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

Phase or Group Delay Measurements

For information on group delay principles, refer to
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
(dφ/dω). Traditionally, group delay has been used to describe the propagation delay (τ
and deviation from linear phase through a 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:
Model Number
ANZAC
MCD-123
"Setting the Electrical Delay" on page
Useful Frequency
Range
0.03 to 3 GHz
Making Mixer Measurements
Phase or Group Delay Measurements
Group Delay
0.5 ns
),
g
2-33