user 41968
Well-known member
- Joined
- Jul 8, 2010
- Messages
- 513
Back in the early 70s, a filter that maintained 90° between its two output was called a "dome filter." I made making two strings of single-pole all-pass filters. For the application, ±5° was close enough so the turnover frequencies were interleaved in roughly 3 to 1 ratios. Use more stages and smaller ratios to hold the 90° arbitrarily tight. These filters were used in radio communications for decades to make quadrature detectors that worked over octaves of frequency range. I used mine to maintain quadrature in a variable-frequency motor drive (from half-speed to double speed) for a 3M "Iso-Loop" tape machine. Using the dome filter, plus a high-pass filter to increase drive voltage linearly with frequency as well, two audio power amps driving the motor windings directly gave it full torque over the 4:1 speed range.
Such a filter is at the heart of the attached circuit I found in a quick web search. For what you're doing, drive the lines marked "L" and "R" together. Output of the filter is on lines marked "I" and "Q" (traditional for In-phase and Quadrature).
Thanks Bill for posting my rendition of the Studer 90° Filter with Weaver alignment. I hope it proves useful to someone. I may do a board for it soon so others can experiment.
At a minimum it can be used to draw pretty flowers on a goniometer.