Circuit : Andy Collinson
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This very simple circuit will generate wide-band noise up to 10MHz. The noise field is about 1 inch circling the inductor. When placed very close to a shielded cable or a radio receiver it can be used to test the screening and shielding ability. Can also be used as a REIN generator for testing cable immunity. Other uses include direct injection to a circuit for filter applications.
Wide Range Noise GeneratorThe circuit is an astable generator built around 2 gates of a CMOS 4011 quad 2 input NAND. The circuit generates a square wave rich in harmonics up to 10MHz. The output is connected to a small radial inductor of 560uH. A 500uH inductor will also work if said value is not available.
The CMOS 4000 range have very fast rise and fall times, are very low power used ubiquitously throughout the electronics world. The field produced by the inductor generates interference which is transmitted at very low power about 1 inch (2.5 cm) away from the inductor. The output signal is shown below:
The whole circuit can be built on a small breadboard with the inductor used to inject the signal (magnetic coupling) into the device under test. To minimize frequency drift the circuit can by powered from a 78L05 regulator, or simply powered from a 9V battery. Nominal frequency will drift due to falling battery voltage.
One way to see the nominal frequency is with a frequency meter. An alternate method is to perform a FFT (fast fourier transform) in the frequency domain. If you have one a modern LCD oscilloscopes it may have an FFT function. The other way (see diagram above) is to run a fourier transform using LTspice.
With a 5V supply the nominal frequency is around 580kHz. Multiples of 580kHz are produced up to around 10MHz and with lower amplitudes to around 100MHz. Although unmodulated, any sound heard in a receiver will be a hiss. Warning. Please keep this circuit away from modems, routers and other computer network equipment. If you are using ADSL, DSL or ADSL2 or FTTC technologies, these systems all use analog radio frequencies. The data is demodulated, but this circuit will interfere if in close proximity, close than 2.5 cm. As mose modem leads are not screened or twisted pair, the interference will interrupt your broadband. If you have a high quality shielded lead then there should be no interference. If you using mobile home broadband 46 or 5G or fibre then there will be no interference.