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INTERMEDIATE LESSON 13

LEARNING OBJECTIVES and NOTES
Transmitters and Receivers Transmitters and receivers consist of a number of sections or BLOCKS. The circuit of a radio can be shown as a block diagram. At a later stage the electronic components making up each block can be studied. Amateurs often like to experiment with a particular block, trying a new circuit.
4a Simple block diagrams of a transmitter 4a.1 Recall and understand the block diagrams of CW, AM, SSB and FM transmitters. 4a.2 Understand the functions of: the microphone amplifier, AM/FM modulators, balanced modulator for SSB, side band filter, oscillator, power amplifier and low pass filter. RF oscillators Block diagram of a CW transmitter. RF oscillator - produces a stable sine wave. This could be a crystal oscillator or a variable frequency oscillator or a synthesiser. It keeps running all the time at low power. If it is turned on and off to create CW the signal will have a chirpy sound. Keying Stage - This allows the RF oscillation through when the key is pressed. It may also reduce key clicks.This can be used to produce Morse Code. Power Amplifier - This amplifies the oscillator signal to a level where it can be transmitted via the antenna. Low-Pass Filter - All stages, particularly Power Amplifiers not only produce an amplified signal on the required frequency, but also harmonics on multiples of the required frequency and spurious signals which are not harmonically related.. The Low pass filter allows the required signal through, but filters out any higher harmonics and spurious signals.
Block Diagram of an AM Transmitter RF Oscillator - generates the required transmit frequency. Modulator – mixes the audio frequency with the RF frequency to produce an amplitude modulated signal AF Amplifier – amplifies the audio from the microphone Power Amplifier - as above Low –Pass Filter - as above This is how AM is produced in a modern transceiver. Another method used in the past was to apply the audio to the PA via a modulation transformer. The modulation transformer modulated the power applied to the PA stage.

Block Diagram of an FM Transmitter RF Oscillator - Produces the FM frequency Audio Amplifier – Amplifies the very low level audio from the microphone. The amplified audio is then used to slightly change the frequency of the RF oscillator to create frequency modulation. Often done with a varicap diode Power Amplifier - as above Low-pass filter - as above
Block Diagram of a single band SSB Transmitter RF Oscillator -generates a wave at the IF frequency. This is usually a crystal controlled oscillator Balanced Modulator – mixes the audio frequency with the RF and removes most of the the carrier, leaving an upper and lower sideband with a suppressed carrier Sideband filter – filters out one of the two sidebands leaving a single sideband. Usually a crystal filter. VFO and mixer- generate the required transmit frequency by mixing the IF and VFO frequency Power Amplifier - as above Low-pass filter - as above
4b.1 Recall and understand the relative advantages and disadvantages of a crystal oscillator and a VFO. Crystal Oscillator - Advantages Very stable to temperature changes Very stable to mechanical movement / vibrations Does not need a buffer stage No complicated frequency scale required. Uses less power. Crystal Oscillator -Disadvantages Fixed on one frequency and so cannot be tuned other than a few Hertz by a Variable capacitor. VFO - Advantages The frequency can be changed – usually by rotating a variable capacitor VFO - Disadvantages Less stable to temperature changes Less stable to movement and mechanical vibrations. Best built into a metal box which also shields it from RF from the PA Needs to be followed by a buffer stage to stop the frequency being pulled by changes in later stages – particularly during changes from transmit to receive. Needs some form of frequency readout. Uses more power.
4b.2 Recall that the resonant frequency of the tuned circuit in a VFO determines the frequency of oscillation The resonant part of this oscillator circuit consists of the inductor L1 and the associated capacitors (variable C1, C2,and fixed C1). By increasing the value of the capacitors or the inductor the frequency will go down. By decreasing the value of the inductor or the capacitor the frequency goes up.
4b.3 Recall that the frequency stability of an oscillator can be improved by rigid mechanical construction, screening the oscillator enclosure, and using a regulated DC supply. Understand that a lack of stability (drift) may result in operation outside the amateur bands. It is very important that VFOs remain on the desired frequency. To achieve this: use good quality variable capacitors with ball bearings Connect up parts with thick wire Wind coils on ceramic formers that cannot be vibrated and are not subject to expansion and contraction Use a stabilised power supply for the oscillator circuit Mount the VFO in a strong, well shielded box If the VFO drifts there is a danger that when operating near band edges the transmission could fall outside the amateur band.
4b.4 Recall that most modern oscillators are digital synthesisers, which are very stable. Since the 1980s there has been an increasing use of digital synthesisers. These have come down in price and are very stable. Originally they were more noisy than VFOs, but nowadays they have a signal purity to equal the best VFOs. The synthesiser also reduces the complex switching found in VFO controlled rigs.