Almost one third of Communication Electronics is precisely the study of circuits containing passive components - Capacitors, Inductors and Resistors. It is almost like no active components without passive components. All these passive components if brought together in a particular circuit, it becomes, more or less a tuned circuit. If tuned circuits turn frequency generators, they are called Oscillators. There are of two types of Oscillators - Variable Frequency Oscillators and Fixed Frequency Oscillators.
Fixed Frequency Oscillators are mostly Crystal based Oscillators - benchmark for stability. It is a quartz crystal with a definite length, width and thickness that decides the frequency of a circuit that contains it.
Crystals are easily susceptible to over heat and pressure. It also may not withstand potential difference beyond a delicate point. In pictures C-6/1A and C-6/1B, circuit diagrams of Pierce and Colpitts Crystal Oscillators are shown respectively.
Only very minute changes only are possible with regard to the oscillation frequency of a Crystal. Those Carrier Oscillators in SSB transceivers really use this possibility also. In fig: C-6/2 as the 22 PF button trimmer capacitor is tuned the oscillation frequency changes.
Using Crystal Oscillator circuits, generation of frequencies of multiples of the crystal frequency (harmonics) is possible. All that we need to do is fitting a tuned circuit that bypasses only the intended frequency. In fig. C-6/3, a 9.676 MHz crystal is used in the oscillator stage of a 145 MHz VHF transceiver. The oscillator frequency is multiplied five times in the first stage and three times in the second stage. Remember that since transistors get easily saturated at higher applied voltages, tuning coils connected to it may not respond at all. It is common in frequency tuned amplifiers and filters. Increase the value of emitter resistance of the transistor in that stage and avoid corresponding problems if there are.
All the coils in the circuit C-6/3, L1 to L4 have to be wound according to the frequency of resonance in that particular stage. To handle different frequencies using the same circuit, either physical replacement of the crystals or diode switching of crystals can be resorted to.
In Fig: C-6/4, see how LSB and USB Crystals for 9 MHz are switched using diodes.
Here, depending upon which diode gets a positive voltage at its cathode and thus gets forward biased, the crystal in the corresponding circuit begin to oscillate. In most cases what we require is a VFO (Variable Frequency Oscillator) able enough to develop a wide range of frequencies, according to user's requirement. But a VFO as stable as a Crystal Oscillator is nearly impossible. Much before we think of assembling a VFO, it is better to ascertain the operating range and the degree of stability the VFO requires.
VCOs (Voltage Controlled Oscillators) that change the frequency controlling the applied voltage, PTOs (Permeability Tuned Oscillators) that change the frequency by varying the inductance of the coil and VFOs that changes the output frequency by changing the capacitance in a tuned circuit are all in general use. Transistors used in Oscillator stages also are to be verified for its features, before deciding to try in such a sensitive critical stage. The base to emitter capacitance (Cbe) and Base to Collector capacitance (Cbc) within the transistor also changes according to the applied frequency. These factors together with the changing Base- Emitter resistance (rb) are taken in determining the ft (maximum handling frequency) of that transistor. It is always better to use a transistor that has 7 times more capacity than the frequency a transistor is handling.
Another important factor that decides the ability of a transistor is beta. It is recorded in units of 'hfe'. It shows how many folds higher a current any transistor is able to reproduce in its' collector when compared to the current in the input. At the same time see that the power difference between the input and output of a signal is marked in decibel(db). Whatever be the amplification available, in experience, especially in the case of sound, only a negligible change would be experienced by our ears, like a signal amplified ten times appear to us just below three times. This is why amplification of sound signals are recorded in db which is marked in relation to the input-output current ratio.
Bipolar transistors are capable of handling Radio Frequencies, keeping Noise Figure low, signal purity high and dynamic range high. A bipolar transistor is a semiconductor device commonly used as amplifiers - current or voltage. Whichever be the transistor, if the input and output impedance are mismatched, even though stage gain will be reduced highly, in effect there could be considerable stability. Two major drawbacks of Bipolar transistors are their input - output isolation and low inout- output impedance. In the case of Dynamic range also Bipolar transistors have their own limitations. In fig: C-6/5, a 3.5 MHz VFO circuit using Bipolar transistors is given.
Whatever be the transmitter, if its' fundamental frequency and output frequency are one, it results in lower stability and could also result in feed back issues that might seriously affect its performance. If we could successfully avoid all harmonics that are generated in the oscillator stage and block all unwanted frequencies that cross over to following stages, it will help us to improve the efficiency of the transmitter amplifier stages, and push out highly pure Radio Frequency in the output. It is mostly because of this reason why most VFOs resort to frequency doubler/convertor circuits.
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