Transcript Transmitters & Receivers

Foundation Course
Transmitters & Receivers
1
EKRS
Karl Davies
Tuned Circuits
2
 Radios depend on the concept of tuned circuits.
 Tuned circuits are built from combinations of Inductors and
Capacitors which have a self-resonant frequency.
 Tuned circuits are thus able to selectively pass or block
frequencies in transmitters and receivers.
 They are the basis of tuners, filters, oscillators, ATUs etc.
Transmitters
3
 Transmitter concept is in the block diagram below: Foundation Licence only permits use of commercial equipment to
minimise the risk of interference and/or out-of-band operation.
 Avoid over-deviating, and operating PAs into poor matches !!
1
2
4
Mic
1 - Audio Stage
3
2 - Modulator e.g. AM, FM, SSB
3 - RF Frequency Generator
4 - RF Power Amplifier
Receivers
4
 Receiver concept is in the block diagram below: RF Front-end is critical to performance. Inductors and
capacitors create selectively tuned circuits.
 RF Amplifier stage dominates the Noise performance
 Detection circuits for decoding AM, FM etc are different
1
2
3
4
1 - Tuning and RF Amplifier
3 - Audio Amplifier
2 - Detection
4 - Loudspeaker
Modulation
5
 Modulation (or Mode) refers to how audio or data
information is superimposed onto an RF ‘Carrier’
frequency
 Remember - the RF Carrier is a sine wave:-
v m/s
v
f

f Hertz
 metres
AM Modulation
6
• AMPLITUDE MODULATION (AM) - The audio signal varies the
amplitude of the RF Carrier
 Note if Audio is
Audio Input
too strong,
clipping and
distortion occurs
 Simple AM gives
RF Carrier
AM Signal
carrier with
lower and upper
sidebands
FM Modulation
7
• FREQUENCY MODULATION (FM) - The audio signal varies the
Frequency of the RF Carrier - its Amplitude stays constant
 Actual amount of
Audio Input
variation is small
& called Deviation
 Signal Amplitude
RF Carrier
FM Signal
is constant and
doesn't carry info.
It’s therefore less
prone to
interference
CW & FSK Modulation
8
• Morse, also called CW, is the simplest form of digital mode.
• FSK, Frequency Shift Keying, is used for higher speed ‘Packet’ data
 Poor Edges can
give ringing or key
clicks
Keyer /Data
 Don't overdrive if
TNCs used for
Packet Data
CW Signal
 Data rates are
limited by
available
Bandwidth
FSK Signal
Earthing/EMC
9
 Good reception especially on HF, as well as EMC performance,
depends on good earthing.
 Ensure shack equipment is run from a common mains earth to
prevent earth loops - use filtered mains boards and ferrite rings
correctly.
 RF Earths for antennas are often separate - consider earth stakes etc.
 Modern Gas & Water Pipes can give high resistance earth.
 AM/SSB can be rectified/detected easily, so is most prone to cause
interference - Operate in a responsible manner!
Operating Precautions
10
 Ensure Transmitter frequencies/modes are setup correctly so
emissions are always in band, and conform to band plans.
 RF power amplifier outputs must be connected to a correctly
matched antenna to work properly. Use of the wrong antenna
can result in damage to the transmitter.
 Excessive AM modulation or FM deviation will cause
distorted outputs, and interference on adjacent channels
 Ensure that Microphone Gain (where fitted) is correctly
adjusted
Extras
11
 Coils/Inductors pass DC but block AC
 Capacitors block DC but pass AC
Units
mH = milli Henry’s
f micro-Farads
Tuned Circuits are circuits with mixtures of coils and capacitors
Extras – Tuned Circuits
12
Series RLC Circuit notations:
V - the voltage of the power source (measured in volts V)
I - the current in the circuit (measured in amperes A)
R - the resistance of the resistor (measured in ohms = V/A);
L - the inductance of the inductor (measured in henrys = H = V·s/A)
C - the capacitance of the capacitor (measured in farads = F = C/V = A·s/V)
q - the charge across the capacitor (measured in coulombs C)
Parallel RLC Circuit notations:
V - the voltage of the power source (measured in volts V)
I - the current in the circuit (measured in amperes A)
R - the resistance of the resistor (measured in ohms = V/A);
L - the inductance of the inductor (measured in henrys = H = V·s/A)
C - the capacitance of the capacitor (measured in farads = F = C/V = A·s/V)