Agilent Technologies 8753ET User Manual page 411

phase difference over the frequency range of interest. (Note: these phase values can be ±N
× 180 degrees where N is an integer.) If two lines are used (LRL), the difference in
electrical length of the two lines should meet these optimal conditions. Measurement
uncertainty will increase significantly when the insertion phase nears zero or is an integer
multiple of 180 degrees, and this condition is not recommended.
For a transmission media that exhibits linear phase over the frequency range of interest,
the following expression can be used to determine a suitable line length of one-quarter
wavelength at the center frequency (which equals the sum of the start frequency and stop
frequency divided by 2):
Electrical length cm
(
Electrical length cm
let:
f1 = 1000 MHz
f2 = 2000 MHz
VF = Velocity Factor = 1 (for this example)
Thus, the length to initially check is 5 cm.
Next, use the following to verify the insertion phase at f1 and f2:
( )
Phase degrees
where:
f = frequency
l = length of line
v = velocity = speed of light × velocity factor
which can be reduced to the following using frequencies in MHz and length in centimeters:
( ) approx
Phase degrees
So for an air line (velocity factor approximately 1) at 1000 MHz, the insertion phase is
60 degrees for a 5 cm line; it is 120 degrees at 2000 MHz. This line would be a suitable line
standard.
(
( )
= LINE –
0 length THRU
( ×
15000
)
= ------------------------------------------------------ -
( )
f 1 MHz +
( × × )
360 f l
= ------------------------------ -
v
× (
0.012 f MHz
= ------------------------------------------------------------ -
TRL*/LRM* Calibration (ES Models Only)
)
)
V F
( )
f 2 MHz
) × ( )
l cm
VF
Operating Concepts
7-73