Phase and amplitude stability checks

This test establishes how much dynamic range is available in the linear channel for clutter cancellation. This is useful for determining the maximum clutter correction that can be achieved.

note Disconnect or otherwise disable the gain control so that the linear receiver is at full gain during these test.
  1. Set Ascope to run as follows:
    Parameter Value
    Pulse Width: 0.5
    PRF: Highest value
    Plot Parameters: T Spec
    Doppler Filter: None
    LOG Filter: None
    Thresholds: Off
    Max Range: 20
    Pulse Samples: 128
    Spectrum Window: Rect

    This creates a Doppler spectrum with a fairly rapid update for searching.

  2. Use the Reflectivity against Range plot (Total dBZ) to select a range where there are strong clutter targets and observe the peak-to-noise level for the strongest clutter targets. You may want to scan the antenna very slowly, stopping at strong clutter targets. Estimate the peak-to-noise level ratio by eye (20 dB per division) for several of the strongest targets by estimating the average noise level. Spectrum averaging helps to reduce the fluctuations. For a 128-point spectrum (Pulse Samples: 128) the clutter to total noise power in dB is the observed peak-to-noise ratio minus 10 log(128), that is:

    dB(Clutter-to-noise) = dB(Peak-to-noise) - 21 dB

    This is a measure of the available dynamic range of the system. If the value is 20 dB, (typical for a magnetron system), then the maximum clutter correction that can be achieved is approximately 20 to 30 dB depending on the number of samples averaged together.

Another way to assess the coherency of the system is to use the SQI and phase noise values displayed in the upper right corner of the spectrum display. The SQI is a number between 1 (perfectly coherent) and 0 (perfectly incoherent) which is related to the pulse-to-pulse phase noise, the peak-to-noise and the clutter-to-noise. The following table summarizes these relationships. Note that the calculations in the table assume you are looking at a pure clutter target with no weather or coherent artifacts such as 50 or 60 Hz or image spectra. For example, an SQI of 0.9800 corresponds to a phase noise of 8.1°, a clutter-to-signal ratio of 16.9 dB and a 128-point peak-to-noise (observable in the spectrum plot) of 37.9 dB.

Coherency relationships
SQI Phase Noise (Degrees) Clutter-to-Noise (dB) 128-Point Peak-to-Noise (dB) 256-Point Peak-to-Noise (dB)
0.99998 0.26 47.0 68.0 71.0
0.99996 0.36 44.0 65.0 68.0
0.99994 0.44 42.2 63.2 66.2
0.99992 0.51 41.0 62.0 65.0
0.99990 0.57 40.0 61.0 64.0
0.99988 0.63 39.2 60.2 63.2
0.99986 0.68 38.5 59.5 62.5
0.99984 0.72 38.0 59.0 62.0
0.99982 0.77 37.4 58.4 61.4
0.99980 0.81 37.0 58.0 61.0
0.99970 0.99 35.2 56.2 69.2
0.99960 1.15 34.0 55.0 58.0
0.99950 1.28 33.0 54.0 57.0
0.99940 1.40 32.2 53.2 56.2
0.99930 1.52 31.5 52.5 55.5
0.99920 1.62 31.0 52.0 55.0
0.99910 1.72 30.5 51.5 54.5
0.99900 1.81 30.0 51.0 54.0
0.99800 2.56 27.0 48.0 51.0
0.99700 3.14 25.2 46.2 49.2
0.99600 3.63 24.0 45.0 48.0
0.99500 4.06 23.0 44.0 47.0
0.99400 4.44 22.2 43.2 46.2
0.99300 4.80 21.5 42.5 45.5
0.99200 5.13 20.9 41.9 44.9
0.99100 5.45 20.4 41.4 44.4
0.99000 5.74 20.0 41.0 44.0
0.98000 8.14 16.9 37.9 40.9
0.97000 10.00 15.1 36.1 39.1
0.96000 11.58 13.8 34.8 37.8
0.95000 12.98 12.8 33.8 36.8
0.94000 14.25 11.9 32.9 35.9
0.93000 15.43 11.2 32.2 35.2
0.92000 16.54 10.6 31.6 34.6
0.90000 18.60 9.5 30.5 33.5