Spectrum DSP M2 User Manual V1.1
The solution is simple: Do NOT tune the AM signal so that the carrier is “dead center”.
It is necessary only to offset-tune by a few hundred Hertz, but it is necessary to do this!
Comments on adjusting the AM RX IQ Balance:
This adjusts the receiver I/Q amplitude balance when in AM mode and is used to minimize the
low-level “tweet” (e.g. tone) that may be heard when an AM signal is tuned in slightly off
center frequency. To null this tone it is recommended that you tune in a strong carrier, offset it
by 500 Hz and then adjust this parameter to minimize the amplitude of this tone.
This adjustment is unlikely to completely eliminate this “tweet”, but it can significantly reduce
it. Note also that the efficacy of this reduction changes with audio frequency in that the
optimal null for a 400 Hz “tweet” tone (e.g. 200 Hz offset from the carrier frequency) will be
different from that of a 1000 Hz “tweet” tone.
AM Transmission: (installation dependent)
AM transmission is possible ONLY when frequency translation mode is active!
When transmitting using AM, the power level will automatically be set to 2 watts to prevent the
PEP from exceeding the maximum “clean” power level available. If one does not transmit,
the power level will not be automatically changed.
If an attempt is made to transmit with frequency translation mode turned off, the transmitter
will key, but there will no transmit out power at all in AM mode!
There is no “TUNE” mode in AM mode!
You should remember several things about AM:
• It is MUCH less efficient than SSB! You will have a 9 dB reduction (1/8
th
) of the “talk” power of
SSB: That's just the way it is!
• The UNMODULATED resting carrier will be 25% of that of the peak power! This means that if
you are used to getting 5 watts peak on SSB, you will get only 1.25 watts when no audio is
present: Sorry about that, but that's just the laws of physics!
The speech processor works in AM mode in exactly the same way that it does in SSB mode and it
should NOT be possible to exceed 100% modulation.
There is presently ONE option for AM transmit mode: In the configuration menu, the item labeled “AM
TX Audio Filter” has the selection of ON and OFF. If it is “ON” (default) the transmit audio will be
“brick-wall” filtered from about 275 to 2700 Hz in the same way that the SSB audio is.
If this selection is set to “OFF” the audio filter is disabled. This has the effect of increasing the fidelity
of the audio – mostly through additional low-frequency components (down below 100 Hz) and
somewhat above 3000 Hz. While this can increase the audio fidelity on transmit, you should be aware
that it can significantly shift the RF energy from the audio spectrum that contains speech intelligence
and reduce the “talk power”.
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