In-service CATV HUM measurement technique

• US 5,394,185 A
• Filed: 01/08/1993
• Issued: 02/28/1995
• Est. Priority Date: 01/08/1993
• Status: Expired due to Term

• Current Assignee: Tektronix Incorporated (Fortive Corp.)
• Original Assignee: Tektronix Incorporated (Fortive Corp.)
• Inventors: Bernard, Kyle L.
• Primary Examiner(s): Kostak, Victor R.
• Assistant Examiner(s): Flynn, Nathan J.
• Application Number:
  US08/001,951
• Time in Patent Office: 781 Days
• Field of Search: 358/21 R, 358/21 V, 358/139, 358/86, 358/155, 358/10, 358/166, 358/167, 358/36, 358/37
• US Class Current: 348/192
• CPC Class Codes: H04N 17/00 Diagnosis, testing or measuring for television systems or their details

Abstract

An in-service hum and low frequency distortion measurement technique for in-service cable television channels having a video modulated carrier frequency obtains peak samples of the horizontal sync signal to effectively filter out the vertical interval frequency. The data record formed by the peak samples is filtered and input to a fast Fourier transform to obtain magnitude values for the fundamental and first harmonic frequency components corresponding to a powerline. From these magnitude values hum and low frequency distortion is computed as a percentage relative to the amplitude of the carrier frequency. The same procedure may be used with an input CW signal in an out-of-service measurement for hum only, the difference between the in-service and out-of-service measurements determining the low frequency distortion due to the video signal.

Claims

1. A method of measuring hum and low frequency distortion for an in-service channel of a cable television system having a carrier frequency modulated by a video signal comprising the steps of:

acquiring a data record for the in-service channel representing peak values for horizontal sync pulses of the video signal; and

performing a fast Fourier transform on the data record to obtain peak-to-peak percentage values of the hum and low frequency distortion relative to a magnitude of the carrier frequency at a fundamental and first harmonic frequency of a powerline providing power to the cable television system.

2. A method as recited in claim 1 wherein the acquiring step comprises the steps of:

configuring a spectrum analysis instrument to center the carrier frequency in zero span mode;

enabling a single sweep mode of the spectrum analysis instrument to acquire the data record; and

low pass filtering the data record so that at least the fundamental and first harmonic frequencies are passed, including removing transient data points from the data record.

3. A method as recited in claim 1 wherein the performing step comprises the steps of:

removing d.c. bias from the data record;

applying a window function to the data record to eliminate scalloping effects on the data record;

performing the fast Fourier transform on the data record to obtain a magnitude spectra;

restoring d.c. bias to the magnitude spectra; and

searching for peak magnitudes at the powerline and harmonic frequencies from which the percentage values of the hum and low frequency distortion are calculated.