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In pursuit of quiet: Noise in linear regulators

Posted: 21 Jan 2016 ?? ?Print Version ?Bookmark and Share

Keywords:Voltage controlled oscillators? VCOs? Phase-locked loops? PLLs? signal-to-noise ratio?

To verify input-referred noise, short the input of the amplifier to ground and measure the noise at the output. Measurements are taken directly with an RMS voltmeter or oscilloscope; noise spectral density is viewed with a spectrum analyser. Measured noise spectral density of the wideband output (shown in figure 4) has 1/f noise with a corner at 200Hz, and white noise characteristics of 5?V/Hz from 200Hz to 1MHz. Dividing this down by the gain of 80dB indicates input-referred noise is 500pV/Hz, slightly above the target. Even with the 1/f component, this calculates to 0.15?VRMS over the 10Hz to 100kHz bandwidth, low enough to allow measurement of 1?VRMS over the same bandwidth with confidence. The measurements correlate well with the peak-to-peak noise measured on an oscilloscope as shown in figure 5.

Figure 4: Noise Spectral Density of Amplifier with Input Shorted Shows 1/f Component. Dividing by 80dB Circuit Gain Yields Input Referred Noise.

Figure 5: Peak-to-Peak Amplifier Noise with Input Shorted (100kHz Range) Correlates to Noise Spectral Density.

Measurements are still not a simple task
Several subtle effects cropped up during the design and testing of this circuit that showcased the difficulty in measuring ultra low noise levels. Shorting the input to ground and connecting the output to an oscilloscope reveals much that cannot be seen with an RMS voltmeter or spectrum analyser. Large signal excursions when using ceramic capacitors for the input filter and second stage filtering showcased their piezoelectric nature from simple finger taps on the bench. This made the case for the switch to solid tantalum capacitors.

What was also apparent was that the noise levels for measurement were so small that extraordinary measures were required to ensure solid results. Placing the amplifier board in front of an older oscilloscope showed a regular 20kHz signal (likely a switching regulator inside the oscilloscope) that was larger in amplitude than the input referred noise. Placing it close to a benchtop multi-meter resulted in a large 60Hz signal. Figure 6 shows just how sensitive the amplifier is when placed just a few inches in front of the powered oscilloscope.

Figure 6: Signal Shown with Input Shorted and Close to Oscilloscope Highlights Sensitivity to Magnetic Fields.

In both cases, moving the board away from the equipment or changing the orientation of the board would change the amplitude of the signal and turning the equipment off would remove the signal. A few loops of wire around the end of a pencil were connected to a function generator to operate as a small antenna at various frequencies. Not surprisingly, some areas of the board showed circuit loops that were magnetically coupling with the inductors and transformers inside the bench equipment. Some layout improvements were made to help minimise loops, but it was immediately obvious that external shielding would be required.

Shielded box construction
Figures 7 and 8 show the internal construction of the shielded box used to house the noise amplifier board. The amplifier board is housed together with six D cell alkaline batteries inside a box constructed from 0.050" thick Mu Metal to give good shielding against low frequency magnetic fields. This is then placed 1/2" inside a box made from 2oz. copper clad, chosen to give good shielding against higher frequencies. These are lastly placed 1/2" inside a steel can (a repurposed biscuit tin3) that gives some initial shielding from magnetic fields. The 1/2" air gaps between boxes help with attenuation of fields. For a discussion on materials useful for low frequency magnetic field shielding, please see Appendix A, Materials for Magnetic Shielding.

Figure 7: Shielded Box Construction Uses Mu Metal Inside Copper Inside Steel Tin to Attenuate Fields.

Figure 8: Details of Shielded Box Construction. Note Only Input Coax Shield is Connected to Metal Can to Prevent Ground Loops.

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