Decibel-o-meter and white noise generator

Posted on Jun 3, 2012

Noise level measured into 75ohm 3.1kHz BW using Siemens D2006 level meter: -80dBU (77.5mV) from zero to 1MHz and drops 3dB on 17MHz. Decrease the first coupling capacitor (68nF) to 10nF to increase the lower limit to 50kHz. The amplifier is copied from one of LA7MI's constructions, combined with a general audio amplifier. A noise level was needed to test vintage receivers and requirement was not beyond 10MHz, but it seems that some further bandlimiting circuit could well be used, particularly since I am not interested in noise spectre below 200kHz. Picture: Zenerdiode connected (E-B junction) MPS918 at left side and the other devices follow as on the circuit diagram. The component values were first calculated, but I used the components which was closest, but it seems to cause no problem because of the heavy DC feedback. Reducing first emitter resistor (from 47 to 22W) to the half value will increases the output by 3dB on the lowest frequencies.

Decibel-o-meter and white noise generator
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Decibel-o-meter and white noise generator - image 1
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Decibel-o-meter and white noise generator - image 2
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The 2nd version built on tinned iron plate, output is measured using Siemens D2006 selective level meter to -66dBU 75W (3.1kHz BW), and 30mV into 50W using LA7MI broadband voltmeter (described further down this page). Suppose this is quite high level and I am not sure if it is worth increasing it further since the main task is to check surplus receivers. Level on 10MHz has dropped by 2dB. The noise generators were housed in NERA surplus boxes, and the 2nd room is not used. Have kept the conhex connectors since they are free and available in very large quantities as well as different cablelengths with connectors. It is some some minor changes of components for the two different versions. It increases the gain by 3dB, without any noticable change of frequency response, but the only measurable difference is that the version has 3dB, so it seems no problem to copy the circuits, the only importance is to choose a device with 6v e-b breakdown voltage. The box measures 100x80x30mm. White Noise generator with MSA0304 booster amplifier. The picture shows the 1st version noise generator which has -68dBU 75 ohm o/p (3.1kHz BW), while the 2nd has -66dBU

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