Dual Channel Simultaneous Measurements

The information provided is for help with Commtest Instruments products:

• vb instrument

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Note: Applies to dual channel vb instruments only e.g. vb2000 and vb3000.

QUESTION

Is the vb instrument capable of measuring two channels simultaneously?

ANSWER

Yes. The instrument can measure two channels simultaneously. Dual channel measurements must be taken in Measure Mode (rather than Record Mode) and then saved to an appropriate machine location.

Because the instrument has to buffer and manipulate twice the data, some of the more memory intensive operations are not available.

For vb instruments running firmware versions prior to 4.50 or 3.50 the following restrictions apply:

• Number of spectral lines is reduced to 800

• Number of time waveform samples is reduced to 2048

For vb instruments running firmware versions 4.50, 3.50 or higher the following restrictions apply:

• Number of spectral lines is reduced to 1600 (3200 for vb3000)

• Number of time waveform samples is reduced to 4096 (8192 for vb3000)

Note: To view your instrument's firmware version, turn the instrument off then on again. The current firmware version is displayed at power up.

Demodulation is also not possible on two channels simultaneously. Since there is never any cross-channel phase to analyze in Demodulation Mode this restriction is not detrimental.

Routes have always only had the ability to record at a single point and therefore single channel. Since routes automate data collection, they are much faster than using Record Mode to collect data, even though the recordings have to be single channel.

Important: The Ascent software identifies recordings by their location and date/timestamp. If the date/timestamp of two recordings are identical at the same location in the instrument, one is discarded. Therefore, when recording the two channels you must store them at different locations. This can be either a different point or different axes. It is good practice to store the two channels in measurement locations that reflect the position and direction in which the sensors were placed when the measurement was taken.

Setup

• To select dual channel measuring, from the Set Parameters Menu set the channel setting to CH1 & CH2.
• Connect accelerometers to both channels and take the measurement as normal.
• To store the measurements, with the spectrum/waveform/tachometer reading displayed, press MENU to open a pop-up window containing the Record option.
• Press [2] to record both channels then select the machine, point and direction for channel 1.
• Press ENTER to store the channel 1 measurement. The vb instrument will beep and display the message 'Recorded CH1 - Now record CH2'.
• Select a different location for the channel 2 measurement. Use SHIFT+BACK to navigate back through the machine and point menus if necessary.
• Press ENTER to store the channel 2 measurement. The spectrum/waveform/tachometer reading will be re-displayed with the message 'Recorded CH1 & CH2'.

Viewing Both Channels

For spectra and waveforms, the channel 1 measurement is displayed by default.

• To view channel 2 press [8]. The currently displayed channel is indicated at the bottom of the screen.

For tachometer displays, channel 1 is displayed on the left and channel 2 on the right.

FAQ ID: 11652 Last Reviewed: 3 November 2005