Circuit Notebook 94 - Audio Test Burst Generator

Here is a circuit which will generate a sine wave signal in 8 steps of increasing amplitude. Each step lasts for 2 seconds with the sequence of steps repeating continuously. The circuit could be used for checking audio level meters, transmitters, limiters, compressors etc.

Circuit Description

The circuit is shown in Fig.1. IC1 (74HC 4060) is a 14 stage Ripple Counter with Oscillator. The 1kHz (approx) signal is generated in the oscillator section of IC1 where the frequency is determined by R15 and C1. The square-wave signal is passed through a low-pass 1kHz filter consisting of IC2a and IC2b (LM358). After this filtering, the output from IC2b is a respectable sine wave. This is fed to the resistor chain R1 to R14. Its purpose is to provide 8 levels of signal including zero and maximum level.

Returning to IC1, the outputs from the last three counter stages (Q11, Q12 & Q13) provide a three line address which is connected to the select inputs of IC3 (74HC 4051) an Analogue Multiplexer. The eight signal inputs of the multiplexer are connected to points on the chain of resistors to select specific signal levels. One input is connected to zero level and another to maximum level. A diagram indicating the relationship between the three line address and signal level is shown in Fig.2. The diagram shows the signal level increasing in equal steps, this would be the case if all the resistors in the chain were of equal value.

Table 1 Separater However, the values of the resistors in the chain are selected to provide specific amplitude levels of the 1kHz signal. In the circuit shown, the resistor values are chosen to provide 3dB increments of signal level. To avoid 'difficult' resistor values, each required value is made up of two resistors selected and connected in series. Using 1% resistors, the overall error in signal level is likely to be less than 0.2dB. The resistor values for 3dB steps are shown in Table 1. and for 4dB steps in Table 2.

Table 2 The sine wave amplitude at the top of the resistor chain is approximately 150mV and a final amplifier stage IC3 (ICL 7611) is provided to enable the maximum output signal to be increased to 0dBu (0.775v r.m.s., 2.2 V p-p). The ICL 7611 is a rail-to-rail op-amp which is needed to provide a 2.2V p-p output from a +5V supply. In use, the maximum output is set to 0dBu by means of the 'Set Level' control RV1. An alternative maximum level could be chosen, e.g. -10dBu, and the 3dB steps would then apply to the new level.

The oscillator frequency of nominally 1kHz (mine turned out to be 987 Hz) can be adjusted by replacing R15 with a 1M0 potentiometer and setting the frequency using a frequency counter. If the frequency is changed, by a significant amount, for example to 600Hz, then the component values in the low-pass filter will also need to changed.

An interesting dB-dBu-dBv Comparison Table is published in the Technical Data section of the Canford catalogue www.canford.co.uk and it is reproduced here in Table.3.

References

74HC 4051 and 74HC 4060 data. Mullard Technical Handbook No.4. Part 5.

Decibel Table RSGB Radio Data Reference Book G. R. Jessop, G6JP, Fifth Edition p.37

dB-dBu-dBV Comparison Table www.canford.co.uk/technical Source: BBC Designs Department Handbook No.3. 186 (1986)

Figures

1. Circuit Diagram Audio Test Burst Generator 2. Multiplexer address and signal levels

Tables

1. Resistor values for 3dB steps 2. Resistor values for 4dB steps 3. dB-dBu-dBV Comparison Table

Fig.1. IQ2 Microcontroller with Terminal Blocks added

Fig.2. IQ2 Layout and connections

Fig.3. Suggested ATV use for the IQ2 Microcontroller