Modulation

Voice Modulation- SSB

AF5NP AM (Amplitude Modulation), Bandwidth, FM (Frequency Modulation), HF (High Frequency), Modulation, SSB (Single Sideband), VHF/UHF

In the last post we explored voice modulation and learned that AM and FM are the two primary modes.  We also noted that single sideband (SSB), a variant of AM, is the dominant form of voice communication on the ham bands below 50 MHz.   This post will delve into the details of SSB.

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SSB is important because it significantly improves the plain AM signal in both efficiency and bandwidth.

G2A06-2019In context of voice modulation, bandwidth means the range of frequencies the signal occupies.  In general, wider bandwidth means higher audio quality but less efficiency, and vice versa.

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The SSB signal is generated using AM but before it is transmitted the carrier and one sideband are removed.  A diagram of an AM signal helps visualize how it starts:

sidebands1

SSB transmission reduces the power required (more efficient) and occupies a smaller slice of the RF spectrum (narrower bandwidth, ~3kHz), when compared to AM.

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The transmitter circuitry is essentially AM but adds carrier and sideband filters at nominal cost.  The SSB receiver is more complex and expensive than AM because it must reconstruct the missing (mirror image) sideband to create a full voice signal, as well as replace the carrier with a local signal.

Because it has to recover the opposite sideband the SSB receiver is somewhat sensitive to tuning to make the voice sound normal or natural.  You will find that voices sound quite different in clarity and pitch when tuning around the transmit frequency.   A common reaction to hearing off-tune SSB is that it sounds like a quacking duck.  A great video demonstrating some of the nuances of SSB sound and tuning can be found in the links at the bottom.

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So SSB transmits only one sideband, but which one, you ask?  The answer is either.  When the upper sideband is transmitted and received, the operation is called upper sideband (USB).

USB waveform

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Conversely, when the lower sideband is used, the operation is called lower sideband (LSB).

LSB waveform

G2A02-2019In ham radio either one is legal to use in the phone portion of the band plan but by convention LSB is used below 10MHz, and USB is used above 10MHz (note that Continue reading →

Voice Modulation- AM & FM

AF5NP AM (Amplitude Modulation), FM (Frequency Modulation), Modulation, PM (Phase Modulation), SSB (Single Sideband)

In the last post we introduced the concept of modulation as one of the five meanings of the term mode.  Voice is the most common and obvious mode of modulation so let’s dive into that a bit as it is one of the basic concepts of amateur radio.  

Modulation is the means to impress information on a radio signal.  It’s how a circuit puts our voice onto the radio signal through a microphone.

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If you are (or were) fortunate enough to receive US Technician license training using the ARRL course book and companion slides you would find some good info as follows:

When information is added to the radio wave, (the carrier) we modulate the wave.
     •Turn the wave on and off (Morse code)
     •Speech or music
     •Data

In the physics of waves there are three characteristics we might control to impart information (three possible modulation methods or modes).

Different modulation techniques vary different properties of the wave to add the information:   amplitude, frequency, or phase.

These three are commonly known as Amplitude Modulation (AM), Frequency Modulation (FM), and Phase Modulation (PM).

AM is relatively easy to understand and visualize.  The RF carrier wave is varied in amplitude according to the audio waveform (as from a microphone) to create a modulated signal.

AM Graphic

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Amplitude modulation is commonly used on the familiar AM broadcast band.  It may occasionally be found on lower frequencies in the HF ham bands.  While it used to be the primary mode of voice communication in the early years, very little pure AM is used nowadays in favor of single sideband (SSB), which is a type of AM. 

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SSB is a big and important topic which merits its own write-up so we will post that soon.

FM is less intuitive and harder to visualize than AM.  The RF carrier is varied in frequency according to the audio waveform (as from a microphone) to create a modulated signal.

FM Graphic

Frequency modulation is commonly used on the familiar FM broadcast band and in ham radio above 28MHz for high quality simplex and repeater operation.

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PM is very similar to FM in the way it operates with only a technical difference between the two (integration of the baseband signal).  Here the RF carrier is varied in phase according to the audio waveform, instead of the frequency.

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Despite the terminology, many (most?) FM radios we routinely use for local communications over repeaters use PM circuitry.   The two are effectively equivalent for ham radio use; interesting discussion here and even more detailed one here.

For the ham radio operator, AM and FM are the two main voice modulation modes to understand.  SSB and PM are simply sub-sets or derivations.  This animated graphic is particularly helpful in visualizing AM vs FM:

AM vs FM animation

Useful comparisons between the two (applies equally to broadcast and ham radio):

  • AM has poorer sound quality (narrow bandwidth); FM better quality (wider).
  • AM is prone to noise interference; FM is relatively immune to electrical noise.
  • AM has narrower bandwidth (more efficient) than FM (trade-off with sound quality)
  • AM circuitry is simpler and cheaper than FM
  • With AM, transmitted power level varies with the amplitude of the signal; with FM transmitted power level is constant, regardless how much modulation (deviation) is applied.

In context of voice modulation, bandwidth means the range of frequencies the signal occupies.

Because of these differences in bandwidth and efficiency, AM and FM are generally used on different Continue reading →

Mode Madness

AF5NP CW, Digital, Mode, Modulation, Operation, Phone, Propagation, Satellite, Split

In ham radio the term mode has at least five distinct meanings.  It’s confusing for even experienced hams so we’ll try to tame some of this madness.

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If you’re a new ham with a handheld or mobile transceiver to talk on the local repeater, mode doesn’t mean much to you.  Your basic radio has no mode controls because it can only do one thing.  In this case you are operating in voice mode using frequency modulation (FM).  Guess what?  These are the first two—and most important—of the definitions of mode.  These are operating and modulation modes.

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Below we will explore the five different contexts for mode found in US license exam questions:

  • Operating
  • Modulation
  • Propagation
  • Satellite
  • Split

Operating Mode-  The most basic definition of mode; a general category of radio transmission and reception.  There are three or four operating modes, depending on how they are categorized.  The common three are:

  • CW (continuous wave), typically for Morse code (radiotelegraphy)
  • Radiotelephony (phone), a fancy term for voice communications
  • Digital, where data is exchanged over the air, requiring computers or machines to interpret signals

For logging and awards these three categories are CW, Phone, and Digital modes.  The ARRL Logbook of The World (LoTW) adds a fourth category:  Image.  Collectively these 3 or 4 are known as mode Groups.

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Modulation Mode-  Modulation is the means to impress information on a radio signal.  It’s how a circuit puts our voice onto the radio signal through a microphone.  There are different forms (modes) of modulation which can be employed within each basic operating mode.

For example, typical modern HF transceivers support voice modes using AM, FM, and SSB modulation modes.  There are a few flavors of CW and dozens of digital modes (and the list keeps growing).  Just look at that mode group list link above.

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To further complicate matters we now have both traditional analog modulation for phone (voice) signals, and digital voice modulation as well.  Many digital modes simply modulate a SSB waveform using specific tones to represent data characters.  We live in an era where computers and radios are really working together to do amazing things.

Operating and modulation modes are hard to separate.  In fact, they sort of overlap and mash together.  Context of the discussion is key here; often it doesn’t really matter.

These two also play a role in the ITU classification of RF signals.  Refer to Types of Radio Emissions link.  Hams may occasionally log their mode according to this or a similar scheme.

Why do so many operational and modulation modes exist?  It’s largely for historic reasons as technology and electronics have advanced over the years.  In the earliest days of radio, only radiotelegraphy existed. Mode had no meaning as CW was the only possibility.   Then came voice technology and a second operating mode was born.  Going from original AM to SSB,  we then had modulation modes, adding FM as an improvement later.  Digital mode entered the scene after voice once people discovered they could encode audio signals to represent data; computer technology has made the digital mode wildly successful, if less personal, in recent years.  Image modes have been around since the early days of television but here again, computers have made them better and easier.

One other reason for different modes is Continue reading →