Breaking the 80 dB barrier with the LOW-LOSS AM band notch filter...
AM NRSC Measurements with a 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.
NRSC bandwidth measurements at 5 KW and above require
emissions measurement 80 db below carrier... but your spectrum analyzer can only see 78 ?
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
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..
Low-Loss Filter Specifications
Frequency range: .535 - 1.705 MHz
impedance: 50 ohms
Maximum VSWR @ 2F notch: 1.2:1
Maximum attenuation @ 2F notch: .4 dB
Maximum operating level: +30 dBm
Minimum Notch depth at .535: 13 dB
Construction: cast aluminum.
Documentation: User guide
Specifications and pricing subject
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