ANSI SCTE 16:2018 pdf free download – Test Procedure for Hum Modulation
6. Background
In a coaxial telecommunications system, where AC power and radio frequency (RF) signals exist on the same conductor, hum modulation of a television signal is measured as a comparison between the modulation envelope (created by line power distortions of the RF carriers), and the peak voltage of the sync tips of the video signal. In a laboratory test environment, the video signal is replaced with a sine wave RF carrier. Under these conditions, the only modulation existing on the carrier is the line power related distortion (created by the device under test) of that carrier. Hum modulation is determined as a function of the line frequency current passing through the device by comparing the peak to peak modulation envelope of the power line distortion to the peak voltage of the carrier by utilizing a diode detector. Due to the non-linear characteristics of any diode detector, it is inaccurate to simply compare the AC modulation voltage and the rectified DC carrier voltage directly due to the voltage drop across the diode. Instead, the modulation voltage is compared to a calibrated change in the rectified carrier. There are two time-domain methods for measuring hum modulation in a laboratory environment, which are detailed in this procedure. These methods are commonly referred to as the 1 dB delta method, and the differential voltage method. These methods are described in Appendix 5.
7.4. Resistive Load Bank: capable of dissipating desired test power.
7.5. Display Devices:
7.5.1. Power analyzer capable of measuring rms current and voltage simultaneously.
7.5.2. Oscilloscope must have a minimum deflection sensitivity of 2.0 mV/div.
7.5.2.1. The oscilloscope used for the test must possess signal averaging capability.
7.6. Power supply:
7.6.1. Ferro-resonant quasi-square wave (trapezoidal wave with < 100 V/ms slew rate).
7.6.2. AC voltage output (50/60 Hz) at desired test voltage.
7.6.3. Rated current output of at least 30% greater than the desired test current.
7.7. Low Pass Filter: DC – 1 kHz(minimum)
7.7.1. Shielded.
7.7.2. High Impedance (5 k Ω or greater).
7.7.3. > 30 dB attenuation above 30 kHz.
7.7.4. This filter may be contained in the differential preamplifier used in the test.
7.8. AM Detector:
7.8.1. 75 Ω Input Impedance.
7.8.2. Frequency Range: 200 kHz to 1.002 GHz.
7.8.3. Maximum VSWR: 1.2:1 to 1.002 GHz.
7.9. Required inter-connect cables and connectors.
7.9.1. Quad-shield 75 Ω coaxial cable, for RF only paths.
7.9.2. Rigid or semi-rigid 75 Ω coaxial cable (1/2” diameter or greater), for RF and AC common paths, with a current carrying capacity of at least 50% greater than desired test currents.