[AMRadio] Side band power with increasedcarrier/was clippertononAM

[Gary Schafer]

Ok Jeff, let's first define how you find PEP. 
You sum all of the voltages present in the signal, square that and divide by
the feed line impedance.
Since there are 3 RF signals present in an AM signal, 2 side bands and a
carrier, we find out what the voltage is on each and add them together.

With a 25 watt carrier the carrier voltage across 50 ohms will be 35.36
volts. Where voltage = the square root of (power X impedance). Ohms law.

Since the power in each side band will be 6 db down (1/4) from the carrier
at 100% modulation that leaves 6.25 watts of RF in each side band. The
voltage on each side band will be 17.68 volts across 50 ohms.

To find PEP we first add all those voltages up. 17.68 + 17.68 +35.36 = 70.7
volts. Now we square the total voltage 70.7 x 70.7 = 4998. divide by the
feed line resistance of 50 ohms and that = 100 watts PEP
Note that the sum of the voltage in both side bands is equal the carrier
voltage in this case. Classic 100% modulation.

Let's now increase the carrier. I am going to use 50 watts for ease of
calculation. We know that if we were to 100% modulate that we would get 200
watts PEP.
But since we want to limit our PEP to 100 watts as that is all our solid
state transceiver will handle.

Again we first find the voltage in the 50 watt carrier. It turns out that it
is 50 volts. 
Now we know that 100 watts PEP would be 70.7 volts total from above so if we
subtract our 50 volt carrier from 70.7 volts that leaves 20.7 volts total
for audio side bands. 
Since that will be divided between the two side bands take half the 20.7
which = 10.35 volts per side band, the maximum that we can run and have the
signal stay under our 100 watt PEP radio limit.
10.35 squared = 107 divide by the 50 ohm feed line and we have 2.14 watts of
power in each side band for our now 100 watt PEP transmitter!

Someone else may want to figure what percent modulation this is. Too lazy as
I don't remember how to do it just now.

This transmitter will be  undistorted but under modulated. But we will have
reached the 100 watt PEP limit. We will still have lots of room on the
negative peak side.

As I said earlier, you can judge by looking at the scope of how many
divisions of audio you have. In this case the carrier will be much greater
than the audio on the scope.

We could crank up the audio to fill out the negative side so that they just
hit the 100% point but the positive peaks will be severely clipped due to
the 100 watt limitation of the transmitter.

73
Gary  K4FMX


> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of Geoff Edmonson
> Sent: Thursday, April 14, 2011 3:37 AM
> To: Discussion of AM Radio in the Amateur Service
> Subject: Re: [AMRadio] Side band power with increasedcarrier/was
> clippertononAM
> 
> On 04/13/2011 08:22 PM, Gary Schafer wrote:
> > Further you can look at the number of divisions on the scope that the
> audio
> > occupies. The more divisions the greater the audio power on the
> signal. If
> > you raise the carrier above the 375 watt level you will not hit the
> 100%
> > mark on negative peaks as soon but you will sooner hit the 1500 watt
> PEP
> > mark and you will have less total divisions of audio on the scope.
> This
> > means less audio power transmitted.
> 
> What that means is, you don't have enough audio power available to fully
> modulate the carrier.
> 
> As the carrier level increases, the need for enough audio voltage to
> properly modulate the carrier increases exponentially.
> 
> 25w out of a typical solid state transceiver, modulated to 100 percent
> with a sine-wave, produces 100w PEP.
> If you increase the carrier level to 40w you'll only see 60w PEP and
> it'll be severely distorted, because there's simply not enough
> dissipation in the device.  Of course, we're talking low-level
> modulation levels, but amplify that with a linear on the output of a
> solid state transceiver, and the problem still exists, only on a much
> amplified scale.  A linear that only produces 1,000w PEP output, is only
> good for 250w -max- carrier, and -should- be dropped to 150~200w to
> ensure enough dissipation available for 100% non-distorted, non
> flat-topped audio.