The basic oscillator circuit for a VCO uses a current to charge a capacitor. When the voltage across the capacitor reaches a preset limit, then it is discharged, and the charging process can start again. This 'relaxation' oscillator produces a crude sawtooth output, which can then be shaped to produce other waveforms (Figure 3.7.2).
By varying the current that is used to charge the capacitor, the time it takes to reach the limit then changes, and so the frequency of the oscillator changes. By using a voltage to control the current, perhaps with a transistor, the oscillator then becomes voltage controlled. This type of circuit forms the basis of many VCOs.
FIGURE 3.7.2 A relaxation oscillator circuit consists of a capacitor which is charged by a current, i, and discharged by a switch when the voltage across the capacitor reaches the point at which the comparator triggers. Two output waveforms are available: the sawtooth voltage from the capacitor and the reset pulses from the comparator.
Simple low-pass filters use RC networks to attenuate high frequencies. By making the resistor variable, it is possible to alter the cut-off frequency. This RC network forms a single-pole filter, which has poor performance in terms of cut-off slope. Two- or four-pole filters improve the performance, but require more resistors and capacitors. This requires separate buffer stages and multiple variable resistors.
One way to produce several variable resistors uses the variation in impedance of a transistor or diode as the current through it is varied. By arranging a cascade of RC networks, where the transistors or diodes have the 'voltage-controlled' current flowing through them, it is possible to make a low-pass filter whose cut-off frequency is controlled by the current that flows through the chain of transistors. This is the principle behind the 'ladder' filters used in Moog synthesizers.
The basic Moog-type filter uses two sets of transistors or diodes in a 'ladder' arrangement (Figure 3.7.3). The important parts of the filter are the base–emitter junction resistance and the capacitors that connect the two sides of the ladder. Current flows down the ladder, and the input signal is injected into one side of the ladder. Since the resistance of the junctions is determined by the current which is flowing, the RC network thus formed changes its cut-off frequency as the current changes. This gives the voltage (actually current) control over the filter.
FIGURE 3.7.3 A typical 'ladder' filter. The current flows through the CV transistor, TR-1, and then through the two chains of diodes. The diodes and the connecting capacitors form RC networks which produce the filtering effect, with the diodes acting as variable resistors. The op-amp amplifies the difference between the two chains of diodes and feeds back this signal, thus producing a resonance or 'Q' control.
Another type of filter which is found in analogue synthesizers is the 'state variable' filter. This configuration had been used in analogue computers since valve days to solve differential equations. Once op-amps were developed, making a state variable filter was considerably easier, and by using field effect transistors (FETs) or transconductance amplifiers the cut-off frequency of the filter could easily be changed by a CV.
A typical state variable filter is made in the form of a loop of three op-amps (Figure 3.7.4). It is a constant-Q filter. Three outputs are available: low-pass, high-pass and band-pass (a band-reject can be produced by adding a fourth op-amp). Other types of multiple op-amp filters can be made: the bi-quad is one example whose circuit looks similar to a state variable, but the minor changes make it a constant-bandwidth filter and it only has low-pass and bandpass outputs.
FIGURE 3.7.4 A typical 'two-pole' state variable filter. This produces three simultaneous outputs: highpass, band-pass and low-pass.