used as 10 single ended input channels. The dataTaker 500/600 series loggers and Channel Expansion Module (CEM-AD) have 10 differential analog input channels, which can also be used as 30 single ended input channels.

Any combination of differential and single ended analog input channels can be used for measuring the frequency or period of signals.

Frequency Measurement

A comparator is used in the frequency measurement circuit of the dataTaker to detect the threshold crossings. The amplitude of the frequency signal should be greater than 100 mV peak to peak, but not greater than the input common mode range of the dataTaker.

For dataTaker 50, 500 and 600 the input common mode range is ñ3.5 Volts to +3.5 Volts.

For dataTaker 505, 605, 515 and 615 the input common mode range is ñ100 Volts to +100 Volts if the internal attenuators are selected.

The frequency signal to be measured by-passes the VCO stage of the analog to digital converter, and is fed directly into the counter stage of the analog to digital converter.

Frequency signals are counted by the analog to digital converter counters for between 16 to 40 mS. The frequency of the input signal is calculated from the mean period of the signal over the sampling time.

The period of the input signal is calculated as the inverse of the frequency.

Frequency vs Amplitude

The range of input frequency signals that can be measured by the dataTaker data logger is dependent on the amplitude of the signal. As the amplitude of the frequency signal decreases, the upper limit for frequency measurement increases.

This relationship between signal amplitude and the limit of frequency measurement is illustrated below

 

 

Figure 60 ñ Upper Limit of Frequency Measurement in Relation to Signal Amplitude

The dataTaker can measure frequencies in the range of 0.102 Hz to 20,000 Hz, and can measure periods of frequencies in the range of 50 µSec to 9.8 Sec, depending on the amplitude of the signal.

Data from frequency inputs can be returned either in terms of the frequency (Hz), or in terms of the period of the frequency (mSec).

Range of Frequency Measurement

By default the dataTaker measures frequency between 33Hz to 20KHz. However frequency can be measured down to 0.102Hz by changing default settings.

The default minimum frequency is set by the maximum period that the logger can wait until at least two pulses have been received to allow calculation of frequency. The 33Hz default minimum is determined by a default maximum period of 30 mSec.

If frequencies less than 33Hz are to be measured, then the maximum period must be increased accordingly. The maximum period is defined by the channel factor for the frequency and period channel types, which by default is 30mSec.

The maximum allowed period is 9800 mSec, which permits measurement of frequency down to 0.102Hz. Be aware that as the frequency decreases, then the time taken to complete a single measurement increases, such that a frequency of 0.102Hz will take 9.8 seconds to get a single reading. This then has implications on maximum sampling rates of other channels, since the logger waits until the two pulses are received.

The method for setting the minimum frequency measurement is illustrated in the example below for measuring frequency using differential connections.

Measuring Frequency as Differential Inputs

When frequency signals are measured as differential inputs, the frequency signal is connected between the +ve terminal and the ñve terminals of the analog input channels of the dataTaker.

The comparator threshold level for frequency measurement is the voltage connected to the ñve terminal of the input channel.

If the frequency of a voltage signal greater than 3 Volts is to be measured using either a dataTaker 50, 500 or 600 data logger, then the signal must be externally attenuated to the common mode voltage of ±3.5 Volts peak to peak before connecting to the analog input channels. The implementation and use of attenuator networks is described in Section II ñ Measuring High Level Voltages. Both sides of the high voltage frequency signal must be attenuated for differential inputs.

The frequency of a voltage signals up to ±100 Volts can to be measured directly by the dataTaker 505, 605, 515 and 615, if the internal attenuators are selected.

Frequency signals are connected to the analog input channels as differential inputs as follows

 

 

Figure 61 ñ Connecting Frequency as Differential Inputs

 

The frequency is measured between the +ve and ñve terminals of the analog input channel.

Frequency inputs connected as differential inputs are read and the data is returned when a Schedule containing the channel is executed.

Using DeTransfer, the command for example

instructs the dataTaker to measure the frequency of signals as follows

measure the frequency connected to analog input channel 1, which by default has a minimum capability of 33Hz

measure the frequency connected to analog input channel 5, for which minimum capability has been set to 1Hz by setting maximum period to 1000mSec

measure the period of the frequency connected to channel 10, , which by default has a minimum capability of 33Hz. The signal as a higher voltage, and is to be attenuated by selecting the internal attenuator channel option A (dataTaker 505, 515, 605 and 615 only)

The F indicates that the signal connected to these channels is to be measured as a frequency, and data is returned in units of Hz. The P indicates that the signal is to be measured as the period of the frequency, and data is returned in units of mSec.

Using DeLogger, frequency and period signals that are connected as differential inputs are measured by the following Program Builder program. The differential connection is selected from the Frequency Wiring Configurations dialog which opens when you have selected the analog input channel.

 

 

If you are measuring the frequency of the signal, then set Measurement Type to Frequency. If you are measuring the period of the signal, then set Measurement Type to Period.

The Longest Period has also been changed to 1000mSec from the default 30mSec, to allow frequency measurement for this channel down to 1Hz.

The dataTaker will read the inputs every 10 minutes, and readings are stopped by entering a H (Halt) command.

Measuring Frequency as Single Ended Inputs

Frequency signals can also be measured as single ended inputs, referenced either to Analog Return or an external common point connected to the SE REF input terminal.

When the single ended frequency signal is connected between the +ve, ñve or T input terminal of the analog input channel and Analog Return, the comparator threshold voltage is the voltage level on the Analog Return terminal of the channel.

When the single ended frequency signal is connected between the +ve, ñve or T input terminal of the analog input channel and the SE REF input, the comparator threshold voltage is the voltage level on the SE REF terminal.

Alternatively for each of the single ended connection methods, frequency inputs can be measured using the internal 2.5 Volt threshold voltage. In this case the voltage level applied to the Analog Return or the SE REF input terminal is not used. This option provides compatibility with 5 Volt TTL and CMOS level inputs.

If the frequency of a voltage signal greater than 3 Volts is to be measured using either a dataTaker 50, 500 or 600 data logger, then the signal must be externally attenuated to the common mode voltage of ±3.5 Volts peak to peak before connecting to the analog input channels. The implementation and use of attenuator networks is described in Section II ñ Measuring High Level Voltages.

The frequency of a voltage signals up to ±100 Volts can to be measured directly by the relay multiplexed dataTakers, if the internal attenuators are selected.

Single Ended Frequency Inputs Referenced to Analog Return

Frequency signals are connected to the analog input channels as single ended inputs referenced to Analog Return as follows

 

 

Figure 62 ñ Connecting Frequency as Single Ended Inputs 
Referenced to Analog Return

 

The frequency on the +ve, ñve or T terminal is measured using the voltage on the Analog Return terminal as the comparator threshold voltage.

The dataTaker 50 does not support single ended frequency measurement on the Excite (T) terminal.

Frequency signals connected to the analog input channels as single ended inputs referenced to Analog Return are sampled and the data is returned when a Schedule containing the channel is executed.

Using DeTransfer, the command for example

instructs the dataTaker to measure the single ended frequencies connected to the channels 2+ and 5ñ, and the period connected to channel 10 T. The comparator threshold is the voltage level connected to the Analog Return terminal.

The F indicates that the signal connected to these channels is to be measured as a frequency, and data is returned in units of Hz. The P indicates that the signal is to be measured as the period of the frequency, and data is returned in units of mSec.

 

 

The dataTaker will read the inputs every 5 minutes, and readings are stopped by entering a H (Halt) command.

Single Ended Threshold Voltage

The measurements in the above example would be taken using the voltage on the Analog Return terminal as the comparator threshold voltage.

The single ended frequency signals can also be measured using an internal 2.5 Volt threshold voltage. The amplitude of the single ended frequency signal referenced to Analog Return is compared with this internal comparator threshold voltage to measure frequency. The internal comparator threshold voltage is 2.5 Volt above dataTaker ground, and so this single ended frequency measurement of signals 5 Volt peak to peak, which provides compatibility with TTL and CMOS logic.

The measurement of single ended frequency signals against the internal 2.5 Volt comparator threshold voltage is specified by the 2V channel option.

Using DeTransfer, the command for example

instructs the dataTaker to measure the frequency on single ended input channels 3+ and 5ñ. The amplitude of the frequency signals is sensed between the input terminals and Analog Return, and compared with the internal 2.5 Volt comparator threshold voltage. Frequency is counted when the signal amplitude exceeds this 2.5 Volt threshold.

Using DeLogger, the internal 2.5 Volt threshold is specified by setting the Single Ended Reference option to Internal 2.5 Volt.

 

 

Single Ended Frequency Inputs Referenced to an External Common

Frequency signals are connected to the analog input channels as single ended inputs referenced to an external common as follows

 

 

Figure 63 ñ Connecting Frequency as Single Ended Inputs
Referenced to an External Common

 

The frequency on the +ve, ñve or T terminal is measured using the voltage connected to the external SE REF input terminal as the comparator threshold voltage.

The dataTaker 50 does not support single ended frequency measurement for the Excite (T) terminal.

Frequency inputs connected to the analog input channels as single ended inputs referenced to an external common are read and the data is returned when a Schedule containing the channel is executed.

Using DeTransfer, the command for example

instructs the dataTaker to measure the single ended frequencies connected to the channels 1+ and 3ñ.

The F indicates that the signal connected to these channels is to be measured as a frequency, and data is returned in units of Hz. The P indicates that the signal is to be measured as the period of the frequency, and data is returned in units of mSec.

The X channel option specifies that the single ended frequency inputs are referenced to the SE REF terminal. The comparator threshold is the voltage level connected to the SE REF input terminal.

This command will read the inputs every 15 minutes, and readings are stopped by entering a H (Halt) command.

DeLogger does not support frequency measurement as single ended inputs referenced to an external common. However the User channel type (DeLogger Version 4.2.15 or later) can be used, and the channel specifications shown above for DeTransfer can be entered.

Measurement Ranges and Accuracy

The dataTaker can measure frequency in the range of 0.102 Hz to 20,000 Hz, and can measure period of the frequency in the range 50 µSec to 9.8 Sec.

The accuracy of frequency and period measurement is 0.0025% of reading.

Error Messages

Frequencies outside the range of 0.102 Hz to 20,000 Hz produce an over-range reading of 0 Hz.

The dataTaker also reports the error condition with the error message ëE11 - Input(s) out of rangeí if the Messages Switch /M is enabled.