Re: [amsat-bb] FM vs SSB

["John Stephensen" <kd6ozh@xxxxxxx>]

There is an extra "PEP" in the second line. The correct version is:

Unprocessed     Processed       RF
Avg.    PEP    Avg.    PEP     BW      Mod.
39      53      35      42       3      SSB
39      53      35      42       6      DSB
45      45      41      41       6      FM
46      46      42      42      12     FM
54      59      43      48       6      AM

Transmitters are peak power limited so 6 kHz wide FM makes the best use of
the power amplifier. If the object is to reduce the average power output
then SSB wins.

A speech processor is very useful on SSB as it reduces the peak power
requirement by 11 dB. On FM you only gain 4 dB.

73,

John,
KD6OZH

----- Original Message -----
From: "Mike Murphree" <mike@tropo.org>
To: <kd6ozh@AMSAT.Org>
Sent: Friday, July 25, 2003 14:23
Subject: Re: [amsat-bb] FM vs SSB


> > Here is some interesting data for transponder designers. The table
> > compares the modulation types by the received signal level required for
> > an
> > articulation index of 30%.
> >
> > Unprocessed     Processed       RF
> > PEP     Avg.    PEP    Avg.    PEP     BW     Mod.
> > 39      53      35      42       3      SSB
> > 39      53      35      42       6      DSB
> > 45      45      41      41       6      FM
> > 46      46      42      42      12     FM
> > 54      59      43      48       6      AM
> > Source: Sabin & Schoenike, "HF Radio Systems and Circuits"
> >
> > The articulation index is the percentage of spoken words received
> > correctly. The audio bandwidth in all cases is 300-3000 Hz. Processed
> > speech is preemphasized by +6dB/octave above 500 Hz to prevent loss of
> > the weaker high-frequency components in the human voice and then clipped
> > to a 7dB peak-to-average power ratio. The power levels are in dB above
> > the receiver IF noise level in one Hz so that they are proportional the
> > power output at the transmitter regardless of receiver bandwidth..
> >
> > Note that narrowband FM requires the lowest peak power level. SSB
> > requires the lowest average power level.
>
>
> Looking at your table, isn't that reversed?
> i.e. FM requires least average power, while SSB requires least peak
> power...  and by definition FM peak power = average power.
>
> It also appears that using a speech processor on a satellite
> may be a good thing as long as you back the power down to a readable
> signal and not relative to some arbitrary value below the beacon.
>
> 73
> Mike W4LNA
>
>
>


----
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