SINGLE SIDEBAND (SSB)
What is SSB, anyway? (You might be wondering.) SSB is method for transmitting and receiving information via radio. A term often used for information communicated via radio is intelligence. Later in this tutorial, we’ll take a closer look at “intelligence” within the context of radio communication.
This tutorial includes:
[] Introduction (you are here)
[] Modulation
[] SSB transmitting hardware
[] SSB receiving hardware
[] SSB transceiver hardware
[] Summary
As commonly used in amateur radio parlance, SSB refers to voice communications, but it can also be used for “digital” forms of communication, such as RTTY, AMTOR, and PACTOR. There are, of course, methods other than SSB to create radio signals suitable for voice communication, such as frequency modulation (FM) and amplitude modulation (AM), but those are topics outside the scope of this tutorial.
Whole books could be (and have been) written to explain SSB. You will find a much shorter explanation here.
Let’s start with continuous wave (CW) communications. “CW” usually refers to the use of Morse code, which is rather strange because Morse code is not continuous at all; it is a radio signal that is turned “on” and “off” to produce dots and dashes. When “CW” is used to refer to Morse code, it really means a non-modulated signal that is turned on and off. Radio jargon can be a bit misleading at times.
A continuous radio wave signal, as related to SSB, is commonly referred to as a “carrier”. A carrier is nothing more, and nothing less than a CW signal, shown in Figure 1 as a red line.
A carrier can be thought of as a no sideband (NSB) signal, and is not really good for much except to imply “here I am” since it carries no other information (unless it is turned on and off as in Morse code.). In order for a radio to be useful, it must have information superimposed upon it. Modulation of one sort or another can do this.
MODULATION
Within the context of single sideband for voice communication, think of modulation as a mixing process that produces radio signals that are the sums and the differences of the carrier and the modulating signals (voice). These sums and differences are the sidebands.
Suppose you used a perfect concert pitch “A” note (440 Hz) to modulate a carrier of 7.250 MHz in the amateur radio 40 meter band. The resulting modulated signal would contain:
The carrier signal, 7,250,000 Hz
The carrier signal plus your “A” note: 7,250,440 Hz
The carrier signal minus your “A” note: 7,249,560 Hz
(. . . and other stuff that we need not bother with for this discussion.)
The combined signals can be conceptualized as shown in Figure 2.
The carrier frequency, 7.250 MHz, is sometimes referred to as the “center of intelligence” (CI) in this context. This is an important concept, as you will see when we get to “offsets”, but don’t worry about that right now.
In Figure 2 the upper sideband is represented by the blue line to the right, and the lower sideband is represented by the green line to the left. If a distant receiver tuned to either sideband, the operator would hear only the 440 Hz tone – not very useful for communications.
As you know, there are many frequencies in addition to 440 Hz that make up the human voice. We don’t need all those frequencies for effective voice communications, so we will limit ourselves to the most useful ones. A complete double sideband (DSB) signal, plus the CW signal is shown in Figure 3.
Notice that this composite contains both sidebands, plus carrier signal.
Alert readers, such as yourself, probably suspect that it is about time to get rid of the CW signal and one of the sidebands in order to achieve a SSB signal, and you are correct. Before we get into that, however, let’s take a look at what we have, so far.
Double sideband can be used for voice communications, but that defeats one of the main reasons for using single sideband, which is spectrum conservation. Notice in Figure 3 that the DSB signal is twice as wide as either sideband. Twice as wide as - - what, exactly? (You might want to know.) Twice as wide as 3 kHz, which has become the “standard” for a SSB signal. Why 3 kHz? Because it has been shown that effective voice communication can be accomplished using only three kHz of spectrum. Actually, useful voice communication can be accomplished using even less bandwidth, but fidelity begins to suffer badly when using less than three kHz.
Be that as it may, SSB allows more signals to be on a given band than would be possible if using DSB, or some other mode that takes up more than 3 khz of bandwidth, such as amplitude modulation (AM) or frequency modulation (FM).
So, how do we get rid of the unwanted sideband and the carrier? Eliminating the carrier can be accomplished by the sideband generation circuitry, let’s concentrate on selecting the sideband of interest.
Clever mixing and filtering is one way to achieve a SSB signal, and that is the method that will be discussed here.
What we need is a filter that will eliminate all but the 3 kHz that contains the signal of interest. Figure 4 shows an idealized representation of such a filter to the right of the DSB signal.
Think of the filter as a narrow doorway through which the radio signal must pass.
Clever mixing (sideband generation) will virtually eliminate the carrier, so we will assume it has been eliminated in the mixing process.
Before we take a look at what the filter does for us, there is an important concept that must be introduced (by me) and understood (by You). That concept is the “center of intelligence”.
Notice, in Figure 4, that the composite signal has a bandwidth of 6 kHz and the filter has a bandwidth of only 3 kHz.
The center of intelligence for the composite signal (7.250 MHz) is 3 kHz from either side of the total bandwidth.
The center if intelligence for the filter is only 1.5 kHz from either side of its bandwidth.
Keep in mind that this is a conceptualization for tutorial purposes – if you were to view the signals on a spectrum analyzer, they would not appear as shown in these drawings (they would be much more “messy”).
Figure 5 shows the filter with a width of 3 kHz superimposed upon the DSB signal.
Notice that a portion of both the LSB and the USL signals are present. This will produce a sound, but it will not be recognizable as a human voice. What to do? WHAT TO DO??
There are two ways to “line-up” the filter with the sideband of interest:
1) We can shift the carrier frequency so that the desired sideband appears in the filter’s aperture. Many older SSB transceivers accomplished the “offset”- by switching between three different crystals in an oscillator to produce one carrier for CW operation, a different carrier for LSB operation, and a third carrier for USB operation.
2) We can shift the frequency of the filter to match the desired sideband.
Shifting the carrier is the method most often used.
No matter how we do it, what we want to end up with is represented in Figure 6.
At last! We have produced a single sideband signal that is 3 kHz wide; in this case, a LSB signal.
Notice that the LSB has been selected by shifting (offsetting) the carrier so that the center of intelligence for the filter is 1.5 kHz below the carrier frequency. LSB is the conventional mode of operation on the 40 meter band even though it is legal and effective to use either sideband for voice communications there, so we will concentrate on LSB even though either sideband will get the job done.
NOTE: The discussion here is about the details of SSB circuitry. Do not confuse this with the operating of SSB communications equipment. For example, if you are operating your commercially built amateur radio SSB transceiver, you simply tune your radio until the “other radio operator” sounds right, and you are good to go.
There is one exception worth noting: when operating in a “channel” mode of operation, such as that required for the 60 meter band, where the 1.5 kHz offset is done by the equipment operator (YOU) using the tuning knob on your radio equipment. If you are operating in the “normal” HF Ham bands you need not concern yourself with offsets when operating your radio – all the offsets are built-in by the manufacturer.
You do, however, need to know and understand about offsets in order to understand the circuitry, which is what this tutorial is about.
I realize offsets can be a bit confusing, and I hope my attempt to explain it has helped to lessen the confusion as opposed to exacerbating the situation.
A quick summary of what we’ve covered so far:
[] We need a “carrier” signal on which to construct our SSB signal.
[] Once the carrier has served its purpose, it will be discarded.
[] The sideband(s) are created by mixing an audio frequency with a radio frequency (the carrier).
[] The sideband of interest, LSB in this case, is selected by:
[] A 1.5 kHz shift in the carrier, and
[] A filter
That’s about all the theory we (You & I) need to know in order to build simple and effective SSB equipment.
Now, let’s take a look at some hardware we can use to accomplish SSB voice communication.
SSB TRANSMITTING HARDWARE
Figure 7 shows a simplified diagram of what it takes to generate and transmit a SSB signal for the 40 meter amateur radio band.
As you can see, two mixers involved here. The first mixer combines the audio signal with the carrier signal to produce an intermediate frequency (IF) of 11.xxx MHz.
The second mixer combines the 11.xxx IF signal with the 4.xxx variable frequency oscillator (VFO) signal to produce the 7.xxx MHz signal that will be transmitted.
Why all this mixing? (You might want to know.) It is true that a very simple SSB transmitter can be built that is less complicated than the one shown here, but it would have severe limitations. For example, it might be able to operate on only one fixed frequency in addition to having no choice of sideband. By using heterodyning (mixing) techniques, we can built transceivers that can operate anywhere in the HF radio spectrum on either USB or on LSB. This makes for a much more flexible and useful piece of equipment.
In older equipment, and in many “home brew” radios, the carrier oscillator in a transmitter is usually crystal controlled, as shown in Figure 7. Direct digital synthesis (DDS) is often used in more recent equipment. DDS is outside the scope of this tutorial.
SSB RECEIVING HARDWARE
In order to receive a SSB signal, you simply (sort-of) reverse the transmitting operations. A simplified diagram of a SSB receiver is shown in Figure 8.
Here, again, we have two mixers, a 4.xxx MHz VFO, an 11.xxx MHz SSB Filter, an 11.xxx MHz IF, and an 11.xxx MHz carrier oscillator.
The first mixer combines the 7.xxx MHz SSB signal from the antenna with the 4.xxx MHz signal from the VFO to produce the 11.xxx MHz IF signal.
The second mixer combines the ll.xxx MHz IF signal with the ll.xxx MHz signal from the carrier oscillator to produce the audio signal that you eventually hear coming from the speaker or headphones.
SSB TRANSCEIVER HARDWARE
The transceiver, which includes both receiver circuitry and transmitter circuitry within a single box has replaced the older technology of a separate box for each function, so let’s take a look at a transceiver. As you examine the simplified diagram of a complete SSB transceiver in Figure 9, notice that much of the circuitry is shared by the receiver and transmitter functions.
This block diagram is based on a design by David Harrison, W6IBC, which appeared in the November, 2007 edition of QST magazine. I don’t know of a more suitable design to illustrate a simple and effective SSB transceiver.
RECEIVER SIGNAL PATH
The red line you see winding its way through the various parts of the transceiver, from the antenna in the upper right-hand corner of the diagram to the speaker in the lower left-hand corner traces the signal path for receiving SSB signals.
[] The signal from the antenna first passes through a low-pass (LP) filter which is common to both the receiver section and the transmitter section of the transceiver. The LP filter attenuates signals above the 40 meter band.
[] Just below the LP filter you see a relay switch that allows the signal to enter the receiver band-pass filter. The BP filter attenuates all except 40 meter signals, including those below the 40 meter band.
[] In transmit mode, the relay switch will ground the input to the RX BP filter to prevent damage to receiver circuits while in transmit mode.
[] Upon exiting the RX BP filter circuit, the 7.xx MHz signal goes to an amplifier which boosts the signal before it enters the SA612 mixer (U1).
NOTE: The LP filter and the BP filter have nothing to do with selecting the sideband – the crystal filter does that for us.
[] Inside the SA612, the 7.xx MHz signal is added to a 4.xx MHz signal from the variable frequency oscillator (VFO) circuit to produce a 11.xx intermediate frequency signal.
[] The 11.xx IF signal is routed to the 11.xx crystal filter by the 74HC4053 multiplexer. (Think of the multiplexer as an integrated circuit with some tiny relays inside.)
[] After passing through the crystal filter, the multiplexer routes the signal to U2 (a second SA612 integrated circuit) which serves as a product detector in receive mode.
[] Inside U2, the 11.xx MHz IF signal is mixed with fixed- frequency carrier oscillator signal which “extracts” the voice signals from the 11.xx IF signal. Even though the carrier oscillator is crystal controlled, notice the variable capacitor, “Adj”, which allows for minor adjustments in order to obtain exact offset.
[] The voice signal goes to an LM386 amplifier, which boosts the signal and feeds it to the speaker.
[] The sound coming from the speaker will be the voice of the person who transmitted the radio signal.
TRANSMITTER SIGNAL PATH
The signal path for transmit mode, represented by a red line in Figure 10, begins at the microphone in the lower right-hand corner of the diagram, then winds its way through the various circuits to the antenna connector in the upper right-hand corner.
[] The “raw” signal coming from the microphone is conditioned in the preamplifier circuit before being fed into the SA612 mixer.
[] In transmit mode, the SA612 is used as a sideband generator by mixing the audio with the carrier oscillator signal to produce an intermediate frequency of 11.xx MHz.
[] The TX IF signal is routed to the crystal filter via the 74HC4053 multiplexer.
[] The multiplexer routes the filtered 11.xx MHz signal to U1.
[] Inside U1, the 4.xx MHz signal is subtracted from the ll.xx IF signal to produce the 7.xx MHz SSB signal.
[] The 7.xx MHz signal passes through a 40 meter BP filter, then goes to the transmitter buffer/driver circuit.
[] The filtered 7.xx MHz SSB signal is used to drive the RF power amplifier.
[] The amplified signal passed through the LP filter to the antenna connector.
NOTE: The BP filter and the LP filter have nothing to do with selecting the sideband – the crystal filter does that for us.
[] Assuming a “good antenna” your 40 meter SSB signal then goes “on the air”, and is available to anyone who happens to be listening.
SUMMARY
As you probably noticed, much of the circuitry in this transceiver is used in both receive and transmit mode:
[] UI serves as a mixer . . .
[] Adding the 4.xx MHz signal to the 7.xx MHz signal to produce the 11.xx MHz IF
signal during receive mode.
[] Subtracting the 4.xx MHz VFO signal from the ll.xx IF signal to produce a 7.xx
MHz signal for transmitting on the 40 meter band.
[] U2 serves as a mixer . . .
[] Providing a 11.xx carrier frequency to extract audio from the 11.xx MHx IF signal
in receive mode.
[] Generating a 11.xx intermediate frequency signal in transmit mode.
[] U3 serves as a multiplexer to implement switching between receive and transmit
modes.
[] The bi-directional crystal filter serves to provide selectivity in both receive and
transmit modes.
You have probably noticed various other items in the drawings that have not been addressed. These items are not unique to SSB:
[] The push to talk (PTT) switch provides for manual switching between receive and transmit mode. This switch is normally located on the microphone.
[] Voltage regulation and distribution. For purposed of this tutorial, they can be assumed to be in place and working properly.
[] Voltage regulation and distribution, as well as T/R circuitry are both assumed to be in place and working properly.
[] Since this tutorial is about “how it works” as opposed to “how to build it”, no schematics or parts placement diagrams are provided.
If, perchance, I receive feedback indicating enough interest in actually building this circuitry, further details will be provided.
A closing thought . . .
Perhaps I should mention that a SSB signal can be generated using a phasing method (as opposed to a filtering method). Both methods produce the same results. The reason I used the filtering method in this tutorial is because I think it is easier to understand.
End of SSB Tutorial