EXFO IQS-1600 High-Speed Power Meter for IQS-500/600 - Manual (Page 30)

Setting Up Your High Speed Power Meter
22 IQS-1600
Setting Channel Display(IQS-1620, IQS-1640)
Selecting a Virtual Channel (IQS-1620, IQS-1640)
In addition to the optical channels, your application offers one virtual
channel for the dual-channel power meter and two virtual channels for the
quadruple-channel power meter.
A virtual channel is a combination (sum or difference when using dB or
dBm as units, product or ratio when using watts or watts/watts as units—
see the table on page 24) of the measured power from any two optical
channels. This feature provides an extremely flexible comparative
measurement display. A virtual channel is used and controlled in the same
way as an optical channel.
The virtual channel is an excellent tool for comparing two optical channels.
For example, if two devices (connected to channels 1 and 2) are supposed
to have identical power levels, displaying a virtual channel (defined as
channel 1 minus channel 2) provides quick monitoring of both devices and
indicates any power fluctuations. You may also want to reference the drift
of a source in real time by cancelling it out in a virtual channel.