Coarse adjustment of gain and offset pots

cautionCaution

Perform this procedure when you have adjusted all signal line drivers and you have checked the I, Q, and LOG video signals with a scope to make sure that they are within the proper limits of the signal processor A/D converters.

The analog-to-digital converter section of each DSP contains 2 potentiometers for each digitized video signal. Nominally, I and Q signals should be between ±1 V and the LOG channel 0 ... 2 V. The potentiometers allow the A/D converter span and offsets to be adjusted around these nominal values so that the full span of A/D values is available to the signal processor.

These pots are on the outside edge of the RVP. When signal processors are used for reflectivity-only processing, then only the LOG video channel must be adjusted.

Otherwise the signal processor could be damaged if an inappropriate voltage is applied.

note For information on the location and use of the adjustment pots, see the section on A/D alignment in RVP10 User Guide (M212604EN).

There are two ways of setting these controls. The recommended procedure is to use the Ascope AI, AQ, and ALOG plots to view the raw time series against range.

For information on setting these controls without Ascope, see RVP10 User Guide (M212604EN).

  1. Run Zauto and check the value of the calibration slope.

    Typical values are 0.3 ... 0.5. The default is 0.5.

    If the calibration slope is 0, set it to the correct value.

  2. Disconnect the gain control output from the processor.

    This makes sure that the linear receiver is operating at full gain. If the processor gain is not used on your system, check with your manufacturer to see how the receiver can be set to full gain at close range.

  3. In the terminal prompt, type: ascope&
  4. Set up Ascope as follows:
    PRF 250
    Max Range 100
    Number of Bins 100
    Plot Parameters AI AQ ALOG
  5. While running Ascope, turn on the transmitter and point the antenna in a direction so that the main bang of ground return can be observed.

    Sometimes it helps to scan the antenna very slowly to get a representative sample of targets.

    1. Make sure all test signals are off.
    2. Adjust the LOG offset pot so that the far range noise is approximately 10 % off the bottom of the plot (as compared to the full range of A/D values).
    3. Select Noise to invoke a new noise sample and verify that the Z offset in the status section is 15 ... 30 units.
  6. Adjust the LOG gain pot so that the saturated main bang at close range is below the high limit.
    1. If there is difficulty observing the saturated main bang close to range 0, reduce the Max Range field to 10 km (6.2 mi).
    2. Readjust the offset as required, because changing the gain can affect the offset.
    tip

    Another technique is to use an RF test signal generator to adjust the A/D range to correspond to the dynamic range of the receiver (typically 80 dB).

    To do this, use Ascope to observe the LOG channel A/D against range plot (ALOG).

    Do not select Noise or leave the Sample Offset Noise Sampling Mode set to Continuous.

    Use the signal generator in CW mode. The transmitter is usually turned off to avoid damage to the signal generator while it is connected.

    Now starting from the minimum value of the signal generator output, increase the signal level until the LOG A/D values just start to increase. This is the approximate noise level.

    Adjust the gain so that, for example, the LOG A/D values are at approximately 250 when the signal generator level is increased to 80 dB above the noise level.

  7. Adjust the I and Q pots until the signal is approximately centered in the display.
    1. Select Noise to verify that they are within ±5 units of 0.
    2. If not, adjust accordingly.
  8. Make sure the gain control is disconnected or otherwise disabled so that the receiver is operating at full gain for this test.
    1. Adjust the I and Q gain pots so that the strongest clutter targets at close range are just below saturation.
    2. Adjust the amplitude of the I and Q signals with the gain pot so that they are equal.

      Slight saturation on the very strongest clutter targets is tolerable because no useful weather information can be obtained in the vicinity of these anyway.

    3. Recheck and readjust the offsets as required.