Transmitters & Receivers

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Transcript Transmitters & Receivers

Foundation Course Transmitters & Receivers

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 !!

Mic 1 2 3 4

1 - Audio Stage 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

1 - Tuning and RF Amplifier 2 - Detection

3 - Audio Amplifier 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 v

m/s f Hertz



metres

AM Modulation

6 •

AMPLITUDE MODULATION (AM) - The audio signal varies the amplitude of the RF Carrier

Audio Input RF Carrier  Note if Audio is too strong, clipping and distortion occurs  Simple AM gives carrier with lower and upper sidebands AM Signal

FM Modulation

7 •

FREQUENCY MODULATION (FM) - The audio signal varies the Frequency of the RF Carrier - its Amplitude stays constant

Audio Input RF Carrier  Actual amount of variation is small & called Deviation  Signal Amplitude is constant and doesn't carry info. It’s therefore less prone to interference FM Signal

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

Keyer /Data CW Signal  Poor Edges can give ringing or key clicks  Don't overdrive if TNCs used for Packet Data  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:

I V

- the voltage of the power source (measured in volts - the current in the circuit (measured in amperes A) V)

R L C

- the - the resistance inductance of the resistor (measured in ohms of the inductor (measured in = V/A); henrys = H = V· s /A) - the capacitance of the capacitor (measured in farads = F = C /V = A·s/V)

- the charge across the capacitor (measured in coulombs C) Parallel RLC Circuit notations: