Audio and Radio Frequency Signal Generators

These instruments are indispensable to the keen amateur but require considerable knowledge of electronic circuitry to design. Unless one is especially interested in the actual design of signal generators it is probably cheaper in the long run to build one from a kit of parts.

The circuit is that of the Daystrom (Heathkit) audio generator which has a frequency range of 10 to 100,000 c/s with sine or square-wave output. Wave-form distortion is less than 0.1 per cent between 20 and 20,000 c/s and the output is controlled by a 10 dB step-attenuator, actual output voltage being to within +/-1 dB. over the entire frequency range.

Circuit Description.

The circuit of this instrument may be divided in four parts, the power-supply, the oscillator, the attenuator and the meter circuit. The power supply is conventional consisting of a mains transformer, full-wave rectifier and ripple filter. The oscillator employs an EF94 pentode amplifier and an EL821 triode connected as a cathode follower. Regenerative feedback from the EL821 to the EF94 cathode is applied through a special tungsten-filament barretter lamp.

Degenerative feedback is applied from the EL821 through a bridged-T network to the grid of the EF94. The resultant oscillation occurs at a frequency where degeneration is at minimum and phase shift is zero. The amplitude of oscillation is maintained at a nearly constant value by the tungsten lamp. The regenerative feedback is applied through a voltage divider comprising the lamp and the oscillator control. An increase in output signal increases the lamp current, the lamp temperature and its resistance, which reduces the amount of feedback applied to the EF94 cathode and stabilises the output. A balanced condition is thus obtained.

The bridged-T network consists basically of two resistors and two capacitors as shown inset in the figure. From the relationship shown it is evident that a decrease in capacities by a factor of 10 will increase the frequency by a factor of 10. As the values of C1 and C2 were chosen with a 10-1 ratio five capacitors can do more economically, what would otherwise need four pairs (eight capacitors) in achieving four decade ranges. The frequency is controlled by the multiplier switch, the 0-10 and 0-100 cycles selector switches. Precision resistors with a 1 per cent tolerance are used in conjunction with the 0-100 c/s switch and 2 per cent capacitors on the multiplier switch. The attenuator reduces the output voltage from the EL821 cathode follower, through a continuously variable 5 kilohm output control and then through a step attenuator. The attenuator system is designed for 600-ohm output on the ranges from 0.003 to I volt and for high impedance output at the 3- and 10-volt positions. The 600-ohms position may be terminated by an internal load for high impedance work, or this load may be disconnected when an external 600-ohm load is used. In the 3- and 10-volt positions the internal load is automatically disconnected. The attenuator operates in steps of 10 dB.

The metering circuit measures the voltage taken from the output control, a portion of this voltage being determined by the meter control and rectified by a half-bridge using two crystal diodes. Nonlinearity of the diodes at low signal-level is compensated by a third diode across the meter. The meter has three calibrated scales: 0-10 volts, 0-3 volts, and 10 to + 2 dB. When the instrument is operated with the proper termination, the meter and attenuator will indicate the output level at the terminals.

(The circuit description and circuit are given by kind permission of Daystrom Limited.)