**
**## Power Supply Baseband Noise Measurements

**October 22, 2008: HP 3048A measurements**

Some three-terminal and discrete series-pass regulators:

HP linear supplies:

Non-HP linear supplies (plus a Norand 15-volt switching supply):

Note inability of HP 3561A FFT analyzer to resolve spurs in the 1-10 kHz decade; this may be fixable with
segment-table optimizations. There's also an unexplained 10-dB discontinuity in the HP 6263B's trace at 100 kHz,
which was repeatable with that supply but not with any of the others.

While the three-terminal and Darlington regulators were tested with a resistive load, the bench supplies were tested driving a 12-volt relay winding, so some of the low-frequency spurious content may be due to stray AC field response on the part of the relay coil. Coupling to the HP 11848A's 50-ohm
baseband noise input was achieved via parallel 0.15 uF tantalum/22 uF aluminum
electrolytic capacitors, followed by a 20 dB attenuator (HP 355D). The resulting 3-dB corner frequency is 120 Hz.

Graph scales are in dB below 1 volt RMS (Sv(f) [dBv/Hz] vs. f[Hz]), normalized to 1 Hz bandwidth. For example, -140 dBv/Hz equals
10^(-140/20) volts, or 100 nv/Hz. To generalize this figure to equivalent input noise, multiply by the square root of the system bandwidth. 100 nv/Hz of Gaussian noise in a 1-kHz bandwidth would correspond to approximately 3.2 uv on a true-RMS indicator, neglecting any noise-equivalent scale factors that might apply to the original bandwidth calculation.

A similar relationship can be expressed in terms of power: since 0 dBm = 224 mv RMS in a 50-ohm system, 1 v RMS is +13 dBm, and 100 nv/Hz would equal -127 dBm/Hz. (Note that 'root-Hz' is now just 'Hz', since power is proportional to voltage squared and sqrt(1 Hz) = 1 Hz.)

The green trace below represents the measurement floor with the DC block/relay coil disconnected from any supplies. Again, 20 dB of range is lost due to the attenuator.

**May 29, 2014**

Steve Hageman makes some similar measurements in his EDN column "The Practicing Instrumentation Engineer": Simple circuits reduce regulator noise floor (.pdf here).
At 1.4 uV/root-Hz or -117 dBv/Hz, Steve's measurement of the LM317 *sans* Radj bypass is a good match for the -137 dBv/Hz figure seen in the green trace in the first plot, allowing for the 20 dB attenuator. I really need to revisit these measurements with a better test setup...

A bit more on the subject from Jason Breitbarth of Holzworth Instrumentation on Marki Microwave's blog.

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