Wayne Kerr AP10030, Руководство пользователя и обслуживания

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Wayne Kerr AP 1kW Power Supplies • Issue 2 41
50ohm
Termination
0.1uF 100V
50 ohm co-axial
cable
-
+
Load
Oscilloscope
Set for differential Input
Ch 1
Ch 2
Power Supply
Output Terminals
Note: Screens are connected
together at terminals.
Fig.8.4 Differential measuring mode connections
The oscilloscope, of 20MHz bandwidth, should be set to ‘ADD’ the two channels with
one channel inverted. Also ensure that both channels are set to the same attenuator levels.
The two leads must be kept close together and the wiring to the terminals around the
blocking capacitors should be as compact as possible to reduce pick up problems. The
common-mode signal rejection capability of the oscilloscope must be checked before
making a measurement. This is done by connecting the leads to the same power supply
terminal whilst the unit is delivering the desired output and checking for any reading on
the oscilloscope. Noise generated by the power supply will tend to be short spikes or
bursts of high frequency noise with a period of approximately 40µs between them.
Ideally the oscilloscope should display no reading at this point. Anything that does
appear is due to insufficient common-mode rejection or noise pick up in the leads. It may
be possible to reduce it by moving he leads or winding them around a ferrite toroid (a
couple of turns) to form a common-mode choke. Otherwise this residual reading will
have to be taken into account when measuring the actual output.
8.3.3 R.M.S. Noise On The Output
This gives a figure indicating the total r.m.s. value of noise on the output, i.e. mains hum
and noise spikes without he ambiguity of an oscilloscope reading. A suitable measuring
instrument is the Hewlett Packard True RMS Millivoltmeter type 3400A which can be
connected directly to the output terminals.