Gain and phase measure the amplification and the phase lag between the stimulus
channel and the response channel at the test frequency. This lag time must be accounted
for.
Gain and phase perform a dual-channel
measurement that requires both stimulus and response signals. A single-channel
measurement reads only the response signal. To account for the the lag time between the
stimulus channel and the response channel:
Identify the detected tone amplitude and phase on the stimulus channel.
Treat the stimulus tone amplitude and phase as references when specifying the
tone relative amplitude and phase of the response channel.
Search for a response channel tone at the same frequency as the stimulus
tone.
You have the option of generating
the stimulus signal from the acquisition device, a separate output device, or an
external source when the stimulus and response measurements are acquired. If you are
using a dynamic signal acquisition (DSA) device, NI recommends that you perform a
dual-channel instead of a single-channel measurement when possible. Performing a
dual-channel measurement takes full advantage of the interchannel gain mismatch and
simultaneous sampling of the analog input channels. Refer to the DSA device
specifications for more information. For example, in single-channel mode, the
measurement uncertainty when using an NI PXI-4461 is 0.1 dB, whereas in dual-channel
mode, the measurement uncertainty is typically 0.01 dB.