Quote:
Originally Posted by Penthode
After thinking about this a bit, I believe the large inductance was to specifically limit the Q bandwidth to roll off at 500kHz. The I channel is 1.5 MHz wide.
Remember the I and Q are transmitted vestigial sideband in quadrature which means extending the Q bandwidth will lead to quadrature crosstalk. Further in this early design both Y and I channels have delay inserted to align with the limited bandwidth Q channel: Y a larger delay than I. Reducing the peaking coil inductance will extend the Q bandwidth which will reduce the delay.
All sets I believe following the 21CT55 used equa-band narrow channel (500kHz) chroma demodulation apart from my CTC5 Deluxe which has a equa-wide-band chroma demodulator. This means the CTC5 Deluxe intentionally introduced quadrature crosstalk in what appears to be an attempt to increase marginally chroma resolution. The crosstalk is noticeable at abrupt color transitions. (There was a 1955-56 RCA review I recall reading about this many decades ago. I need to look for it).
So I am going to investigate this further. Certainly I can do some tests with the peaking in circuit resonance with the technique outlined in my Measurements Lab. Grid Dip Meter applications notes pertaining to video amplifier design which outlines how to determine peaking coil inductance in circuit.
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I think you've got it right. Since the Q coil is large, it's resonant frequency probably is also more critical - too low and you could bite into the 500 kHz bandwidth and get ringing. Going the other way, a little quadrature distortion could be hard to see on a small screen.