[Boatanchors] SSB and carrier insertion?

[Gary Schafer]

Hi Brian,

It sounds like you are familiar with Terman. You will find it all in his 
books.

I did not say that an AM detector used both the upper and lower part of 
the envelope. I said that with double side band AM the two side bands 
are coherent in the detector and add together. With SSB AM there are no 
coherent side bands to add.
Terman tells you all about that. It is also in most of the ARRL handbooks.

As far as "lack of punch" I will agree it is a rather "quaint technical 
term". :>)
The lack of punch or loudness in this situation usually comes from the 
transmitter often being mis-adjusted for proper operation. Too much 
carrier and flat topping of audio and or not enough audio. People often 
run the audio lower to keep the distortion down with SSB AM.

Distortion in the detector is due to the nature of the envelope detector 
process. Terman shows all the math under his chapter on detectors. What 
you hear mostly is 2nd harmonic distortion that doesn't get canceled 
with SSB AM as it does with double side band AM.
What it boils down to is (in the detector) the side band energy 
modulates the carrier. However as the modulation level gets nearer to 
100% the carrier starts to modulate the side band energy. That = distortion.

You can see this easily by modulating an SSB AM transmitter with a 
single tone and looking at the envelope on a scope. You do not get a 
sine wave waveform out as you do with double side band AM.

Even in detecting SSB with an envelope detector the BFO must be many 
times the ssb signal to avoid distortion in the detector. If the ssb 
signal is near the same level as the bfo (local carrier) signal there 
will have quite a bit of distortion in the recovered audio.
SSB AM has the same situation. The carrier is not many times the 
strength of the ssb audio part. Exactly the same detection process takes 
place whether the carrier is inserted locally (bfo) or comes along with 
the side band information.

As I said before, in the detector, if both signals are near the same 
strength each modulates the other producing unwanted products.

Again Terman tells you all about it under detectors.


Maybe I was a little confusing when I said "you need a 6 db carrier 
reduction when running a SSB transmitter on AM".

Lets start at the beginning: With a regular old 100 watt carrier output 
plate modulated AM transmitter that is modulated 100% on positive and 
negative peaks, the Peak envelope power output is 400 watts. Ok so far?
That's 6 db above the carrier level for PEP.

Using a SSB transmitter that has a peak envelope power (PEP) output 
capability of 400 watts, we want to run it on AM. With maximum carrier 
inserted it will put out 400 watts carrier. The same as its PEP capability.

We must reduce the carrier level by 6 db or to 100 watts out to run it 
on AM. That will allow the peak envelope power from the modulation 
process to reach 400 watts with 100% modulation.

If both the above transmitter examples are run in the manor described 
the two signals will be indistinguishable.

Terman will also tell you the same.


73
Gary  K4FMX



Brian Clarke wrote:
> Hi Gary,
> 
> That's really intriguing. Have you a reference for any of this material? I have 
> been teaching broadcast engineering for some years and I've not heard any 
> of this before - I'ld like to put it in front of my aspiring broadcast engineers.
> 
> What kind of AM detector uses both the upper and lower envelopes 
> simultaneously?  
> 
> I suspect that the lack of punch, another quaint 'technical term', may be 
> more related to the lack of bass which comes with trying to achieve a filter 
> with steep sides that only responds to one sideband. A normal AM 
> detector can go right down to near DC if required.
> 
> What is there in the detection process that gives rise to more distortion 
> with one kind of signal than another? I assume you mean waveform [ie, 
> amplitude vs time] distortion rather than frequency or phase distortion.
> 
> And 6 dB carrier reduction - compared with what? And audio peak power 
> 6 dB higher than the carrier in normal AM - now let me see if I understand 
> that - if I have a DSBAM transmitter with a 100 W carrier, then I must have 
> 400 W of audio peak power - so, as it's DSB, I would have 200 W per 
> side. Silly me - I always thought that I required only 50 W total for the two 
> sidebands for 100 % modulation. I wonder if Fred Terman ever saw the 
> maths for that one?
> 
> All very intriguing.
> 
> 73 de Brian, VK2GCE.
>   Gary said:
> 
>   SSB AM does not have quite the audio punch that full AM does. Without 
>   the other side band the detected audio at the receiver is 6 db down from 
>   what a double side band AM signal would be. There is 3 db less audio 
>   power being transmitted and the detector has another 3 db of loss not 
>   having the coherent other side band present to add in the detector.
> 
>   The detector in the receiver will also have more distortion as 
>   modulation approaches 100% with single side band AM from what it will 
>   with double side band AM. This is why it sounds a little funny compared 
>   to full AM. In essence the detector gets confused as to whether the 
>   audio is modulating the carrier or the carrier is modulating the audio 
>   signal.
> 
>   When operating SSB with carrier or double side band with carrier, with a 
>   side band transmitter the carrier needs to be reduced 6 db in order for 
>   it to operate properly. This allows the amplifier in the transmitter to 
>   achieve the peaks of the audio power.
> 
>   A normal AM transmitter has audio peak power 6 db higher than the 
>   carrier (4 times the power)
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