YT1NT wrote:
>Adding attenuation has no influence on dynamic range.
Theoretically true Sinisa, but perhaps not
if the range is improperly positioned. Let's look
at actual measurements made by W8JI on arguably the
worst band for dynamic range...160 meters. There
you have the strongest local signals and weakest
DX signals, which would logically be the band where
the MicroTune (uT) preselector might be most useful.
W8JI data from http://www.w8ji.com/receiving.htm :
My local wintertime 350Hz BW noise (after amplifier) compared to a sample
of signals on one night was:
Noise -127dBm
9H1BM -122dBm
OM0WR -95dBm
DF2PY -88dBm
WA8OLN -78dBm
W3GH -60dBm
W4ZV -32dBm
In all of the following numbers, I am using the
3.5 MHz data for both the 9000 and Orion since it closest
to 1.8 MHz. Required dynamic range is 95 dB (-127 dBm to
-32 dBm) if I want to copy noise floor (MDS) signals. For
the actual signals above I only need 90 dB to be able to copy
9H1BM (note "Max" indicates maximum signal the receiver
can tolerate without generating IMD products at the MDS level).
Radio MDS IMDDR3 Max
9000/uT -117dBm 94dB -23dBm
I've got great dynamic range, but where did that 9H1BM
signal go? Into the noise because my MDS is limited. OK,
now I'll add a preamp (pa) to compensate:
Radio MDS IMDDR3 Max
9000/uT/pa -124dBm 88dB -36dBm
Oops, adding that preamp brought 9H1BM back above the
noise floor, but now I have potential IMD products at -121 dBm
from strong stations like that pesky W4ZV. Note that adding the
preamp also degraded my IMD performance (to be expected
since amplifiers nearly always add nonlinearities).
So how does Orion do under the same conditions?
Radio MDS IMDDR3 Max
Orion(nopa) -124dBm 92dB -32dBm
I can hear 9H1BM and also tolerate strong signals at the
-32dBm level without generating IMD products at my MDS.
So does MicroTune really give IMD results that
are "the best we've ever measured"? I don't think so, at
least in the above real world example. Stay tuned for
1 to 2 kHz spacing results which should be interesting.
I predict you'll see a lot more of those * (noise limited)
measurements like we saw for the 5 kHz BDR results.
73, Bill W4ZV
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