Given the elaborate schemes to reduce noise, it is surprising to what lengths you need to go when you actually want to produce some.
It is possible to use a specialist noise diode, or the reverse- biased base- emitter junction of a transistor, for the noise source: The silicon BE diode will break down at about 8 Volts, so the power supply must be comfortably higher than that. The emitter is fed through a resistor of a few tens of kiloOhms.
Considerable voltage gain will be needed to bring the noise signal up to usable levels, without, at the same time, amplifying mains hum to an equal extent. It may also be possible to low- pass filter an ultrasonic oscillator, which must not have a particularly stable amplitude or frequency (that shouldn't be too difficult.)
White noise carries the same power at all frequencies, while pink noise has constant power per frequency octave, sounds similar to falling rain, and is more appropriate for audio tests. Generating pink noise directly is probably very difficult, but white noise can be low- pass filtered to obtain the desired result: Designing the filter is not a run- of- the- mill task, as 3 dBs of voltage loss must be sustained per octave, rather than the more customary 6 dBs of final attenuation rate registered by a single stage filter.
Power supply connections both chips: Vcc (5 Volts) pins 14, ground pins 7. If the extra gate is not used, it had better be fed from either supply line. Internal connections made for the 4006: 13 to 4, 12 to 5, 10 to 6. (For a start, any of those can provide the tap.)
Although they have never been intended to produce random pulses, digital noise generators can easily be produced by using logic gates: Amplitude- related problems (hum from excessive amplification) do not exist in this case. A counter can become more of a random- number generator by feeding the clock input from a switch which has not had its contacts debounced.
A digital oscillator can be sampled at well below the Nyquist rate to yield an effectively random bit. Of course, both oscillators must not be particularly stable. (This option clearly bars using a frequency divider in order to derive the sampling frequency from the frequency to be sampled.) A D- type flip- flop is a simple sampling circuit, in this instance.
Random numbers are unpredictable by definition, but sometimes this not the exact requirement: Pseudo -random numbers appear, for all intents and purposes, random, but are actually predictable. Simply, the function which calculates pseudo- random numbers is, or soon becomes, too complicated to identify.
And so, to figure 25: The pseudo- random number generator (PRNG) is strongly reminiscent of CRC generators. Obviously, one input to the Exor gate is no longer a data input, but is driven from a tap along the shift- register. There is little (if anything) which is spectacular about the circuit: Choosing the tap position at random will not maximise the length of the pseudo- random sequence.
There are three spare gates on the HC86 chip; what shall be done with them? An oscillator is needed for the 4006 clock input. As the Exor chip does not feature Schmitt trigger action, it is much easier to form an oscillator by using two gates. This is not a quality oscillator, but will suffice for this purpose: Performance is eventually restricted by the input and output impedances of the gate, and other factors, but not at 2 kiloHertz.
Astute visitors should complain that if the shift register is completely filled with zeros, the circuit will fail. The spare gate can be introduced just before the shift register input to invert the level supplied, if a possible all- clear condition is detected. A multi- input Nor gate can decode as many of the shift register stages as possible, though not all of them directly. (The 4006 is an 18- stage shift register on a 14 DIL package. It is made of 4 smaller, 4 or 5- stage registers, so only 6 of the stage outputs are easily accessible.)
It is easy to listen to the shift register output on a high impedance earphone: Using an output series resistor of 470 Ohms should prevent unpleasant surprises. Sometimes, the circuit sounds like strong wind, although at other times there is a more regular, (and probably annoying) pattern.
| Find a fast and easy way to get rich, then tell half the population about it. |