White Noise Generator– Sound Check

by: Alan Lippett,Stafford

Source: EPE June 04

HAVING a need to align the centre frequency and bandwidth of an 8MHz filter in a radio set, it was decided to find an alternative to using an expensive Spectrum Analyser. This is not a common item of test gear for your average experimenter and is not something the writer owns. A cheaper alternative, although not as accurate, is to feed the output of a noise generator into the filter and adjust the filter for maximum output. A “white noise” generator is a useful piece of inexpensive test gear and it was decided to explore this further. Investigating the existing literature suggests that a Zener diode was a good noise source. Some tests with a few “spares box” Zeners were disappointing, providing quite a low output even after amplification. A very useful alternative to the Zener, using an “inverted” transistor, is shown in the White Noise Generator circuit diagram Fig.1. The noise source itself is the BC549 transistor followed by two 2N3904 transistors, used for amplification of the noise.

Fig.1. Circuit diagram for a White Noise Generator.

The interesting characteristic of this circuit is that TR1 is connected the wrong way

round. Usually, an npn transistor’s collector (c) is positive with respect to its emitter (e). It was found that by connecting it in this way results in a great rush of “white noise”. I do not recommend this treatment for an npn transistor under normal conditions, but as an experiment it worked very well. Potentiometer VR1 needs to be adjusted for maximum output, as measured at the collector of TR3, or simply take a length of wire from your receiver antenna socket and place it a few centimetres away from the generator’s noise output. Tune the receiver to any frequency up to 30MHz and the noise should be apparent. The output level can be adjusted by potentiometer VR2. This measured about 5V peak-to-peak output up to 30MHz, so I expect it could be used in the v.h.f. region. Also, the high output could prove useful in driving passive r.f. bridges for measurement of circuit impedance or antenna characteristics. Using a BC549 is not critical and other types could work as well or even better in this circuit. This is a matter for experiment. Potentiometer VR1 can be replaced with a fixed resistor once you are satisfied with the output.