[AMRadio] SSB AM revisited


Gary Schafer garyschafer at comcast.net
Thu Mar 17 16:05:18 EST 2005


SSB AM has been discussed before and was always thought the transmitter 
to have audio power down 3 db from regular double side band AM 
transmitter. This is because of only transmitting one side band and thus 
half the power in the side bands.

A DSB AM transmitter has each side band 6 db down from the carrier.

In thinking about it I was trying to come up with how we get the same 
PEP power with SSB AM as we do with DSB AM.
It turns out that you can run 4 times the side band power in a single 
side band as you can when using double side band AM.

With SSB AM 100% modulation the side band power equals the carrier 
power! It is not 6 db down as each side band is with DSB AM.

If you look at PEP with AM, the voltage into the antenna doubles with 
modulation to make PEP 4 times what the carrier power is.
With SSB AM the voltage also doubles to make PEP 4 times carrier power. 
But in this case it takes the same amount of power in the side band as 
there is in the carrier!

Just to verify all this I hooked up the spectrum analyzer, scope and PEP 
watt meter to the old 20A. Here is how the numbers work out:
		
On AM DSB.
carrier power=          2 watts.
carrier volts=         10 volts.
side band power each = .5 watts.
side band volts each =  5 volts.
PEP of transmitter =    8 watts.

On SSB AM.
carrier power =         2 watts.
carrier volts =         10 volts.
side band power =       2 watts.
side band volts =       10 volts.
PEP of transmitter =    8 watts.

When figuring PEP of an AM transmitter you must add the voltage of each 
component together and then square it and divide by the resistance it 
feeds. You can not add power directly. In the case of DSB AM the voltage 
of the carrier gets added plus the voltage of the upper side band plus 
the voltage of the lower side band. In the example above that equals 20 
volts. 20x20=400. divide by 50 = 8 watts PEP.

With SSB AM there is only the carrier and one side bands voltage to add 
together to find PEP. In the above example 10 volts carrier plus 10 
volts side band = 20 volts. Squared= 400 divide by 50 = 8 watts PEP.
But as you can see there is twice the voltage in the side band or 4 
times more power in it than with double side band am. This still gives 
the same PEP.

With 6 db more audio power in SSB AM than in one side band of DSB AM the 
recovered audio should be the same as with DSB AM. DSB AM was always 
thought to have a 6 db advantage because of having the second side band 
and being coherent in the detector. Seems this evens things out.
Of course there are the distortion issues in the detector with SSB AM to 
deal with.

With a little thinking you can see where you might be able to run more 
carrier power with SSB AM and not having to reduce the audio level as 
much as you would with DSB AM and have the PEP still stay within limits.

Here are some numbers with the carrier increased to 3.34 watts (1.67 
times) and audio down only by 3 db.

carrier power=          3.34 watts.
carrier volts=          12.93 volts.
side band power =       1 watt.
side band volts =       7.07 volts.
PEP of transmitter =    8 watts.


73
Gary  K4FMX







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