Spectrum DSP M2 User Manual V1.1
• 10 kHz: Pre-detection bandwidth: +/-10kHz (20 kHz total); Post-detection bandwidth:
10 kHz.
• 7.5 kHz: Pre-detection bandwidth: +/-7.5kHz (15 kHz total); Post-detection
bandwidth: 10 kHz.
• 6 kHz: Pre-detection bandwidth: +/-6kHz (12 kHz total); Post-detection bandwidth:
10 kHz.
• 5 kHz: Pre-detection bandwidth: +/-5kHz (10 kHz total); Post-detection bandwidth:
10 kHz.
Other (“non-wide”) bandwidths are available:
• 3.6 kHz: Pre-detection bandwidth: +/-3.6 kHz (7.2 kHz total); Post-detection
bandwidth: 3.6 kHz.
• 2.3 kHz: Pre-detection bandwidth: +/-2.0 kHz (4.0 kHz total); Post-detection
bandwidth: 2.3 kHz (300-2600Hz, adjustable).
• 1.8 kHz: Pre-detection bandwidth: +/-2.0 kHz (4.0 kHz total); Post-detection
bandwidth: 1.8 kHz (500-2300Hz, adjustable).
Some explanation is required for the 1.8 kHz and 2.3 kHz modes as you'll note that the pre-
detection bandwidth appears to be a bit on the narrow side to accommodate the sidebands
that extend out beyond the filter (e.g. greater than the +/-2kHz bandwidth). If one tunes the
receiver to the center frequency of the AM signal when these bandwidths are enabled the
audio response will be limited to just 2 kHz by the pre-detection filter. If one off-tunes from the
center frequency, this +/- 2 kHz bandwidth – which encompasses 4 kHz - may be shifted to
include the higher audio frequencies of one or the other sidebands of the AM signal.
Because it is always necessary to off-center tune an AM signal to obtain the full audio
bandwidth permitted by the 1.8 kHz or 2.3 kHz post-detection filter, one of the two sidebands
(upper or lower) may be encompassed in the narrower bandwidth. This “quirk” may also be
used to advantage in the presence of QRM (interference) by selectively tuning for one
sideband or the other, moving away from the source of the interference.
In this version the filtering when in AM mode has been re-done: The Hilbert transformers,
which have a bandpass response, are replaced with low-pass filters (e.g. response down to
DC) that have their low-pass cut-off frequency selected according to the desired bandwidth.
Post-detection, there is additional audio filtering applied to reduce the wideband noise that
inevitably results with envelope detection of weak signals.
The “Zero-Hertz” hole problem if operating with “Frequency Translation” disabled:
This (and all “sound-card”) type SDRs have a “hole” at zero Hertz – right in the middle of the
display. This is the inevitable result of AC coupling to the A/D converter (codec) and cannot
easily be helped without added design complication.
What this means is that if you tune in an AM signal “dead center” its carrier will fall into this
“hole” and disappear which effectively turns it into a double sideband with no carrier –
which is to say, it is no longer AM! If an AM signal is tuned dead-center, it will sound
distorted – much like an SSB signal tuned on an AM receiver!
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