By William Gregory Kostka, Gillcable TV
An active CATV system frequency response is typically measured by using either a high-level or a low-level sweep-but both of these methods have inherent limitations. Gillcable's on-going gated mid-level sweep project is developing a new method which can improve resolution and readability, while simultaneously minimizing system interference.
High-level sweeps provide good signal to noise ratios and are easy to detect, but they significantly interfere with existing cable signals. Because of this interference, the sweep repetition rate is usually low, thus impairing readability. While low-level sweep is relatively non-interfering, its low level reduces the signal to noise ratio thus impairing resolution. Furthermore, any higher level cable signal will mask the sweep and also impair readability.
Gillcable's gated mid-level sweep project is based on selective spectrum sampling; it is not an analog sweep, but a series of samples at arbitrary frequencies. The sample points can be set to avoid any critical cable frequencies. But beyond just that, to avoid interfering with television pictures, the signal source will only generate signals on an occupied channel during video blanking intervals.
Since the gated signals only minimally interfere with existing signals, sweep level can be set high enough to deliver good signal to noise ratios for a high-resolution display.