Noise Suppressor II
– "Here's how it works"
SLX
, Inc.
512 Ashford Road
Silver Spring, MD 20910
Phone: 301-589-2265
Fax: 301-587-3228
Web: www.SLX-inc.com
The main point that is stressed continually in the documentation is to use as much PreAmp gain as
possible (up to 30 dB in 10 dB steps) for BOTH the "Max" (i.e., enhancing weak signals) and "Null" (i.e.,
suppressing noise signals) as long as you don't observe clipping (same as signal "flat topping") on your UT
instrument's display. This will give you the very best signal/noise response.
The BandWidth (BW) switch (for each Filter) has 3 positions -- Low, Mid, Wide. You will normally find
that Low, or sometimes Mid, to be best for "Gain"ing weak signals and Wide to be best for "Null"ing
noise signals. Play around and see for yourself.
Keep in mind that "noise" is defined as any undesired signal. It can truly be a noise spike or grain back-
scatter, but it can also be any legitimate signal -- such as a geometrical reflector -- that is undesired.
There are two ways to improve the S/N ratio -- either by boosting a weak signal via the Max setting or
by suppressing an undesirable signal via the Null setting. Here's how each approach works:
(1) Set one of the Filters for "Max" (and leave the other Filter Off for the time being) and dial the
FREQuency switch until you observe the weak signal of interest to increase in amplitude. Dither the
FREQ switch until you achieve the max response (remember that the PreAmp will have been set as high
as possible and the Filter's "Max" setting provides an additional 15 dB of Gain just in that band). Try
both the Low and Mid BW settings to find the optimum response. Then turn on the other Filter ("Max"
setting) and set its three switches to the identical settings thereby adding another 15 dB in just that
band (and nowhere else). You now have created 60 dB of gain (PreAmp = 30dB; Filter A = 15dB; Filter B =
15dB) in
one specific frequency band
prior to your UT unit seeing the signal. Cut down the gain in your UT
instrument until the desired signal comes down out of saturation and you should see a dramatic
improvement in the S/N ratio.
(2) Set one of the Filters for "Null" (and leave the other Filter Off for the time being) and dial the
FREQuency switch until you observe the "grass" surrounding the desired weak signal decrease in
amplitude. Dither the FREQ switch until you achieve the greatest decrease (again using the PreAmp). You
should find the BW setting of Wide gives the best result. You have now created a notch filter with a 20
dB attenuation in just that specific frequency band. Now turn on the other Filter ("Null" setting) and set
its three switches to the identical settings, thereby producing another 20 dB drop in just that band (and
nowhere else). You now have created a 30 dB signal amplitude increase (via the PreAmp) and a 40 dB
attenuation in just
one specific band
. Therefore, your desired signal (and everything in the sweep
including the grass) enjoys a 30 dB enhancement while the undesired signal in the band suffers a 40dB
attenuation. The net gain is now 30 dB for everything outside the specific notch band and a -10 dB "gain"
(30 dB gain - 40 dB attenuation) for everything inside the specific band. Again, you should see a dramatic
improvement in the S/N ratio.
One final tip involves the use of both Filters in the "doubling up" mode (as described above) when
suppressing noise in the "Null" mode. Try setting the FREQ switch for the 2nd filter about twice as high
as the optimum setting found in the first filter. Sometimes you can pick up a harmonic of the noise signal
and cause even more of a reduction.
Copyright © 2006 SLX, Inc. All Rights
