W4BQF wrote:
>The 600Hz filter will give you advertise IMD performance at
it's stated bandwidth, but not down to 100hz..
There is actually no advertised IMD performance...just
some published measurements made by Sherwood and unpublished
by Sherwood and others.
Here is why you really want the 600 Hz engaged instead
of the 500 or 250, even at bandwidths down to 100 Hz. Signals
spaced to the side of your center frequency can create IM birdies.
Assume you are tuned to 7.040 and have 2 strong (S9+30) signals at
f1 7.041 and f2 7.042. You are trying to hear a weak station on 7.040
using your DSP BW of 100 Hz, but an IM product is created by 2*f1 - f2
= 14.082-7.042 = 7.040...right where you are attempting to copy
a noise floor signal at Orion's MDS of -125 dBm (preamp off). With
the 600 Hz filter, Orion's dynamic range is ~95 dBm for 1 kHz
spacings (~10 dB better than the 1000 Hz filter according to Sherwood).
This means the S9+30 signals (-43 dBm) create an IM birdie at -43 dBm
-95 dB = -138 dBm which is well below your -125 dBm noise floor. There
are few published measurements of the 500 Hz filter, but YT1NT measured
his at about 25 dB worse IMD DR than his 1000 Hz filter. The 1000 Hz
filter at 1 kHz has IMD performance of ~85 dB, so it's conceivable
that the 500 Hz filter could be as bad as 60 dB. Now let's look at the
IMD effects again assuming 60 dB DR. -43 dBm (interfering signals at
S9+30) - 60 dB DR = -43 dBm - 60 dB = -103 dBm, resulting in an IM
birdie +22 dB above the noise floor! You will definitely hear that,
even with a 100 Hz DSP since it is exactly within your passband. By
the way, I would estimate most current receivers have somewhere in the
range of 60-70 dB dynamic range for 1 kHz spacings. This is the main
reason why I have an Orion, and why the 600 Hz filter is a big deal
for very closely spaced interfering signals. My example above could be
off by 10 dB for the 500's dynamic range, but that still leaves a birdie
12 dB above the noise floor.
Remember that even though the 500 & 250 filters have narrower
bandwidths than the 600 Hz, they have VERY POOR IM performance.
Even with DSP settings down to 100 Hz, you will hear these IM
birdies because they can fall right within your 100 Hz bandpass if
the spacings are just right. Sinisa's results at 700 Hz spacings
actually showed the 250 Hz filter was not far behind his 1000 Hz
filter, but I've seen other private measurements that indicate it
is even worse than the 500 Hz example I used above, and these same
measurements confirm the poor performance of the 500 noted above.
The ONLY time I would consider using the 500 or 250 Hz filters
would be on a dead band in very high QRN when I might want to disable
DSP filtering by setting it to 6000 Hz, and simply use roofing filters
as my only filtering. You might want to do this to avoid some strange
effects that can be introduced by narrow DSP settings when trying to
copy extremely weak signals in very high QRN situations like 160/80
meters in Summertime lightning QRN, but these are very rare situations
for most of us.
73, Bill W4ZV
P.S. I did this in a hurry so I could have made mistakes above.
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