Calibrating the DSA to Measure FM Deviation

When and where to use:

The Bessel Null technique is only applicable to sine waves used for testing and calibration. The DSA cannot be used to measure peak deviation with anything other than a sine wave. In other words, this cannot be used with voice to check the deviation. This procedure should be performed before starting to optimize/calibrate the simulcast system.
NoteThe RF Analyzer, RF Generator, and Channel Analyzer referenced in the steps are part of the service monitor.

Procedure:

  1. Connect the Service Monitor demod output to the DSA input.
  2. Set up the service monitor to generate an RF signal into the RF Analyzer (loopback operation).
    NoteSome service monitors, such as the GDI 2600 series, must be placed in Duplex Generate mode to support loopback.
  3. Set the RF Generator modulation source frequency and FM deviation to the values in the table below depending upon your system channel bandwidth:
    Bessel Null Calibration Setup
    System Bandwidth Modulation Frequency FM Deviation
    12.5 kHz 1039.6 Hz 2.500 kHz
    25 kHz 905.8 Hz 5.000 kHz
  4. Set the Channel Analyzer display for a Span of 10 kHz.
    NoteThe GDI 2600 series does not support a spectrum analyzer span small enough to see the carrier null. You can use an external spectrum analyzer to set the deviation using the Bessel null. If a suitable spectrum analyzer is not available, set the GDI generator modulation 5% low (2.375 kHz or 4.75 kHz) and calibrate the DSA to this level.
  5. Vary the RF Generator Deviation (not the Modulation Frequency) to obtain the minimum level of the carrier frequency (null). The RF Generator is now generating an FM signal with exactly the FM Deviation in the table above.
  6. Note the level of the Modulation Frequency signal on the DSA. This level is the Service Monitor demod output level that equals the maximum peak deviation allowed (100% system deviation). The 60% system deviation level is -4.44 dB lower than the level noted above.
  7. Turn off the RF Generator.