HP 8753E manual Measuring Phase Distortion, Deviation From Linear Phase

8. Press (-Ref) ~~~~~~~~~~		ad turn the front panel knob to increase me ele&ricd
	>,...>,......... .......... ............ ..<.:.,.,.:.......... i.i..i:<<<Y .:......:::.. ,.:.:..,... ..ss

length until you achieve the best flat line, as shown in Figure 2-35.

The measurement value that the analyzer displays represents the electrical length of your device relative to the speed of light in free space. The physical length of your device is related to this value by the propagation velocity of its medium.

Note Velocity factor is the ratio of the velocity of wave propagation in a coaxial cable to the velocity of wave propagation in free space. Most cables have a relative velocity of about 0.66 the speed in free space. This velocity depends on the relative permittivity of the cable dielectric (cr) as

Velocity Factor = I 6

where Ed is the relative permittivity of the cable dielectric

You could change the velocity factor.to compensate for propagation velocity by
.Gst.. .: “” i;“.	.;;;;..;;;;; ..>>..		(enter the value) @. This will help the
pressing (GJ :,&lRJE~ ~~~~~~~~~~~’
//1,...:..:......:::.;:: .A.. ;...A..s	:::....::........i............~.,.,.,.,. .....A.,..i;::.:	.._...... .,.

analyzer to accurately calculate the equivalent distance that corresponds to the entered electrical delay.

CENTER 134 000 000 MHZ SPAN 2 000 000 MHZ

Figure 2-35. Example Best Flat Line with Added Electrical Delay

9. To display the electrical length, press:

In this example, there is a large amount of electrical delay due to the long electrical length of the SAW filter under test.

Measuring Phase Distortion

This portion of the example shows you how to measure the linearity of the phase shift over a range of frequencies. The analyzer allows you to measure this linearity and read it in two different ways: deviation from linear phase, or group delay.

Deviation From Linear Phase

By adding electrical length to “flatten out” the phase response, you have removed the linear phase shift through your device. The deviation from linear phase shift through your device is all that remains.

242 Making Measurements