Fabricating and defining calibration standards for TRL/LRM

When calibrating a network analyzer, the actual calibration standards must have known physical characteristics For the reflect standard, these characteristics include the offset in electrical delay (seconds) and the loss (ohms/second of delay). The characteristic impedance,

$@@@: Z@., is not used in the calculations in that it is determined by the line standard. The reflection coefficient magnitude should optimally be 1.0, but need not be known since the same reflection coefficient magnitude must be applied to both ports.

The thru standard may be a zero-length or known length of transmission line. The value of length must be converted to electrical delay, just like that done for the reelect standard. The loss term must also be specified.

The line standard must meet specific frequency related criteria, in conjunction with the length used by the thru standard. In particular, the insertion phase of the line must not be the same as the thru. The optimal line length is l/4 wavelength (90 degrees) relative to a zero length thru at the center frequency of interest, and between 20 and 160 degrees of phase difference over the frequency range of interest. (Note: these phase values can be fN x 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) = (LINE - 0 length THRU)

(15000 x VF)

Electrical length (cm) = fl(MH~z) + f2(MHz)

let:

fl = 1000 MHz

f2=2OOoMHz

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 fl and f2:

Phase (degrees) =

where:

f = frequency l=lengthofline

V= velocity = speed of light x velocity factor

(360x f x 1)

V

which can be reduced to the following using frequencies in MHz and length in centimeters:

0.012 x f(MHz) x l(cm)

Phase (degrees) approz =

V F

So for an air line (velocity factor approximately 1) at 1000 MHz, the insertion phase is

60degrees for a 5 cm line; it is 120 degrees at 2000 MHz. This line would be a suitable line standard.

648 Application and Operation Concepts