Break the 80 dB barrier - Chris Scott & Associates -

Breaking the 80 dB barrier with the LOW-LOSS AM band notch filter...

AM NRSC Measurements with a spectrum analyzer

NRSC bandwidth measurements at 5 KW and above require emissions measurement 80 db below carrier... but your spectrum analyzer can only see 78 ? 

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 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 dbc, keeping in mind the 78 dB sideband noise limitation.  The resulting carrier attenuation is shown in the plot.  The CS&A low-loss notch filter produces  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 classic version of the filter provides deeper notch depth for difficult harmonic or intermodulation product measurement. The attenuation curve relative to the NRSC mask is graphed below.

For those wanting both functions in one unit, a combined notch filter is available which includes dual outputs, allowing both the deep notch and the low-loss functions in the same unit..

The low-loss notch filter does not attenuate beyond + - 75 khz

Low-Loss Filter Specifications

Specifications and pricing subject to change
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