Topband: RE: Minimum discernible signal ?

[Tracy, Michael, KC1SX]

Tony,
    Allow me to introduce myself as the ARRL's Test Engineer.  First, I have
to note that the comment you refer to is one I hadn't heard before, and I
suspect the context of the discussion that accompanied it may have some
bearing on the exact meaning.

On the bench, mds is mds.  The output of the receiver consists of the
receiver's internal noise, plus the constant unmodulated tone from the
signal generator.  This is the "stick" by which all radios are measured for
QST's Product Review data tables.

Unfortunately, it is impossible to duplicate "real world conditions" on the
test bench - an approximation is all that can be achieved.  To further
complicate matters, "real world conditions" are different for every ham, so
any attempt to duplicate such conditions would only apply to a fraction of
hams.

In the ham shack, the radio is connected to an antenna and what you hear
from the speaker is a combination of receiver noise and local noise
(atmospheric and man-made).  In rare circumstances, the receiver noise might
dominate (my grandparents' 100-acre QTH in ME comes to mind), but in the
vast majority of cases, local noise predominates (even on a "quiet band in
winter").  While atmospheric noise is very random (similar to the receiver
internal noise), man-made noise sources are often pulse-type or otherwise
very periodic in their characteristics.

Where DSP really shines is in suppression of noise with a periodic nature.
The same general mathematical trends repeated appear in the waveform
analysis and this is therefore distinguishable from intelligence-contained
modulated signals.  Therefore, DSP can improve the noise floor observed in a
real-world condition.

However, this is NOT the _receiver's_ noise floor.

Also, there is another point to be made here.  The noise from the receiver
that is heard by the ear is proportional to the bandwidth.  Dropping the
bandwidth from 500 Hz to 50 Hz will result in a 10-dB reduction in noise
(assuming "brickwall" bandwidths here...) if the signal occupies no
bandwidth in itself (i.e., the unmodulated signal generator).  If the signal
occupies bandwidth (as all modulated signals do), then there is a further
reduction in the noise heard when the signal is received.  Yes, 6-dB is
indeed a notable improvement, if your signal to noise ratio is only 3-dB to
begin with.

So you see narrow bandwidths can make a dramatic difference in how weak a
signal can be copied by the ear.  Most rigs do not have that capability and
on some of those that do, the resulting passband shift makes you retune when
you narrow up the filter.

Dynamic range is a measure of what happens to your rig's ability to receive
weak signals in the presence of strong nearby (assumed to be undesired)
signals.  While this is a critical issue to multi-multi contest operating,
it can be important even to casual weak-signal DXers if they have a strong
station nearby or have an antenna system that provides enough gain to create
problems with strong distant signals.

Fortunately, the rig's attenuator can be of help here.  If your rig is
capable of receiving a -140 dBm signal on the bench and your local noise
level is -130 dBm, adding 10 dB of attenuation will not make a difference in
the weakest signal you can copy but will reduce problems from strong
signals.

Actually, I should note that -140 is typical only of a rig with a preamp
turned on and it is preferable to turn the preamp off before adding
attenuation because the preamp adds some noise of its own.  When operating
W1AW for Field Day when we went Class A a few years ago, I found 20 dB of
attenuation worked best for the conditions I was experiencing on 80-meters.

Of course, I should also note that it is better to have a rig with too much
gain where you can reduce it rather than to have a rig with not enough.

73, Michael Tracy, KC1SX, ARRL Lab


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