GSP-830 User Manual 134 Internal reference signal, median stability. Available when the ±1ppm stability module (page174) is installed. The MED icon appears at the bottom of the display. Internal reference signal, high stability (reserved for future use). GSP-830 as slave (external reference signal) Panel operation 1. Press the Option key. Option 2. Press F4 (Ext Ref Freq) to enable external reference signal.Ext Ref Freq10MHzF 4 3. Use the Left/Right keys or Scroll knob to select the external reference frequency. 4. Connect the external reference signal to the input terminal on the rear panel. 5. The external reference signal icon appears at the bottom of the display. Signal type TTL signal Frequency 10 preset frequencies are available (units in MHz). 1.0 1.544 2.048 5.0 10.0 10.24 13.0 15.36 15.4 19.2 Status check 1. Press the System key. System 2. Press F6 (More). More...F 6 3. Press F4 (System Config On). System ConfigOn OffF 4 The system configuration window appears. The External Reference Signal sign shows the status: - enabled, - disabled.