> Where can I find a " Dummies" explanation of the " Dynamic
> ranges" and other associated terms regarding receiver specs.
Others have suggested looking at the ARRL Handbook, and
the ARRL Lab test procedures.
The tests and specs can be used to compare the performance
of the rcvrs in crowded band conditions, particularly when
there are very strong signals present. Very strong signals,
S9 plus a bunch; bunch defined variously: S9 +30dB, S9+40dB
over nearby weak signals, say down around S2, or -115 dBm,
or at 0.4 micro volt for example.
Suppose the strong signal were 40 dB over S9 (or at -33dBm),
the overall dynamic range in power between these two signals is
just the difference in power levels, in this case 82 dB delta.
Consider the Ten Tec Omni VI+. The ARRL labs 28 page extended
report found that on 14 MHz, the minimum discernable signal
of the VI+ was about -133 dBm [0.05 microvolt] (CW using
500 Hz IF bandwidth).Minimum discernable = just able to hear
the tone above the noise. So a S9+40 dB signal at -33 dBm is
100 dB stronger than the MDS of the tested Omni.
What does that mean. Probably if the two signals were within
a few kHz of one another, you would NOT be able to hear
(discern) the weaker signal because the base band of noise
around that weak signal would be raised because of the
presence of the nearby strong signal. Note that the ARRL labs
only since the middle to 2001 has been measuring this "blocking
dynamic range" with the signals closer that 20 kHz. At 20 kHz
spacing the Omni VI+ has a BDR, per ARRL of 123 dB. But
our interest in crowded bands is what happens when that
strong signal is with in 5 kHz or less of the sought after
MDS signal. In the most recent ARRL lab test reports,
5 kHz spacing data blocking dynamic range is being given.
The other strong signal problem is when two or more very strong
signals are near your operating frequency. Even if they are not
within your selected operating IF bandwidth (your filter selection),
these strong signals can still produce signals well above the
MDS level right there inside your IF passband!! These phony
(spurious) signals are called "intermediation distortion" or IMD's.
They are generated in the "not quite perfectly linear" stages of
your rcvr. Every semiconductor junction can introduce some level
of nonlinear transfer function behavior. Each one will reduce
the "spurious signal free dynamic range" of your rig.
These spurs are, in fact, more troublesome than the noise increase
from a single near by strong signal, for obvious reasons, hi.
There are two specifications which tell us what the IMD performance
of a rcvr will be: the Intercept Points and the so called 3rd order
IMD spec, or IMD3 spec. The higher the Intercept Points, in dBm,
the "cleaner" the band will be; clean from spurious signal generation.
3rd order intercept points of various rigs are around the order
+15 to +20 dBm or so, top of the line gear, somewhat higher still.
The IMD3 numbers can be anywhere from 70 to nearly 100 dB.
Ten Tec is publishing a number of 101 dB for the Orion IMD3,
and +24 or +25 dBm typ for the 3rd order intercept point.
Hope this too long explanation might help, hi. Also, others
may even find that my explanation needs "clarification", hi.
73, Jim KH7M
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