fars presentation on modulation

8/6/2019 FARS Presentation on Modulation

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Modulation and Deviation

Dave Platt, AE6EO

Foothills Amateur Radio Society

October 26, 2007


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Modulation

What is modulation?

Why do we modulate?

Spectrum: carrier and sidebands

AM

FM and PM

Data modulations (PSK, QAM) DOs and DO NOTs


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What is modulation?

Modulation is a change or alteration in asignal

Any aspect of the signal can be changed:

amplitude, frequency, phase, timing orrepetition rate of pulses


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Why do we modulate?

We modulate because we wish tocommunicate

An unmodulated signal conveys no

information other than Hi, I'm here Changes in the signal convey information

Interpreting the changes is a matter of

convention: some may be significant,others can be ignored


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Unmodulated (CW) carrier

-2.000

-1.500

-1.000

-0.500

0.000

0.500

1.000

1.500

2.000

Wa

veforma

mplitude


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Unmodulated (CW) carrier


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Spectrum

Most forms of RF modulation involve thegeneration of an RF carrier signal, whichis then modified in some way

Modifying (modulating) the carrierproduces sidebands other RFfrequencies which carry some energy

The specific sidebands created (frequencyand amplitude) depend on the modulationtype and the content of the signal


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Spectrum and content

The amount of information you canconvey depends on the bandwidth of thetransmission, the noise level of the

transmission channel, and the amount oferror you're willing to accept (the fidelity)

There is a fundamental limit (the Shannonlimit) beyond which you can't pack moreinformation into the channel


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Spectrum and content

Some modern digital data transmissionsystems come very close to reaching theShannon limit

Ham voice (phone) modes don't comeclose

Digital cellphone comes closer, thanks to

digital coding/compression


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Overcoming noise

You can use more bandwidth

You can reduce the noise level (moresensitive receiver, directional antennas,

higher transmit power) You can live with it

You can use less bandwidth and accept a

lower information rate (e.g. CW, PSK31) It's all about tradeoffs


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Amplitude modulation

Change amplitude of a carrier signal

Can be a simple on/off keying (amplitudeis either none or full on), or more

subtle. AM usually refers to a linear change in

carrier amplitude, proportional to the

amplitude of an intelligence signal (mostcommonly voice or music)


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Amplitude Modulation

Multiplies the carrier's amplitude, by theamplitude of the modulating signal (plus aconstant)

where m is the modulation level (0-100%)

Vt=sin 2 f

ct 1 m sin 2 f

mt


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AM 50% modulation

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-1.500

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0.000

0.500

1.000

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Waveformam

plitude


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AM 100% modulation

-2.000

-1.500

-1.000

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0.000

0.500

1.000

1.500

2.000

Waveformam

plitude


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expands to the sum of three terms

m

2

sin 2 fc

fm

t

sin 2 fct

m

2 sin 2 f

cf

m t

Vt=sin 2 f

ct 1 m sin 2 f

mt


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The total signal consists of

the original carrier (full amplitude), plus

two sidebands, located above and belowthe carrier frequency by a distance equalto the modulating frequency, each with

an amplitude of up to half that of thecarrier (and thus each has up to of the

power in the carrier)


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AM, 1kHz tone, 50% modulation


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AM, 1 kHz, 100% modulation


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AM creates an RF signal whose totalpower varies from moment to moment...

... but the power level at the carrier

frequency doesn't change! At full modulation, total power is 50%

more than the carrier power

Peak power (top of the modulated-carrierwaveform) is 4 times the average carrierpower


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Envelope maxima occur when the twosideband signals are in phase with oneanother and with the carrier

Envelope minima occur when thesidebands are in phase with one another,but 180 degrees out of phase with carrier

Envelope center occurs when sidebandsare absent or are 180 degrees out ofphase with one another


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AM is easy to generate, either directly(plate modulation) or by amplifying a low-level AM signal (linear amp required!)

Any non-linear method of combining twosignals, or amplifying a signal containingmultiple frequencies, has the effect ofsquaring or multiplying components and

thus generates AM sidebands


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Demodulating (detecting) AM uses thesame technique, in reverse

Multiplying the RF sidebands by the

carrier frequency creates new sidebands,including a set down at the original audiofrequency

Since the RF signal contains both thesidebands, and the carrier, all we have todo is multiply it by itself!


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As noted before, almost any nonlineardevice will perform some amount ofmultiplication or squaring, and thus serve

as a product detector Thus, AM is pathetically easy to detect

you can just follow the RF envelope with arectifier (diode, crystal radio, razor-blade-

and-pin, braces and fillings, corroded wirejunction) and a low-pass filter


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Prone to noise interference, preciselybecause it's amplitude sensitive

Not very efficient. At least 2/3 of the

power is in the carrier and carries nointelligence. Sidebands are identical(symmetrical) and therefore carry thesame intelligence

Not popular on HF because of widebandwidth, heterodyne whistle noise


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Sideband modulation

expands to the sum of two terms

1

2 sin 2 f

cf

m t

1

2 sin 2 f

cf

m t

Vt=sin 2 f c t sin 2 fm t


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Single-sideband modulation

Fundamentally similar to AM

Generate a low-level AM signal, theneliminate the carrier and one set of

sidebands, leaving only the componentsof the other sideband

Amplify the remaining sideband signal

and transmit Can use balanced mixers, crystal filters,

etc. for carrier and sideband rejection


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More power-efficient than AM all of theenergy goes into information-carryingsidebands, none into the carrier

Redundant sideband is filtered awaybefore amplification, uses no power

RF bandwidth same as baseband (audio)

bandwidth many QSOs fit into limitedspectrum

Requires linear amplification (like AM)


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Harder to demodulate than AM becausethere's no carrier.

Must re-inject a carrier signal (local

oscillator) to mix with the incoming RF,multiply, and thus shift the sidebandsignal down to the original audiofrequencies

If LO doesn't match original carrierfrequency, say hello to Donald Duck!


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Frequency modulation

Frequency of carrier is varied, depending onmodulating signal's value. Amplitude(envelope) of modulated signal does not

change

This expands into an infinite series involving

Vt=sin 2 f

cm sin 2 f

mt t

sin 2 fc

N fm

t


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-1.000

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0.000

0.200

0.400

0.600

0.800

1.000

W

aveforma

mplitude


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Frequency modulation usually done byusing a voltage to vary the value of areactance (capacitor or inductor) in anoscillator or tuned-amplifier circuit

Modulated signal is then amplified,multiplied up to a higher harmonic, and/orheterodyned to the final frequency

Amplifier stages need not be linear!


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Like AM, FM creates sidebands bothabove and below the carrier frequency

Each modulating frequency creates

multiple sidebands, separated from thecarrier by the modulating frequency andall of its multiples... all the way up toinfinity (in theory)

Sideband amplitude depends on amountof modulation the modulation index


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Modulation index = peak carrier deviationdivided by modulating frequency

FM signals are inherently wider than AM

signals having the same intelligencebandwidth, due to the presence ofmultiple sidebands

At high modulation index, 3 5 sidebandsmay have significant power


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Total signal power doesn't change... yetthe power for the sidebands has to comefrom somewhere

Frequency modulation shifts power awayfrom the carrier frequency, into thesidebands

At certain modulation indexes, the powerat the carrier frequency actually drops tozero all the power is in the sidebands!


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FM, 1 kHz signal, 1 kHz dev.


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FM, 1 kHz signal, 1.5 kHz dev.

k i l k d


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FM 1 kH i l 3 kH d


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FM 1 kH i l 4 kH d


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FM 1 kH i l 5 kH d


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FM 5 kH i l 2 5 kH d


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R i i FM


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Receiving FM

Amplify, mix RF down to convenient IF Bandpass filter (filter width roughly equal

to channel spacing; sharp selectivity)

Amplify / limit (~70 dB of gain) Frequency-to-voltage converter (phase-

locked loop, tuned-transformer

discriminator, crystal discriminator, pulse-counting zero-crossing detector)

Slope detection works, but not well

Receiving FM


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Receiving FM

High IF gain, limiting process eliminatesmost changes in signal amplitude

RF noise doesn't interfere unless it's right

at the zero-crossing or is nearly as strongas the signal

Limiting also creates capture effect -weaker signals on the same or nearbyfrequency are largely eliminated

Wide and Narrow FM


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Wide and Narrow FM

Wide informationbandwidth (15 kHz)

High maximum

deviation (75kHz)

Wide channelspacing (200 kHz)

High modulationindex

Limited audiobandwidth (3 kHz)

Small maximum

deviation (3-5kHz)

Narrow channelspacing (15 kHz)

Low modulationindex



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FM is legal for use in HF phone bands(except 60 meters) with limitations

Modulation index at highest audio

frequency must not exceed 1.0 Total bandwidth may not exceed that of

equivalent AM signal

Has little benefit, hence almost nobodyuses it (a few 10-meter repeaters?)

Commercial FM stereo


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Commercial FM stereo

Add left and right signals to createmonaural signal (L+R)

Subtract left and right signals to create

stereo difference signal (L-R) Generate a 38 kHz audio subcarrier,

frequency modulated by L-R

Mix subcarrier, 19 kHz pilot signal, L+Rmonaural audio together

Frequency-modulate RF carrier

Pushing it a bit too hard


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Pushing it a bit too hard...

Modulation level (AM or FM) correspondsto peak amplitude of audio signal.

AM: sideband frequencies remain fixed.

No additional sidebands until signal triesto exceed 100% modulation and thecarrier is clipped or cut off.

AM overmodulation creates audibleintermodulation sidebands, and splatteroutside of assigned frequency range.

Pushing it a bit too hard


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Pushing it a bit too hard...

FM: multiple sidebands always exist,although levels are insignificant at lowmodulation index

Increasing modulation causes higher-order sidebands to gain strength. Thesesidebands can fall outside of thereceiver's IF passband and are lost to the

receiver (causes distortion), and... ... fall into adjacent channel's bandwidth

(interference)

Avoiding overmodulation


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Avoiding overmodulation

Best radios turn down the gain bywatching peak levels (selectivecompression) little added distortion

Many radios simply clip audio signal (e.g.using diodes), then apply low-pass filter adds more distortion

Bad or poorly-adjusted radios don't limitenough or at all, may over-modulate

Talking too loudly is Bad


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Talking too loudly is Bad

Can overdrive HT mic and audio path Pushes HT into audio clipping/limiting,

creates audio intermodulation

Overmodulates the FM signal, deviatesbeyond desired channel bandwidth

Can cause internal overdrive and

distortion in the receiver due to designproblems (discriminator clipping ornonlinearity)

PL/CTCSS tone issues


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PL/CTCSS tone issues

Subaudible tone for repeater access ispart of the signal modulating the carrier

Typically adds 500-750 Hz of deviation

Too much (or distorted) PL can be audible If injected before limiter (bad!), excessive

audio level and clipping can suppress or

mask PL tone, lead to talking off of therepeater (PL detector drops out)

Good practice for FM


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Good practice for FM

Check your transmitter's frequencyaccuracy

Check transmitter's deviation limit (e.g.

when talking loudly) and PL tone level Adjust / repair radio if necessary

Talk softly enough that you don't push the

HT into audio limiting Talk across the mic, not into it


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Digital / data AX 25 packet


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Digital / data AX.25 packet

AX.25 packet radio on HF is like faster(300-baud) RTTY

AX.25 packet (1200-baud) on FM uses

FSK of audio signal, fed into standard FMmodulator

AX.25 packet (9600-baud) uses FM(drives modulator directly) with carefully

adjusted deviation levels and filtering

Digital/data - PSK31


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Digital/data PSK31

PSK31 uses Binary Phase Shift Keying.The phase of a single carrier frequency isreversed periodically to indicatetransitions between bits

Low baud rate (31 baud!), narrowbandwidth (under 100 Hz)

Computer generates BPSK-modulated

audio signal, fed to SSB modulator



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Digital/data PSK31

Requires that receiver track carrierfrequency exactly ( a few Hz)

Fine-tuning is done by software, which

uses phase error in received signal to lockonto center of carrier

Can maintain lock on signal, tolerableerror rate even when signal is buried so

deeply in noise you can't hear it



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Digital/data PSK31

Requires excellent linearity of the SSBtransmit chain (including computer soundcard and audio-isolation interface)

Overdriving radio in any way causessplattering (excessively-wide sidebands)which will make the operator veryunpopular!

Best used with careful adjustment, lowtransmit power, signal-quality checks

Digital/data - MFSK


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Digital/data MFSK

Multiple-tone FSK like RTTY but with 8 or16 tones rather than 2 (carries multiplebits of data per symbol)

Phase-continuous shifting between tones Fairly narrow bandwidth (~350 Hz)

Convolutional forward error correction

Sounds a bit like a calliope

Questions?


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Ques o s

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