Oscillators have critical roles in both Receivers and Transmitters. They decide the quality of output signals in them. Which is the best oscillator for generating a quality Radio Frequency signal? There are various types of oscillators like VFO (Variable Frequency Oscillator), VXO (Variable Crystal Oscillator), VCO (Voltage Controlled Oscillator), PTO (Permeability Tuned Oscillator), PLL (Phase Loop Lock) and LMO (Linear Master Oscillator or Frequency Synthesiser). Each one has its' own subdivisions and variations with unique features of its' own. What we mostly consider before choosing one are our nature of requirement, limitations in investment, availability of components and technicalities of the circuitry.
The stability, clarity and strength factors of the signals are very critical when the oscillator circuit is attached to a transmitter. Stability may be defined as the capacity of an energy line to remain in equilibrium under steady/unsteady load conditions, and its ability to regain a state of equilibrium under disturbances from varying conditions. An RF signal passing through an active circuitry always carries with it all its' harmonics too. Harmonics are multiples of the fundamental wavelength while subharmonics are integral submultiples of the fundamental frequency. When a 7MHz signals is fed into a tuned circuit for 14 MHz, 7 MHz signal also will be there in the output along with all the harmonics of 14 MHz.
The stability and clarity features of an oscillator circuitry are not totally dependent on the type of circuitry used. They are influenced by outside signals and quality of power supply too. This is why we prefer frequency doublers (where original oscillator frequency and the output signal frequencies are not one and the same), fix oscillator modules inside heavy aluminium boxes with no unnecessary holes on it and keep oscillator modules reasonably away from linear amplifier/power supply sections. In a receivers, if the size of the Frequency Oscillator Module box is small, while mixing the output signal with the received RF, Receivers may experience a 'jamming' effect.
For best stability, there are people who go for Convertors. In a Convertor there are a Master Oscillator and a Sub- Oscillator. Master Oscillator will be variable while Sub-Oscillator is Fixed (mostly Crystal). In SSB exciters this format is inevitable. If this pattern is accepted, instead of the DC Receiver in Transceivers high quality BC receivers are possible. In fig: C-8/1 modules 1 to 7 belong to the Receiver modules 8-12 including 1belong to transmitter.
VCO (Voltage Controlled Oscillators are also seen in common use. Here also, it is the effective capacitance of a tuned circuit that is changing. But it is not a variable capacitor that we use for it. Instead, the varying junction capacitance of a solid state diode in tune with the applied voltage is what that is exploited here. Those special high speed diodes that are manufactured for this purpose are called Varicap (Variable Capacitance) Diodes, VVC (Voltage Variable Capacitors) and Tuning Diodes. In a common circuitry, what the VFO requires to make a change of 100 KHz is a change of 10 PF to 20 PF in the value of the tuning capacitor. Since similar low value capacitors are un available, shrewd home brewers open small value 2J PVC variable capacitors and remove a few plates.
In VCOs it is linear potentiometers that are used at tuning. The proportional frequency tuning favour and the mechanical convenience along with it are positive factors that compel home brewers to opt VCO circuits. Two important negative features of a VCO are that they need a highly stabilised power supply and the junction capacitance stability is not as steady as that of a simple capacitor used in VFOs. Still, for RIT (Receiver Incremental Tuning for frequency shifting), it is VCOs are preferred.
be it in a VFO or a RF Linear Amplifier, inter-stage isolation is important to avoid mutual interferences. LF, HF, VHF signals that pass through Power supply lines are grounded using a decoupling networks. Capacitors at near by values of 0.001μF, 0.01μF found frequently on power supply lines mean this. To maintain the audio quality, the value of these decoupling capacitors are compromised in Audio amplifier stages. Below shown is the circuit and assembling details of a 3.5 MHz doubler VFO designed by the author.
The beta (β) factor of transistors are not as stable in Transistors as that in amplifier Valves. Even if the oscillator transistor used is rated at high 'hfe', it may not oscillate at 5.1 or 6 volts. To identify problems due to this raise the supply voltage to 9 and then check output RF voltage using RF probe or RF probe. Still if the it is not oscillating, assuming that it should have oscillated at 6 Voltage, check all capacitors in the oscillator circuit. Soldering quality capacitors in parallel at the print side will help us locate the open capacitor.
Once the oscillator stage has begun functioning, adjust the tuning frequency to the desired range and adjust the tuning width of the tuning capacitor also. The capacitor in parallel to the coil decides the frequency and the trimmer in series to the tuning capacitor decides the tuning width. In RF coupling circuits, lose coupling is preferable. Unless we use low value capacitors (5 PF t0 10 PF) for coupling in oscillator buffer stages, a pulling effect is more likely.
Buffer stages are not amplifiers at all. The purpose is to isolate the oscillator stage from the disturbances of the loading/pulling effect of the later stages.It is always better to use only low tolerance (2.5%) components (esp. capacitors) in delicate stages like the oscillator stage.
Buffer stages are not amplifiers at all. The purpose is to isolate the oscillator stage from the disturbances of the loading/pulling effect of the later stages.It is always better to use only low tolerance (2.5%) components (esp. capacitors) in delicate stages like the oscillator stage.
The bandpass filter coils used in C-8/3 can be easily tuned to both 3.5 MHz and 7 MHz. If you tune the filter coils for maximum RF output what we get could be 3.5 MHz. To change the output to 7 MHz, while tuning filter coils, use a frequency counter for best results. Whatever be the VFO we assemble, both the applied voltage at the oscillator stage and the value of the resistor connected to the ground in series to the RFC connected to the Source point of the FET are to be adjusted according to the 'hfe' of that particular transistor. Thus we can set the voltage between the Drain and Source of that transistor just above what ie required for an oscillation. This is how we control or limit generation of unwanted signals and thus increase the stability of the stage.
Another easy technique is connecting VFO to a tuned 7 MHz amplifier at its input and tune the VFO filter coils to maximum out put at the 7 MHz amplifier output. The best of all is applying a 7050 KHz crystal Oscillator signal at the base of the doubler stage transistor (BF195C) and tuning the Filter stages for maximum output. While doing this the oscillator stage in the circuit should be disconnected an inactivated. Always remember that the first filter coil is more important and determining.
As told before, having a Crystal Oscillator of 7050 MHz is helpful in fixing all tuned circuits coming in that range, and evaluate the performance of VFOs too. Every time we check the efficiency of a VFO, the power supply should necessarily be a battery source. All components working excellent but having no output RF means that the input at the doubler transistor base is considerably low. here, either increase the oscillator RF voltage or increase the stage gain of buffer stage. It is the RF amplifier stage that decides the amplification percentage of the above VFO. To increase the output RF voltage, use a 150 Ω Resistor and a 1μH RFC in series to that at the collector of 2N2222 transistor. So far we were discussing only on Frequency Generators commonly used by Hams.
As told before, having a Crystal Oscillator of 7050 MHz is helpful in fixing all tuned circuits coming in that range, and evaluate the performance of VFOs too. Every time we check the efficiency of a VFO, the power supply should necessarily be a battery source. All components working excellent but having no output RF means that the input at the doubler transistor base is considerably low. here, either increase the oscillator RF voltage or increase the stage gain of buffer stage. It is the RF amplifier stage that decides the amplification percentage of the above VFO. To increase the output RF voltage, use a 150 Ω Resistor and a 1μH RFC in series to that at the collector of 2N2222 transistor. So far we were discussing only on Frequency Generators commonly used by Hams.
Along with two simple circuits used to check FETs are show here through C-8/4 A and C-8/4B.
In C-8/4A, at 0-.25mA Range the multi meter should show 6-10mA while in C-8/4B the multimeter reading should be 4 mA. 
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