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Assume that we are making a measurement at 1 GHz and that the
difference in path-length between the two signals is exactly 1
wavelength. This means that test signal is lagging the reference signal
by 360 degrees (Figure 3-8). We cannot really tell the difference
between one sine wave maxima and the next (they are all identical), so
the network analyzer would measure a phase difference of 0 degrees.
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 measurement at 1 GHz.
The network analyzer will display this phase difference as –36 de
-
grees.
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 signals. 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 char
-
acteristics may be much smaller than the phase change due to
electrical length difference.
37xxxE OM 3-7
NETWORK ANALYZERS, A PRIMER NETWORK ANALYZERS
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
Wavelength
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
+180
+90
0
-90
-180
1.1
1.2
1.3
1.4
FREQUENCY,
GHz
MEASURED PHASE
Figure 3-10. Electrical Delay