Wideband Noise
Generator
Circuit : Andy Collinson
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Description
This very simple circuit will generate wideband noise across a very small area. 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.

Widerange Noise Generator
The circuit is an astable generator built around 2 gates of a CMOS 4011 quad 2 input NAND. It needs no explanation apart from the output is connected to a 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 1cm 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 of the modern LCD oscilloscopes it may have an FFT function. The other way as shown above is to use LTspice to produce the FFT for you.


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. Please keep this circuit away from modems, routers and other computer network equipment as it could cause interference.
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