IEC 61580-3:1997 pdf download – Methods of measurement for wave guides -Part 3: Variation of group delay

IEC 61580-3:1997 pdf download – Methods of measurement for wave guides -Part 3: Variation of group delay.
This part of IEC 61580 is applicable to the variation of group delay of a wave propagated in waveguide or waveguide assemblies. The objective of the test procedures given below is to ctiaracterize the group delay variation of a wave propagated in waveguide or waveguide assemblies.
2 General
Group delay variation of a waveguide versus frequency, describes the variations of the derivative or the slope of the phase characteristic within the bandwidth, Group delay variation, often referred to as group delay distortion also, is found in all waveguide with wave propagation modes having a non-linear dependence of frequency on wavelength. Furthermore, dimensional variations along wavegulde. single or multiple reflections, filter structures, multi-mode propagation, etc. can cause additional group delay variations.
Signals of non-zero bandwidth transmitted through media exhibiting large group delay variations can be altered In such a way that. In the extreme case, the Information contained in the bandwidth cannot be restored.
3 PrincIple
Three main measurement principles are currently used:
a) static numerical differentiation:
b) AM measurement:
C) FM measurement.
Methods a) and C) require expensive and/or complicated test equipment, therefore assembling a test set-up from single components can not be recommended- However, dedicated test equipment does provide accurate group delay variation measurements with convenient data presentation.
Method b) can be assembled from current RF-laboratory test equipment.
For the test, an unmodulated or modulated RFslgnaI Is sent through the waveguide under test (WLIT). At the output, the phase of the RF signal (or of the demodulated signal) is compared with the reference phase at the input of the WUT.
NOTE — The comparison of groi delay variation measurements obtained by direct phase measurement (and subsequent computation) w1h test results (tom a modulated carrier method may be difficult. Compensation tot th. modulation bandwidth in respect to the discrete step-width in tlwi computation routine lot group da4ay calculation may b necessary.
4 Test s.t.up
Two test set-ups are described below. Figure 1 describes a test set-up to be used for any of the methods a), b), or C) allowing direct measurement of group delay variation, Figure 2 describes a test set-up to be used for method b). It can be assembled from standard laboratory test equipment. Additional computation for deriving the correct group delay values is necessary.
4.1 Test set-up I
This test set-up uses a network analyzer which directly measures the phase at two closely spaced trequencies and then computes the slope (approximation of the derivative).
REMARK — Atlentiøn should be paid to the interval between the two frequencies (called aperlurel. Increasing th. apwilur. ends 10 average Out hrwi grain variations in the group delay data because Ito fine grain variations in the phase data are lost Errors will occur when the,. is more than 180 of phase shill between adiaceril frequency points. However, for cl% resolution uncertainty, one must maintain I phase shut beiween frequency points.