This section describes the AM/FM audio demodulation capability (available with Option 24) and key characteristics of analog amplitude modulation (AM) and frequency modulation (FM). The AM/FM audio measurement provides the following features:
• Ability to listen to audio and sweep trace data simultaneously with built-in speaker or headphone. See Figure: AM/FM Audio (RF Spectrum).
• Selectable demodulation frequency with ability to tie frequency to markers.
• Record audio to WAV file
• Selectable modulation modes such as:
• AM
• Upper sideband (US Band)
• Lower sideband (LS Band)
• Narrowband FM
• 6.25 kHz
• 12.5 kHz
• 25 kHz
• FM Wideband
Refer to the previous sections in this chapter for details and menu overviews of general spectrum analysis measurements including setting up frequency and bandwidth parameters, sweep settings, trigger types, attenuator options, and preamp settings. Once the basic measurement is set up, select the AM/FM Audio measurement via MEASURE > MEASUREMENT > AM/FM AUDIO. Select a desired measurement view via MEASURE > VIEW.
Note
The AM/FM Audio measurement (Option 24 or Option 509) remains unresponsive when PCIe is selected as Data Out Format in IQ streaming mode. Refer to IQ STREAMING Menu.
AM/FM Audio – RF Spectrum View
The RF spectrum view displays the carrier signal power of the demodulated AM/FM audio signal in frequency domain.Toggle DEMOD MARKER button to identify the peak value of the carrier power. You can also record the audio signal by setting the duration time. Make sure to toggle on the Audio button to listen to the signal by adjusting the volume accordingly.
2. The trace display area can show multiple traces, each with a unique color. Each trace can be set to Clear/Write, Average, Min or Max Hold, Rolling Average or Rolling Min, or Max Hold. Each trace can have Peak, RMS/Average, or Negative detectors. Traces can be set to Active, Hold/View, or Blank. Refer to Setting Trace and Cursor Parameters. The shaded vertical section marked with green dashed line illustrate the demodulation channel bandwidth.
3. The demodulation marker is the same as a regular marker and can be applied to any trace using the Marker Menu. Refer to Setting Up Markers.
The default view of AM/FM Audio measurement is RF spectrum view. The AM/FM audio demodulation SETUP menu is available in MEASURE > MEASUREMENT > AM/FM Audio > SETUP.
Toggles the density display on or off. Density displays the relative time density of a signal in a color scale ranging from blue to red. Signals that persist longer or more often during the acquisition time are indicated in red color. Brief transient signals are shown in blue color.
PAUSE DENSITY
Pauses the display density, press RESTART DENSITY to resume displaying density.
AUTOSCALE
When toggled on, automatically adjusts the DENSITY SCALE TOP and DENSITY SCALE BOT settings of the density display based on the amplitude of the signal.
DENSITY SCALE TOP
Sets the percentage of signal time for the top (red) density scale.
DENSITY SCALE BOT
Sets the percentage of signal time for the bottom (blue) density scale. Any signal that appears at a rate lower than the DENSITY SCALE BOT setting will not appear in the density plot.
DENSITY RES
Density resolution sets the FFT size of the RTSA capture. Normal resolution sets a 512 point FFT; High resolution sets a 1024 point FFT. Higher resolution gives finer frequency detail to the density display, but increases POI. Lower resolution will result in a lower POI, but gives less frequency detail to the density display.
PERSISTENCE STATE
Toggles the persistence state between Variable or Infinite. Infinite persistence means the density plot will not fade away after a signal is no longer present. It is useful for seeing very fast and infrequent signals. Variable persistence sets a user defined time for the density colors to fade. Longer persistence values can be used to best illustrate the signals being observed.
PERSISTENCE
Sets the decay time of the density display.
RESET PERSISTENCE
Resets the decay time of the density display.
ACQUISITION TIME
The acquisition time sets the update interval for each spectrum trace, spectrogram line, and density display. During the acquisition interval, spectral data from multiple FFTs of the input signal are taken and combined. The acquisition time is adjustable to allow resolving multiple, brief signal events in time and to observe longer term behaviors.
PRESET SETUP
Presets all values on the SETUP menu to default values.
Note
For simultaneous audio demodulation and trace sweep, the set span must not be greater than the maximum capture bandwidth based on the installed bandwidth option (No option: 20 MHz; Option 103: 55 MHz; Option 104: 110 MHz) and the tuning range must not cross more than one internal local oscillator tuning points. These conditions will be indicated by a transient status message at the top of the display.
AM/FM SETUP Menu - RF Spectrum
AM/FM SETUP Menu (AM/FM Audio – RF Spectrum)
DEMOD FREQ
Sets the audio demodulation frequency.
DEMOD MKR
Toggles the tracked marker on and sets the demod frequency to track the marker frequency. Use the MARKER Menu to adjust the marker parameters.
MARKER TRACKED
Selects which marker to track. When DEMOD MKR is enabled, the demodulated frequency tracks the indicated marker.
MODULATION
Selects the modulation type to demodulate the RF signal. Available modulation types are AM, US Band (upper sideband), LS Band (lower sideband), FM Narrowband (6.25 12.5 and 25 kHz), and FM Wideband.
AUDIO
Toggles the demodulated audio output (sound) on or off. The demodulation frequency must be within the current sweep for audio demodulation. Note that AM/FM audio demodulation cannot be used in conjunction with IQ Streaming or when the IQ streaming output port is set to Data Out.
VOLUME
Sets the percentage of audio output volume or mutes the audio.
RECORD DURATION
Sets the audio recording time. The maximum recording time is approximately 27 hours, depending on available storage.
RECORD
Starts audio recording. Recorded audio files (.wav) are saved to the Waves folder. Audio will be recorded for the recording duration or will stop when AUDIO is toggled off.
SQUELCH LVL
Sets the RF threshold level for audio playback. This is typically used with weak or intermittent signals to mute the static sound when the signal is below the squelch threshold.
