AM NRSC MEASUREMENTS WITH SPECTRUM ANALYZER

Many modern spectrum analyzers, including the Tektronix 2710 / 2711 / 2712 series, produce residual phase noise at approximately 78 below the on-screen reference, and have degraded accuracy in the lower 10 db of the display. Some older analyzers are capable of displaying only 70 db on screen. A fundamental rule is that the accuracy of the test equipment must exceed the required limits of the device under test. Therefore, in order to quantify an 80 db test limit, one must be able to measure beyond 80 db.

The solution, implemented by the Chicago Radio Engineers, is to reduce the carrier level independently of the subject sidebands, and then increase the analyzer's sensitivity another ten dB. This places the last 10 db of analyzer resolution squarely between 80 and 90 db, allowing forays down to -88 db, keeping in mind the 78 dB sideband noise limitation.

Using a low-loss notch filter from Chris Scott and Associates allows the Chicago Radio Engineers to produce negligible attenuation at + - 75 khz and beyond, allowing direct measurement of sideband energy without filter slope correction factors. Experience also shows that this carrier notch depth is sufficient to prevent the intermodulation-induced high harmonic syndrome as well. The filter provides deeper notch depth for difficult harmonic or intermodulation product measurement. The attenuation curve relative to the NRSC mask is graphed below.

Low-Loss Filter Specifications

• Frequency range: .535 - 1.705 MHz
• Impedance: 50 ohms
• Connectors: BNC
• Maximum VSWR @ 2F notch: 1.2:1
• Maximum attenuation @ 2F notch: .4 dB
• Maximum operating level: +30 dB

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