Compute Unbalanced Response (Channel Quality)
determines whether to compute the unbalanced channel response (in addition to the balanced channel response, which is always computed). This mode requires a special stimulus signal generated by the 89600 VSA software (see Save to File…).
Several new metrics and trace results are available for measuring unbalanced (and balanced) channel response:
Summary trace - IQ impairment metrics (only available when Compute Unbalanced Response is enabled):
Balanced and Unbalanced Channel Frequency Responses - the balanced response is the part due to reflections/multipath. The unbalanced response is the part due to IQ impairments.
I and Q Channel Frequency Responses - computed from the Balanced and Unbalanced responses, expressing the response separately for the In Phase and Quadrature Phase components of the channel.
Inverse versions of Balanced, Unbalanced, I and Q Channel Frequency Responses - you can use these to predistort the I/Q samples before sending through an I/Q modulator. Or you can correct I/Q samples received from an I/Q demodulator.
Impulse Responses - corresponding to the frequency responses.
Making an Unbalanced Channel Response Measurement
To make a successful measurement of the balanced and unbalanced responses, make sure you do the following:
- Configure the VSA measurement Center Frequency to be the same as the LO of the I/Q modulator
- Enable Compute Unbalanced Response
- Set number of tones and spacing as desired
- Update the stimulus with the desired Predefined settings
- Export and download the stimulus to your signal generator using the Download to Source button or Save to File...
If you have selected a Source in the Source menu, the Download to Source button will be enabled and can be used to download the stimulus directly to the source. Note that this will connect to the source and download the waveform, but will preserve the state of other source settings.
The Predistort checkbox will compensate the stimulus for the currently measured channel such that the measured channel after predistortion will tend towards being flat and having no IQ impairments.
The Predefined section will allow you to set the stimulus to a predefined value of constant (0 dB) amplitude and the selected phase characteristic. Note that Parabolic phase will cause the stimulus to resemble a chirp. Random phase will simulate an OFDM Orthogonal Frequency Division Multiplexing: OFDM employs multiple overlapping radio frequency carriers, each operating at a carefully chosen frequency that is Orthogonal to the others, to produce a transmission scheme that supports higher bit rates due to parallel channel operation. OFDM is an alternative tranmission scheme to DSSS and FHSS. signal with high PAPR peak to average power ratio: Peak-to-average power ratio is the ratio of the peak envelope power to the average envelope power of a signal during a given period of time. (peak to average power ratio).
See Also
