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.
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