sound test. signals and waveforms what is a signal? need not be electrical morse speech video...
TRANSCRIPT
Sound test
Signals and waveforms
What is a signal?
Need not be electrical
Morse
Speech
Video
Contains information
Signals have shapes – waveforms
Water waves – height Audio – sound pressure
Audio – electrical voltage
Electrical waveforms are variations in voltage
AC mains has a waveform but is not a signal.
BITX20 bidirectional SSB transceiver
BITX20 bidirectional SSB transceiver
LO BFO
Mic
Mixer
MixerIF FilterRF Filter
Antenna
Transmit direction shown
Summary of our radio waveforms
Audio Frequency (AF)
Beat Frequency Oscillator (BFO)
Intermediate Frequency stage (IF)
Local Oscillator (LO)
Radio Frequency stage (RF).
Lets look at the waveforms
We start with the input Audio Frequency
Many of the waveforms are sine waves
Later we will look at why
Why Sine waves are important
They are natural
They are as fundamental as the circle
All other waveforms can be broken down into sine waves (More on this later)
A graph of a Sine Wave
Time ->
2 4 6 8 10 12
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Another natural Sine Wave generator
1 2 3 4 5 6
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Plotting waveforms
ScaleAxesOriginTime axis AmplitudeFrequencyNegative axesScope time base
-6 -4 -2 2 4 6
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The sound of waveforms
The note A above Middle C is defined to be 440 HzHere is a pure sine wave at 440Hz
0.001 0.002 0.003 0.004
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Time in seconds->
Vol
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Other waveforms
Here is a Square wave at 440Hz (A above Middle C)
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Time in seconds->
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Fundamentals and Harmonics
In general fundamental frequencies are sine waves.
Any waveform can be broken down into a fundamental sine wave and its harmonics.
Harmonics are 2,3,4 etc (i.e. integer) times the fundamental frequency.
A square wave can be shown to consist of a fundamental (of the same frequency) plus only odd harmonics.
The harmonic content of a square wave
A square wave has a 3rd harmonic of amplitude 1/3 plus a fifth of amplitude 1/5 etc.
If it is a perfect square wave these go on forever.
(Being a symmetrical waveform it has no even harmonics)
We will add the harmonics one at a time and inspect them.
3rd Harmonic
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Playing just the harmonic
Time in seconds->
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3rd Harmonic added
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5th Harmonic also added
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7th Harmonic also added
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Fundamental and odd harmonics up to 15
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There are a total of 7 notes playing
Compare our original square wave
A Square wave at 440Hz (A above Middle C)
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And compare our original pure sine wave
Here is our pure sine wave at 440Hz again
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Linear and non linear systems
We have seen that waveforms can be broken down and rebuilt by adding sine waves.
This only works well for linear systems (i.e. if you can trust addition.)
For example if in your system doubling the input signal doesn’t double the output signal you have a non-linear system.
Non linear systems
In a linear system when you apply a sine wave of frequency F you just get a sine wave of frequency F out.
In a nonlinear system you also get some harmonics at frequencies 2F, 3F etc. (only odd ones if its symmetrical)
E.g. if you seriously overdrive an amplifier with a sine wave you will get something like a square wave.
Non linear systems
In a linear system when you apply two sine waves of frequency F and G you just get frequencies F and G
In a nonlinear system you also get sine waves at frequencies F+G and F-G.
(You also get all the harmonics and all the sums and differences of the harmonics)
The ideal mixer
Another day we will look at the electronics of mixers.
An ideal mixer multiplies rather than adds waveforms.
If you feed two sine waves at frequencies F and G into a multiplier you just get sine waves at frequencies F+G and F-G and no harmonics.
Rather than prove this using maths this lets look and listen.
The inputs to the ideal mixer
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2000Hz
2200Hz
The output from the ideal mixer
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200Hz
4200Hz
and
Comparison sounds to check the output
200Hz
4200Hz
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Some maths
Did you notice the output waveforms were 90 degree phase shifted sine waves of half the amplitude?
For many purposes this makes no difference
Sin(f)* Sin(g) = Cos(f-g)/2 – Cos(f+g)/2
My last graphs allow for the phase shift. A mathematician would call them cosines but they are still sine waves.
BITX20 bidirectional SSB transceiver
LO BFO
Mic
Mixer
MixerIF FilterRF Filter
Antenna
Transmit direction shown
Questions?