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  • Writer's pictureJim

Hurry up and wait

Sometimes, things move just a little too slowly. For example, electricity in a coax cable crawls along at the speed of light times the cable velocity factor. When attempting to phase-align a number of analog signal channels using cables of varying lengths, the pesky physical laws can become a nuisance.

Trombone RF delay line

It's commonplace to length-match traces on PCB designs to insure phase alignment. But it's difficult to maintain such precise alignment to devices under test via system cabling. Consequently, phase adjusters or delay lines are sometimes used in RF systems to trim signal path lengths to meet requirements. The folks at Colby Instruments have some nifty solutions, including RF waveguides whose length can be adjusted programmatically via a stepper motor. Another variant allows discrete programmed delays via cable segments selected via solenoids.

Colby CPDL-100A

The Colby delay lines used for research in my lab (PDL-10A and CPDL-100A) allow phase adjustment from 0 to 16125 pS with a resolution of 0.5 pS. I've written a a small applet in C++ under QtCreator to control these instruments individually or in cascade. The Windows x64 individual control applet is available for download here.

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