[AMRadio] Re: Carrier with one sideband


Gary Schafer garyschafer at comcast.net
Thu Feb 7 20:58:40 EST 2008


Nice display Jim!

But put it in the double side band position and reduce the carrier to -60 db
or so and notice that you have exactly the same wave form as that of the SSB
with carrier at -6db, which is equal to the side band energy in this case.

It is not the transmission of the single side band with carrier that gives
the distortion but the envelope detector in the receiver.
With only one side band and carrier the detector can't make up its mind
which signal is supposed to be the carrier and which is the modulation when
you have higher levels of modulation. The result is high levels of second
harmonic distortion generated in the detector. As long as the modulation
percentage is kept to a lower level the distortion is minimal and it sounds
fine.

If you use a receiver with a rather sharp filter and listen to a regular AM
signal and tune off to one side (which is often done when heavy qrm is
present) you generate the same kind of signal at the detector in your
receiver. You greatly reduce one side band of the signal reaching the
detector and it sees SSB with carrier.

73
Gary K4FMX

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of Jim Tonne
> Sent: Thursday, February 07, 2008 7:48 PM
> To: Discussion of AM Radio in the Amateur Service
> Subject: Re: [AMRadio] Re: Carrier with one sideband
> 
> 
> Gents:
> 
> First, this is my first post to this interesting group and I
> hope you take what I have to say constructively.
> 
> This business of SSB on AM, etc., is of interest to me
> on a technical basis.
> 
> But let's do a computer simulation of this thing.  To
> this end I have written a computer program which
> allows you to select various AM modes and the
> program then shows you the transmitted envelope
> and the associated spectrum.
> 
> It is temporarily posted here:
> http://tonnesoftware.com/ModTutor.exe
> 
> That file is just a barefoot executable without all of
> the usual Window$ garbage.  It should run on most
> Windows computers.  If not e-mail me and I'll make
> a full-blown install routine.
> 
> I am suggesting you download the program and
> run it.  Click on the opening screen option "Single
> sideband" and then click on "Ideal lower sideband".
> 
> See the transmitter output as a "carrier".  Then click on
> Carrier= 0 dB.  Then on Carrier=-6 dB.  Then on
> Carrier=-12 dB.
> 
> Not real pretty, is it?
> 
> Now click on "Double sideband" and on Carrier=0 dB.
> 
> I think the folks on this group will find the routine as
> interesting as others have.
> 
> - Jim WB6BLD
> 
> 
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