Spectrum DSP M2 User Manual V1.1
Frequency Modulation (FM) mode reception and transmission:
IMPORTANT:
• The FM mode is installation dependent and is disabled by default.
• The noise blanker and DSP noise reduction and notch are disabled in FM mode
WARNING:
• Transmitting on FM means that a continuous carrier is being generated. Be absolutely
certain that the final transistors on the Spectrum DSP are adequately heat-sinked and
do not get too hot and also that your power supply is capable of the current being
drawn.
• It is recommended that the lowest transmit power be used on FM that will achieve a “full
quieting” signal for the receiving station's (or repeater's) receiver.
FM (Frequency) modulation is now available experimentally on the Spectrum DSP and while it is
believed to work properly, it likely has a few “issues.”
Repeater operation is possible if one uses the “SPLIT” mode and sets the receive and transmit
frequencies in separate VFOs. By “swapping” the VFOs, one can effect a “reverse” function and
listen on the input frequency while transmitting on the output – useful for checking for a simplex path.
The “FM” produced by the Spectrum DSP is compatible with PM, which is to say that for modulation, a
6dB/octave pre-emphasis is applied and a 6dB/octave de-emphasis is done on receive audio: This is
the worldwide standard for narrowband frequency modulation on amateur and commercial
frequencies.
To enable FM:
• You MUST have Frequency Translate enabled: It will not work without it!
• Set the menu item “FM Mode Enable” to ON.
When in FM mode, the “RFG” control becomes “SQL” (Squelch) – also operated by middle band
swiping. The higher the number, the “tighter” the squelch and a setting of “0” unconditionally opens
the squelch. You will note that when the squelch opens, the “FM” on the mode indicator will change
color: More on this later.
Two “modes” of FM are available: The default is “Narrow” with +/- 2.5 kHz peak deviation (w/1kHz
modulation) and “”Wide” with +/- 5 kHz peak deviation, the former being commonly used on HF and
the latter being that which is used on VHF bands except for those instances where the “narrow” (+/-
2.5 kHz) mode is specifically used by local option.
These two modes are more or less interoperable, with the following provisions:
• Operation of “Wide” (+/-5kHz) on “Narrow” frequencies will result in “loud” audio, possible
“squelch clamping” and splattering onto adjacent “narrow-spaced” channels.
• Operation of “Narrow” (+/-2.5kHz) on “Wide” frequencies will result in chronically “low” audio,
reduced “copyability” under weak-signal (noisy) conditions and everyone telling you to talk
louder!
There is also a selection of receiver bandwidths:
P 57
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