[AMRadio] The Physical Reality of Sidebands in the AM signal


Gary Schafer garyschafer at largeriver.net
Wed Oct 5 23:27:32 EDT 2016


Hi John,
Long time since I have talked to you.

As far as two signals interfering with each other consider the forward and
reflected waves on a transmission line. There is constructive and
destructive interference between the two. They add and subtract at
particular points but they do not mix to produce a new wave. 

An easy way to view AM modulation is to look at it as a mixer. Two signals
in and 4 signals out (sum, difference and the two original signals, audio
and RF) as any mixer gives you provided it is not a balance type of mixer. 
If you have a carrier at say 1 MHz and modulate it with a 100 KHz signal you
can easily tune to each of the output signals of the "mixer" and you will
not know that any other signal exist out of the mixer. None of the other
signals will have any influence on any other output signal including
shifting phase or otherwise of individual signals.

When looking at the output of the "mixer" as an AM signal, (looking at the
carrier and two side band signals)
The phase shift (rotation) involved with an AM signal is because of the
relationship between the carrier frequency and the side band frequencies. 
They are all on different frequencies. When looking at the whole signal
using the carrier frequency as the reference the phase is constantly
changing "between the signals at the rate of the difference in frequency"
between them. But on their own they do not change in phase.

So I would say that the phase change does not necessarily take place in the
receiver but the phase relationship is already established by the time it
gets to the receiver.

The receiver detector is another mixer that combines the received signals
along with their phase relationships to give us the demodulated audio.

As to the total power of the modulated AM signal the average power is the
sum of the three signals. Just as you can see the RF ammeter increase in the
transmission line of an AM transmitter when modulation is applied, you will
also see the S meter on a receiver increase by 1.76 Db with 100% modulation.
The receiver S meter measures total received power.

Multiple signals in the ether do not inter fear with one another and create
heterodynes because there is no non-linear element for them to mix in. 
If you feed two equal level signals into an antenna that are on the same
frequency they will add to produce twice the average power out when in phase
(constructive interference) and they will subtract to produce zero output
when they are out of phase (destructive interference). 

73
Gary  K4FMX



> -----Original Message-----
> From: AMRadio [mailto:amradio-bounces at mailman.qth.net] On Behalf Of John
> Coleman
> Sent: Wednesday, October 05, 2016 9:44 AM
> To: 'Donald Chester'; amradio at mailman.qth.net
> Subject: Re: [AMRadio] The Physical Reality of Sidebands in the AM
> signal
> 
> On the other hand.
>   You might imagine a 100 watt transmitter and to make this a simple way
> to
> see let us say it is a plate modulated class C rig.  When the Rig is not
> modulated the receiver receives a signal carrier of say 100uV and the S
> meter reads the same regardless of the bandwidth.  Now modulate the rig
> to
> say 100% with a 3 khz tone.  The receiver still sees the same S meter
> sig
> strength regardless of bandwidth.  However if the receiver is set to say
> a
> 10 khz bandwidth with a center notch of 1 khz and then center tune to
> the
> carrier  and has a notch filter of very deep notch capability so as to
> completely get rid of the carrier so the un-modulated sig is now zero on
> the
> S meter.  Now when the rig is modulated with the 3 khz tone, the S meter
> will read the algebraic addition of the two sidebands less the carrier.
> This will be less than the carrier and the S meter should read 50uV
> which
> would be the peak of the algebraic addition of the two sidebands.
> 
> I theory a transmitter could be constructed with  carrier generator
> applied
> to one antenna and then two other transmitters adjusted above and below
> the
> carrier rig of .25 power of the carrier transmitter and applied to two
> other
> antennas.  If the phase relationship is correct then a receiving station
> miles away would hear a AM sig.  and if constructed just right turning
> on
> the two sideband rigs could make the sig from the carrier frequency
> disappear from the receiver.
> 
> Sidebands have phasing components that add and subtract from the
> carrier.
> The sideband rigs must be exactly the same freq and phase apart from the
> carrier rig as long as there is a difference between the carrier freq
> rig
> and the sideband rigs sine wave modulation would be detected at the
> distant
> receiver.  But in order to block the carrier over a long period of time
> the
> two sideband rigs would need to be adjusted to the same freq as the
> carrier
> rig and held at a proper phase as to cancel any signal that the receiver
> hears.
> 
> Here is where the uncertainty thing comes in.  It is in the act of
> receiving
> that causes the signal to appear to be canceled.   It has been proven
> that
> any electromagnetic waves, radio or light, does not interfere with each
> other except in the detection.  The interference occurs in the receiving
> antenna or apparatus. which BTW is a very good thing else the ether
> would be
> full fo noise caused by all the radio wave interfearing and hetrodyning
> creating signals whos bandwidth would never end.
> 
> John, WA5BXO
> 
> BTW.  Two lasers of different freq have been shined on some suface to
> produce a third color but only when they or shined on the same spot.
> crossing the beams does nothing.
> 
> 
> 
> 



More information about the AMRadio mailing list

This page last updated 17 Nov 2017.