R.F. Local Oscillators.

R.F. Local Oscillators.

Much design work has been undertaken as regards the local oscillator. At first the variable capacitor made from sections of plate metal to mesh into each other provided the variable capacitance. Other receivers used a tuneable inductance, by retracting in movement the tune core of an inductor.

Electronic tuned oscillators can be achieved by motorising the mechanical movement of the otherwise manually adjusted capacitance or inductance components.

Using transistors to utilise the capacitance of the diode junction also electronic tunes an oscillator. By using a variac capacitance diode for which the presented capacitance is governed by the reverse biasing the diode junction with a tuning voltage, the oscillator’s centre of resonance tuning can be altered. In fact any diode with a large enough capacitance within the diode depletion area can be used.

Stabilising an oscillator is a design subject of much work. To begin with a “huff puff” stability circuit was used, then frequency synthesiser designs using divide down counters to a reference frequency, locked the oscillator to changes of frequency in steps of divide counter increments to the frequency steps of the reference frequency. This type of synthesiser is otherwise known as a “Phase Locked Loop” (PLL).

The latest design thought are that of Direct Digital Synthesise “DDS”, in which a reference stable crystal oscillators output is divided down to output with a waveform made up from phase angle voltages programmed into a memory feeding a D/A convertor. These phase voltages when ran at a frequency rate would produce the stable oscillator output carrier signal.

The DDS design has the advantage that even a 1Hz frequency alteration in the D/A output cycle waveform would have an instant alteration. A DDS circuit does not have a reference feedback loop, as so there is not a time constant to the immediate effect of the alteration in the divide down sequence, as would be the case with a divide by “N” conventional frequency synthesiser PLL.

The spectral purity of an oscillator is of some importance, as so is the frequency stability of the oscillator. Free running oscillators such as a crystal oscillator has a good spectral purity, as the only fault here would by the temperature stability and any other distortions that are created by the oscillator circuit, such as the 1/f noise from the transistor amplifier. A Phase locked loop uses a variac capacitance diode, which the modulated fine tuning voltage of the error signal created by the PLL process, tends to low frequency modulate the oscillator signal output. The 1/f noise also affects the low frequency noise next to the carrier signal from the oscillator the combination thus creates more noise within the carrier signal than a free running oscillator. A DDS synthesiser does not use a variac diode to tune the oscillator, but instead uses a wave form synthesise of phase signal voltages though a D/A convertor to recreate the carrier signal. The DDS is not an oscillator in the original sense of the meaning, due to the waveform synthesise technique, and thus the low noise of a tuning voltage is not present. Some DDS synthesiser chip designs produce an “I” and “Q” output carrier signals.

A technique that can be used to improve the spectral purity of a PLL tuned oscillator is to use a cleaner loop principle of tuning an oscillator. This is a variac tuned oscillator that is not phase locked looped to a reference frequency audio tone, but directly to an RF carrier signal as the reference signal to phase lock against. Normally a DBM mixer could be used here to phase a reference between the two carrier signals, one from the PLL oscillator, and the other from the cleaner oscillator as the feedback carrier signal. The error signal voltage would thus change to the phase slip between the two carrier signals, and then amplified as an error tuning voltage for the cleaner oscillator. The main source of the tuning voltage for the cleaner oscillator would come from a D/A convertor to pre tune the cleaner oscillator, the carrier phase slip error voltage then phase locking the cleaner oscillator. The output of such a circuit is that of similar to a crystal oscillator, but one that can be tuned to any set frequency.