Single Sideband

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A typical all mode Tranceiver

USB,LSB,CW,AM,FSK

 

   Single sideband is not a band! It is not a frequency! It is not a portion of a band!
It is not a rock group! It is not.......what you may have thought!

   Single sideband is more properly called a mode.
   It is a very efficient method of superimposing your voice or other information on a radio wave and the transmission of that radio wave.

   The method by which audio, (information), is impressed on a radio signal is called modulation. To modulate a radio wave is to add information to it that can be received on a receiver for some useful purpose.

   There are two types of modulation that most people are familiar with, AM (amplitude modulation), and FM, (frequency modulation), for which the AM and FM broadcast bands were named. You have used FM modulation on the 2 meter ham band and most likely used AM modulation when you were a kid using toy walkie talkies. You may also have used single sideband on other occasions also, but since you are reading this, you want to know more.

   When you are in the AM mode, your voice modulates, (is superimposed), on a carrier wave at a certain frequency in your transmitter and is transmitted over the air waves.

   The carrier wave is used to "carry" the audio information to the AM receiver where it is detected and transformed back to an audio signal that we can hear representing the original information (voice) that was spoken into the microphone.

   In an AM modulated radio signal, the carrier, is continuously transmitted. Due to the nature of the way AM is produced in the transmitter, two identical modulating signals are attached to the carrier wave, these are called the sidebands. They are a mirror image of each other, identical in every way.

   Any audio that you hear on an AM receiver is from the two sidebands. When the radio transmitter you are tuned to is not transmitting any sound, you can still hear from the speaker and see on your S meter that a signal is present due to the background noise being quieter than either side of that frequency. This is the carrier you are detecting being detected by your receiver.

   These two modulating (audio) sidebands are located on either side of the carrier wave, one just above it and the other just below. As a result, the sideband located just above the carrier frequency is called the upper sideband and that which is located just below the carrier frequency is called the lower sideband.

   The audio sidebands that form an AM broadcast signal are quite important. They contain the "information or audio" intended for the receive station. Although AM signals were transmitted almost exclusively for decades, it was discovered with experimentation that the AM signal could be modified yielding much better results!

   Many methods were experimented with and ham radio operators often used both sidebands without the carrier using special circuits in the transmitter to eliminate the carrier wave while still leaving the modulation to be transmitted.

   This is known as double sideband (DSB) without the carrier. DSB was typically used in the earlier experiments because it was much easier to filter out just the carrier than to filter out the carrier and one of the sidebands. Soon the experimenters were able to filter out the carrier and either of the sidebands to yield what we now know and use as Single Sideband! So we are using a single side band....meaning one side band.

   Using special circuits and filters, single sideband transmissions can consist of either the lower sideband (LSB) or the upper sideband (USB). If you listen to an SSB signal on an AM receiver, the voices are altered and sound very muffled, garbled and distorted. Some people even say "Donal Duck" sounding when tuned improperly in the sideband mode.

  

 

 Enter the SSB receiver.

   Since the receiver still needs the original carrier to “demodulate” or decode the signal, you must have a special SSB receiver to listen to these transmissions. This is accomplished in the SSB receiver by circuits that re-insert a very low level carrier wave back with the lower or upper sideband signal and magically, the audio that was transmitted is restored in the receiver with almost identical reproduction of the original voice. Tuning the SSB receiver is very touchy and critical to make the voices sound natural. If you are tuned off of the transmitter frequency, depending on which way you go, the voices will be higher or lower pitched, resulting in that "Donald Duck" sound. You will tune with ease with some practice.

   Your receiver MUST be in the same "mode" as the transmitted signal or the whole process does not work!
It the transmitter of the other station is in the USB mode, your receiver MUST be in the USB mode and vice versa.
How do you know which "mode" to use?
   On HF and by agreements worldwide, all stations transmitting SSB use LSB on 160 meters through 75 meters, USB on 60 meters, back to LSB on 40 meters and then all bands above 40 meters use USB. This agreement makes life easy when switching bands. Every one knows which modes are used on which bands.
   Here is a sample audio file. At first you will hear a station in the USB mode on 20 meters properly tuned........then the receiver is switched to the AM mode with a station transmitting in the SSB mode......then back to SSB with tuning slightly off frequency and retuning to the correct frequency by "ear". You will notice how the voice pitch changes as the tuning of the receiver gets closer to the transmit frequency of the person transmitting.....
Click here for the audio. Mp3, 149KB, 1:16 seconds. (I did not get a chance to "ID" the stations heard.)
   Since the fidelity of the SSB voice transmission has been altered somewhat through various filters in the process of producing the sideband that is not too wide, usually only the most important portions or characteristics of the voice frequencies needed to communicate are allowed through, and this causes the lack of true AM or FM fidelity to the transmission, but the communication, (understandable), portions of the voice characteristics remain, which is all that is needed in the first place.
It is a "communications" mode, not wide band HI FI commercial broadcast FM radio, CD quality mode!
   The information contained in the average human voice needed to understand the voice is contained within about the first 3000hz of the human hearing range. Frequencies of the human voice beyond this range are not needed for communication purposes and are filtered out in the modulation process. So the average bandwidth of a SSB signal is about 3000hz wide with all of the voice characteristics needed within that range to be understandable.

The Power Ratio factor
   Back to AM for a bit. When producing that AM signal we were talking about, the end result is that it was discovered that approximately half of the transmitter power is "wasted" on the carrier and the rest of the power is divided between the two sidebands. As a result, the actual audio output from a 1000 watt AM transmitter (500 watts of carrier + 250 watts on each sideband) would be the same as a 250 watt SSB transmitter in it’s effectiveness.

The Efficiency of SSB Transmitters

   In the above, we learned that it would take 1000 watts of AM to be as effective as 250 watts SSB. This is a 4 to 1 ratio. The reason for the efficiency of SSB, is that all of that power that was used to produce both sidebands and the carrier are now used in only one sideband at the transmitter, and when you account for the receiver re-adding only a very, very tiny portion of that power back into the equation, you are increasing the efficiency about 16 times better than the standard AM transmitter! It is one reason why long distances can be covered effectively with SSB using much less power than AM.

SSB surprises for the new user!
   When you tune around a ham band where single sideband is used, one thing may startle you compared to listening to AM or FM. Two stations occupying the same frequency can talk at the same time without those terrible squeels and tones caused by two carriers beating together! Since there are no carriers transmitted....no tones. If you are familiar with the terms, "Pileups" or "Double"....you will understand what this means.
   So as not to get too technical, those tones are caused by the differences in the two AM station frequencies that fall within the audible range when added or subtracted from each other creating the difference heard as an audio tone.

Here is an example:
Station 1 transmits on 7.200000mhz exactly using AM.
Station 2 transmits on 7.201000mhz exactly using AM
Station 3 is your receiver tuned to 7.200000 exactly on AM.
When you subtract the difference between the two frequencies of station 1 and station 2, you get 1000hz which is an audio tone that you hear from the receiver. If one of the two stations adjusted their frequency by a tiny amount, you would hear the difference change in the frequency of the audio tone.
   When you are in either sideband mode and you're just setting there listing to the background noise and all of a sudden you hear a tone in the background, you are hearing the "carrier" that is being re-inserted in your receiver beating against the carrier of the other station and producing the difference frequency if he is slightly off of the frequency you are tuned to. If you tune your receiver on top and exactly on his transmit frequency...the tone will simply vanish because the difference frequency between your receiver and his transmit frequency are so close or exact that you can not hear the low audio frequency. Most ham transceivers do not reproduce audio frequencies below about 100hz or so, so even if your ears could hear that frequency, the radio is simply just not producing it.
One more thing that may surprise you....When you key your mic and are in the SSB mode....look at your watt meter......no output! Remember.....there is no carrier produced in the transmitter when using the sideband mode.....so no carrier will be registered on the meter. If you scratch your finger across the face of the mic or speak into it, you will then see the meter register the "modulation".

The instant you "modulate" the transmitter with your voice, you will see that the meter deflects showing you that now you have output....this is normal so don't worry that your transmitter is not operating....it is...and very effectively!

   Now that you have learned more about how SSB works just remember that the SSB mode of transmission is the predominant mode of transmission used by most hams to effectively and efficiently work the world!

Meet you on 10 meters.....28.4mhz......USB! 73!

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