[time-nuts] Measuring power supply noise

[Robert Atkinson]

There was a thread about power supply noise a while ago and the difficulty of measurement. I've just got hold of a piece of test equipment that I've been after for a while that is good for comparative checks. It's an HP 3552A Transmission Test Set. Designed for testing telephone lines, its basically an AF signal generator, level meter, frequency meter, monitor amplifier and line iterface in a portable box. Ignoring the generator, the line interface will allow direct connection to supplies of up to 75V and the level meter measures noise down to -90dBm (600ohm) with low pass filters available. The frequency meter is PLL based and locks on to the predominent frequency. Its specified for 40Hz to 60kHz but responds down to 10Hz. The line interface also has a switchable constant current "hold" circuit that lets you add a 28mA load. The speaker allows you to listen to the noise. A quick check of a switching supply gave -42dBm at 450Hz off load and -45dBm
 at 1.5KHz at 28mA. A nice general purpose audio tester that is not well known. Cost was less than $40.
 
Robert G8RPI.