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RF Vector Impedance Meter Uses – Application Note 19

 

The Tomco TE1000 RF Impedance analyzer is a powerful, easy to use test instrument providing outstanding performance at a very affordable price.

View TE1000 Impedance Analyzer specifications

Hints for making accurate impedance measurements.

The TE1000 RF impedance analyzer is capable of extremely accurate measurements of a wide range of impedances. However, as with any high frequency measurement, a certain amount of care must be taken to ensure that the results are not "contaminated" by stray impedances. Always observe the following precautions when using the TE1000 impedance meter (or any other impedance meter):

  • Minimize any lead lengths between the probe and the impedance to be measured. Even a few millimeters of wire can be significant when measuring low impedances at high frequencies.
  • Connect the load as close as possible to the base of the probe tip. This is the point that the TE1000 impedance meter uses as its reference.
  • Avoid having any excess lead length "hanging off" the probe tip: such lead length acts as a small antenna and appears as a capacitor in parallel with the impedance being measured. This can be significant when measuring high impedances at high frequencies.
  • Keep your fingers clear of the probe tip when making a measurement. Hold the probe body and earth ring only.

Measuring the length of a coaxial cable

Connect the TE1000 impedance analyzer to the cable, and terminate the cable with a "perfect" reflector - an open circuit is usually the easiest. If the total electrical length of the cable is less than one half-wavelength then the angle of the reflection coefficient (in polar format) is twice the electrical length of the cable:

Electrical length in degrees = angle of reflection coefficient / 2

Furthermore, if the velocity factor, Fc, of the cable is known then the physical length of the cable can be calculated from:

Physical length = ( electrical length in degrees x Fc x 3 x 108 ) / (360 x frequency in Hz )

For example, if the reflection coefficient is 0.5<40° then the cable is electrically 20° long. If the velocity factor of the cable is 0.66 and the frequency is 10MHz, then the cable is physically 1.1 meters long.

Measuring the loss in a length of coaxial cable

Connect the TE1000 impedance analyzer to the cable, and terminate the cable with a "perfect" reflector - an open circuit is usually the easiest. Then the loss in the cable is equal to the return loss divided by two. In other words, the return loss is the total loss in the cable, to the end and back again. 

 

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