TRANSMITTER FUNDAMENTALS

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TRANSMITTER FUNDAMENTALS
P-117
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Audio Frequency Definition

• Acoustic, mechanical, or electrical
  frequencies corresponding to normally audible
  sound waves which are of the frequencies of
  approximately 13 to 13,000 hertz

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Radio Frequencies
VLF Very Low Frequency 3 kHz to 30 Khz
LF Low Frequency 30 kHz to 300 Khz
MF Medium Frequency 300 kHz to 3 Mhz
HF High Frequency 3 Mhz to 30 Mhz
VHF Very High Frequency 30 Mhz to 300 Mhz
UHF Ultra High Frequency 300 Mhz to 3 Ghz
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The Electromagnetic Spectrum
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Oscillators

• The oscillator is the heart of a transmitter.
• It creates the frequency that the transmitter
  emits.
• Oscillators can be fixed, (crystal controlled) or
  they can be variable, such as a variable
  frequency oscillator (VFO)

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Block Diagram of a Simple Transmitter
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CW Transmitter
Page 131
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Example of Chirp

• Chirp is caused by an unstable oscillator
• The frequency changes slightly every time the key
  is pressed.

As a point of interest, the letter C is added to
the RST report to let the sending station know
that they have chirp on their signal. EG
UR RST IS 559C 559C
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AMPLITUDE MODULATION

• Amplitude modulation changes the amplitude
  (strength) of the transmitted signal.
• A modulator stage is added to a transmitter that
  uses the signal from the microphone to change the
  transmitted signal in a corresponding manner.
  (It impresses the audio frequency on top of the
  radio frequency).
• The stronger the audio signal, the more the
  amplitude of the carrier change.
• When no audio is applied, the transmitter
  produces an unmodulated carrier like a CW
  transmitter.

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AM Demonstration
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Simple AM Transmitter
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FM Fundamentals

• Frequency Modulation changes the frequency of the
  oscillator. The amplitude always remains the
  same.
• Like an AM transmitter, a modulator stage is
  used, but with an FM transmitter, it is connected
  to the oscillator.
• FM is best for local communications because the
  audio is hi fi and can be clearly understood
  when the signal is weak. Because of this it is
  mostly used for line of sight VHF and UHF work.

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FM Demonstration
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FM Transmitter Diagram P-145
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Single Sideband Fundamentals

• SSB was developed as a more efficient method of
  transmitting audio frequencies.
• Unlike AM modulation, there is no carrier signal.
  All of the energy goes toward transmitting audio
  frequencies.
• An SSB uses less bandwidth than an AM signal. (3
  khz vs 6 khz)
• SSB has become the de facto standard for HF
  Amateur communications.

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First - What is a sideband?
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SSB Transmitter Diagram P-133
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SSB Operating Notes - 1

• Like AM and FM, SSB signals can be overmodulated.
• The result is distortion and splatter.
• SSB Transmitters have a built in control to
  automatically adjust the speech level. (Called
  the automatic level control ALC)
• The ALC cant handle audio signals that are
  excessively strong.
• SSB transmitters have a meter that shows how the
  ALC is working. If the ALC is going into the
  red, then the operator has to talk softer, or
  turn down the microphone gain.

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SSB Operating Notes - 2

• Since there is no carrier, the power in an SSB
  signal. It is measured using Peak Envelope Power
  (PEP)
• PEP is the maximum power in the signal peaks.

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Receivers
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SSB/CW Receiver
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FM Receiver
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Receiver Characteristics

• Selectivity
• Sensitivity
• Stability
• Image Rejection

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Selectivity

• - Is a measure of a receivers ability to
  separate received signals.

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Sensitivity

• Is a measure of a receivers ability to detect
  weak signals. Known as the signal to noise
  ratio measured in Decibels. (There are two types
  of noise.. Internal and External) The formula
  for determining the performance is
• SIGNAL NOISE
• NOISE

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Stability

• Stability is the resistance to frequency drift
  caused by temperature, voltage variations and
  proximity to the body.
• Modern receivers are usually measured in parts
  per million. EG 0.5 PPM
• Older non digital receivers used crystal
  calibrators to make sure they were on frequency.

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Image RejectionThe resistance to receiving
unwanted signals
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Cross Modulation

• Caused by extremely strong signals overloading
  the RF stage. Can cause minor to severe
  distortion.
• Modern communication receivers have an attenuator
  to lower the gain of the RF amplifier

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Bandwidth

• Describes how wide the filter is in the front end
  of the radio.
• The bandwidth must match the bandwidth of the
  desired signal.
• - CW could be 500 Hz
• - RTTY with a shift of 170 Hz would need 250 Hz
• - SSB is typically around 2.4 Khz
• - Amateur FM is typically 10 Khz (/- 5 Khz)