Phase Or Group Delay Measurements; Phase Measurements; Phase Linearity And Group Delay - Agilent Technologies 8753ET User Manual

Making Mixer Measurements

Phase or Group Delay Measurements

Phase or Group Delay Measurements
For information on group delay principles, refer to "Setting the Electrical Delay" on page 1-38.

Phase Measurements

When you are making linear measurements, you must provide a reference for determining phase by splitting
the RF source power and send part of the signal into the reference channel. (This does not work for
frequency offset measurements, since the source and receiver are functioning at different frequencies.)
To provide a reference signal for the phase measurement, you need a second mixer. This mixer is driven by
the same RF and LO signals that are used to drive the mixer under test. The IF output from the reference
mixer is applied to the reference (R) channel of the analyzer.

Phase Linearity and Group Delay

Group delay is the rate of change of phase through a device with respect to frequency (d/d). Traditionally,
group delay has been used to describe the propagation delay ( ), and deviation from linear phase through a
g
linear device. However, this parameter also contains valuable information about transmission delay and
distortion through a non-linear device such as a mixer or frequency converter. For example, flat group delay
corresponds to low modulation distortion (that is, carrier and sidebands propagate at the same rate).
Phase linearity and group delay are both measurements of the distortion of a transmitted signal. Both
measure the non-linearity of a device's phase response with respect to frequency.
In standard vector error-correction, a thru (delay=0) is used as a calibration standard. The solution to this
problem is to use a calibration mixer with very small group delay as the calibration standard.
An important characteristic to remember when selecting a calibration mixer is that the delay of the device
should be kept as low as possible. To do this, select a mixer with very wide bandwidth (wider bandwidth
results in smaller delay). The accuracy of this measurement depends on the quality of the mixer that is being
used for calibration and how well this mixer has been characterized. The following measurement must be
performed with a broadband calibration mixer that has a known group delay. The following table lists the
specifications of two mixers that may be used for calibration:
2-34