Bredhurst Receiving and Transmitting Society |
Part 1 |
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4. Transmitters and Receivers 4a Simple block diagrams of a transmitter 4a.1 Recall and understand the block diagrams of CW, AM, SSB and FM transmitters. Students are not being able to recall these simple block diagrams and understand what they are.
In the CW Tx the morse key is connected into the "keying stage" indicating that the RF oscillator remains undisturbed whilst the keying is carried out.
With the AM Tx a modulator is required after the RF oscillator to mix the oscillator signal with the audio signal.
In the SSB Tx the signal is applied to a balanced modulator (also called a balanced mixer) and there is a sideband filter to remove the unwanted sideband.
The FM Tx is the only one where the audio signal is mixed directly with the RF oscillator as it says in the name Frequency Modulation. It is the frequency that is changed and to do this the RF oscillator must be acted upon. Note:- In ALL cases the Power amplifier stage has a low pass filter following and prior to the antenna connection to reduce unwanted signals from radiating. 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. Audio Frequency (AF) amplifier also called the microphone amplifier
AM / FM Modulator
Balanced Modulator or Balance Mixer is used to create SSB
Students are not being able to recall that the mixing of RF and AF signals produces an output which are known as SIDEBANDS. Side Band Filter
Radio Frequency (RF) oscillator
Power Amplifier (PA)
Low Pass Filter (LPF)
4b RF oscillators 4b.1 Recall and understand the relative advantages and disadvantages of a crystal oscillator and a VFO. There are two basic types of RF oscillator, the crystal controlled oscillator and the Variable Frequency Oscillator (VFO). The oscillators have characteristics which determines which is the best for a particular application. The key characteristics are: Crystal Controlled Oscillator
Variable Frequency Oscillator (VFO)
4b.2 Recall that the resonant frequency of the tuned circuit in a VFO determines the frequency of oscillation. In the section on Inductor the term "tuned circuit" was used (for a refresher click on the link for a new page to open). It is the resonant frequency of the tuned circuit which determines the frequency of the oscillation. 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. Once the transmitter is set to a frequency an oscillator continues to output a constant frequency. We refer to the ability to remain on the set frequency as stability. The frequency of the oscillator can change due to:-
Even crystal oscillators can be affected by such variations although they are generally better in this respected than tuned circuit oscillators. Therefore, good RF oscillator design demands: Rigid construction - To prevent changes in frequency mechanical movement of inductors and capacitors due to vibration. A regulated (stabilised) DC supply to the oscillator - To minimise changes in frequency due to changes in supply voltage. Screening - To limit the effects of stray capacitance and to limit leakage of unwanted RF from or into the RF oscillator and to limit the effect of changes in temperature due to air movements. 4b.4 Recall that most modern oscillators are digital synthesizers, which are very stable. In modern equipment, many of the limitations in the two basic types of oscillators (crystal and variable) can be generally overcome by the use of the modern frequency synthesiser.
There are a various designs of frequency synthesiser which use a master crystal
oscillator as a reference for a variable frequency oscillator. The resultant
output signal is therefore, accurate, stable, and variable.
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