Page 16 Hum Bug Reference Manual
Quest Scientific
C.
The output of the Hum Bug has too much
baseline noise
(random, wide-band
noise).
1.
Are you attempting to use the Hum Bug on very low level signals (less than 1 mV)?
If so, circuit noise from the Hum Bug may be detected on the output.
•
Compare the input and output on two channels of your oscilloscope. The
baseline noise
should look identical on both signals for levels down to 1 mV.
Below this level additional circuit induced noise may appear on the output. If
the baseline noise on your input signal is below 1 mV and the additional noise
introduced on the output is a problem in your application, it may be necessary
to increase the gain of your amplifier.
2.
The
baseline noise
on the output of the Hum Bug is greater than on the input, even
when its amplitude is greater than 1 mV.
•
Directly ground the input connector of the Hum Bug and observe both the
input and output at the same time. Is the
baseline noise
on the output more
than a few mV even when the input is grounded? If so, there may be a
problem with grounding. Refer to section D1.
3.
The baseline noise is only a few mV when the input is grounded but much larger than
expected when an input signal is connected to the Hum Bug.
•
Perhaps the grounding of the Hum Bug and associated equipment is the
problem. See section D1.
•
Are you connecting the Hum Bug to an unusual piece of equipment? If so,
check the output impedance of the device. Although the Hum Bug will adapt
to a wide range of input characteristics, a signal source with a very high
impedance could present a problem. This problem can be solved by passing
the signal through a suitable impedance-matching amplifier (buffer) before it
enters the Hum Bug.
D.
The Hum Bug adds 50/60 Hz noise to the output.
1.
Incorrect grounding is the most likely cause of this problem.
•
Confirm that the grounding prong on the power cable is connected to the
building ground at your preferred grounding point. Avoid using more than 1
service outlet for your setup. A good way to bring all of the instrument
grounds in your system to one point is to use a power bar. The ground on the
power bar becomes the final common ground connected to the wall outlet.
However, poor ground connections are a frequent problem with inexpensive
power distribution bars.
•
Make sure that the signal ground on your probe preamplifier is not attached to
the Hum Bug other than through the BNC cable connecting the two
instruments. This helps prevent ground loops.
