Topband: Sherwood data

Tom Rauch w8ji at
Fri Feb 16 10:14:14 EST 2007

>         There are 3 key issues involved:
> 1.  IMDDR3 (two-tone dynamic range)
> 2.  BDR (single tone dynamic range)
> 3.  Phase noise (which can override both)

All a very good explanation Bill, but there is a fourth, 
fifth, and sixth issue.  Also I think it is fair to mention 
people in a position to design radios often do not listen to 
external input and design for themselves and what they think 
others want. They then get ruffled and want to force 
end-users to agree that whatever they did is perfect and so 
the radio must always work perfectly in any situation and no 
one is allowed to point out flaws!

So adding to the good list you made.....

4.) AGC can affect readability of weak signals near noise. 
Humans do NOT want a perfect AGC when copying weak signals 
near noise. They do not want a perfect AGC in pileups 
either. Designing a receiver with no volume change for 
signal level change is a mistake for HF DXing or contesting 

5.) Selectivity. There has to be a good compromise between 
skirt rolloff and "ringing".

6.) User interface.  If you can't easily use it for what you 
are doing or understand it, then the best performance is 

Then there is another issue on top of all that. A two tone 
test is great in a radio that only passes a narrow slice of 
signals, but when a radio passes a very wide swath the 
testing must also include all the expected multiple signals.

While looking at close spaced and wide spaced testing with 
two tones works with a radio that only passes a narrow 
window to sensitive stages, it does not work when the radio 
passes everything within 50kHz on a very crowded band. This 
is because the accumulated power of all the signals in a 50 
kHz window is significantly higher than the power of all the 
signals within only a few hundred Hz area of the band.

Suppose we have two radios, one with 100 dB close spaced and 
wide spaced spec, and another with 80 dB close spaced and 
140dB wide spaced specs. When the band is nearly empty the 
100dB DR spec radio will probably always be better. When the 
band is congested, the 80 dB close spaced spec radio might 
actually be better with close spaced signals than the 100dB 
radio!!! This is because the 100dB radio has to handle ALL 
the signals with that spec, both close and far in frequency. 
The 80dB radio might only have to handle three or four 
strong signals, and the rest fall into the -140dB wide 
spaced area and can be ignored.

Engineers run into this problem designing broadband 
amplifiers. It's a whole lot easier to design an amplifier 
that only handles two carriers than twenty. Receivers are no 
different, and we can't force it to be otherwise through 

73 Tom

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