Fluke PM6690 - Coupling; Filter

Coupling
Switch between AC coupling and DC cou
-
pling by toggling the softkey AC/DC.
Use the AC coupling feature to eliminate un
-
wanted DC signal components. Always use
AC coupling when the AC signal is superim
-
posed on a DC voltage that is higher than the
trigger level setting range. However, we rec-
ommend AC coupling in many other measure-
ment situations as well.
When you measure symmetrical signals, such
as sine and square/triangle waves, AC cou-
pling filters out all DC components. This
means that a 0 V trigger level is always cen-
tered around the middle of the signal where
triggering is most stable.
Signals with changing duty cycle or with a
very low or high duty cycle do require DC
coupling. Fig. 3-4shows how pulses can be
missed, while Fig. 3-5shows that triggering
does not occur at all because the signal ampli
-
tude and the hysteresis band are not centered.
NOTE: For explanation of the hysteresis band,
see page 4-3.
Filter
If you cannot obtain a stable reading, the sig
-
nal-to-noise ratio (often designated S/N or
SNR) might be too low, probably less than 6
to 10 dB. Then you should use a filter. Certain
conditions call for special solutions like
highpass, bandpass or notch filters, but usu-
ally the unwanted noise signals have higher
frequency than the signal you are interested
in. In that case you can utilize the built-in
lowpass filters. There are both analog and dig-
ital filters, and they can also work together.
n
Analog Lowpass Filter
The counter has analog LP filters of RC type,
one in each of the channels A and B, with a
cutoff frequency of approximately 100 kHz,
and a signal rejection of 20 dB at 1 MHz.
Accurate frequency measurements of noisy
LF signals (up to 200 kHz) can be made when
the noise components have significantly
Input Amplifier 3-3
Input Signal Conditioning
0V
5V
DC Coupling
AC Coupling
Fig. 3-3 AC coupling a symmetrical sig
-
nal.
Fig. 3-4 Missing trigger events due to AC
coupling of signal with varying
duty cycle.
Fig. 3-5 No triggering due to AC coupling
of signal with low duty cycle.
Fig. 3-6 The menu choices after selecting
FILTER.