Transcript TRANSMITTER FUNDAMENTALS

TRANSMITTER
FUNDAMENTALS
P-117
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
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
The Electromagnetic Spectrum
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)
Block Diagram of a Simple
Transmitter
CW Transmitter
Page 131
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”
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.
AM Demonstration
Simple AM Transmitter
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.
FM Demonstration
FM Transmitter Diagram
P-145
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.
First - What is a sideband?
SSB Transmitter Diagram
P-133
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 can’t 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.
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.
Receivers
SSB/CW Receiver
FM Receiver
Receiver Characteristics
• Selectivity
• Sensitivity
• Stability
• Image Rejection
Selectivity
- Is a measure of a receiver’s ability
to separate received signals.
Sensitivity
-Is a measure of a receiver’s 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
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.
Image Rejection
The resistance to receiving unwanted signals
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
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)