3
Figure 1. A 10x probe connected for monitoring the output signal
Short-circuiting this amplifier will not break
down the amplifier, due to the extremely
fast current limiting circuit that has been
employed.
If output monitoring is required, it is
recommended
to
connect
a
10x
oscilloscope probe to the output. A special
BNC to probe tip connector is usually
supplied with the probe (Fig. 1). However,
the user can choose a different way of
connecting the oscilloscope, as long as
care is taken with the high output voltage.
Using a non-coaxial cable can cause
overshoot in the oscilloscope reading.
Noise
The noise of the amplifier (~840
µ
V
rms
in
DC – 10MHz) is lowest when a low-
impedance source is used, such as a pre-
amplifier output or a 50
W
function
generator output. An easy way to assess
the noise performance of the amplifier,
without picking up interference, is to
connect a 50
W
coaxial load resistor to the
input (Fig. 2). The noise voltage at the
output can be measured using a sensitive
amplifier.
The output noise will be lower when the
bandwidth of the amplifier is reduced,
which happens when a significant
capacitive load is connected to the output
(see Fig. 16 on page 12 for a detailed
measured curve).
Figure 2. If assessment of the amplifier
noise is necessary, connect e.g. a 50
W
coaxial load resistor to the input to provide
a low-impedance input connection
Offset adjustment
The WMA-100A model provides an offset
control knob to enable the amplifier to
generate offset voltages over the full
output range (Fig. 3). The offset control
can be switched to ‘Off’ with a small rocker
switch on the front panel to obtain the
lowest noise and highest DC stability of
the amplifier. Turning the offset control to
‘On’ enables the DC control knob. The DC
offset voltage reacts to adjustments of this
knob in a second. With the offset control
turned to ‘On’, the noise voltage level of
the amplifier becomes slightly higher:
~1150
µ
V
rms
instead of ~840
µ
V
rms
.
