Specifications

analyzer would measure a phase difference of 0 de
-
grees.
Now consider that we make this same measurement
at 1.1 GHz. The frequency is higher by 10 percent so
therefore the wavelength is shorter by 10 percent.
The test signal path length is now 0.1 wavelength
longer than that of the reference signal (Figure 3-9).
This test signal is:
1.1 X 360 = 396 degrees
This is 36 degrees different from the phase mea
-
surement at 1 GHz. The network analyzer will dis
-
play this phase difference as –36 degrees.
The test signal at 1.1 GHz is delayed by 36 degrees
more than the test signal at 1 GHz.
You can see that if the measurement frequency is
1.2 GHz, we will get a reading of –72 degrees, –108
degrees for 1.3 GHz, etc. (Figure 3-10). There is an
electrical delay between the reference and test sig-
nals. For this delay we will use the common industry
term of reference delay. You also may hear it called
phase delay. In older network analyzers you had to
equalize the length of the reference arm with that of
the test arm to make an appropriate measurement
of phase vs. frequency.
To measure phase on a DUT, we want to remove this
phase-change-vs.-frequency-due-to changes in the
electrical length. This will allow us to view the
actual phase characteristics. These characteristics
may be much smaller than the
phase-change-due-to-electrical-length difference.
NETWORK ANALYZERS, NETWORK
A PRIMER ANALYZERS
MS462XX OM 3-7
REFERENCE
SIGNAL
SPLITTER
TEST
SIGNAL
MICROWAVE
SOURCE
PHASE
DETECTOR
LONGER BY
ONE WAVELENGTH
LENGTH (360 degrees)
Figure 3-8. Split Signal where Path
Length Differs by Exactly One Wave
-
length
REFERENCE
SIGNAL
SPLITTER
TEST
SIGNAL
MICROWAVE
SOURCE
SAME PATH
LENGTH -BUT-
WAVELENGTH
IS NOW SHORTER
1.1 WAVELENGTHS = 396 degrees
PHASE
DETECTOR
Figure 3-9. Split Signal where Path
Length is Longer than One Wave
-
length
+180
+90
0
-90
-180
1.1
1.2
1.3
1.4
FREQUENCY,
GHz
MEASURED PHASE
Figure 3-10. Electrical Delay