[TenTec] Effect of roofing filter selection on Orion II NB

[Bill Tippett]

NQ5T:

 >Perhaps "on the order of something greater than zero" would have been a more
correct statement.  In any case, there is additional improvement.  It is
clearly audible, and there is a clear difference between the signal+noise
output to noise-only output with NR on versus NR off.

         You can be misled when using your ears.  NR
reduces the gain which reduces both the signal and
noise.  However when your brain's DSP hears the
reduced signal against reduced background noise,
it fools you into thinking S/N is improved.  This
is why you must use measurements and not depend on
your ears.  Remember that your ear/brain gives you
an effective DSP filter of about 40 Hz, so be very
careful when making comparisons by ear only.

 >So I remeasured it, just a minute ago.  Very simple.  Used a TS-585D/U Audio
output meter on the radio's output (one channel to monitor, one channel for
the meter.  (1) With NR off set the baseline noise level to read 0dB on the
meter.  (2) Tuned to signal, read output level in dB.  (3) Turn on NR
(signal level will drop) and adjust audio output to restore the signal level
reading in (2).  (4) Read background noise.

         There are some key points you omitted.
First AGC must be OFF, otherwise Orion's AGC will
readjust the audio output.  Second, is the TS-585
a True-RMS responding meter?  An average-responding
meter will not give correct measurements of noise.
ARRL uses an HP 339A Distortion Analyzer which
is True-RMS responding.  ARRL's test procedure
is on pages 35-36 below and measures the signal
level (MDS) which yields a 3 dB S+N to N ratio
at 500 Hz bandwidths, but you can use the same
approach to compare measured S+N to N for 100 Hz
BW's with NR turned On and Off.

http://www2.arrl.org/members-only/prodrev/testproc.pdf

                                 73,  Bill  W4ZV