Posted by
Bill VanAlstyne W5WVO on
May 17, 2007; 4:01am
URL: http://elecraft.85.s1.nabble.com/K3-IF-AF-Dsp-questions-comparisons-tp447571p447585.html
Hi Brian,
Well... The REAL standard, the one that EVERYBODY agrees on, is that S9 = 50
uV. How many dB an S-unit is depends on where you start from to get there.
Let's say that an S-unit is 6 dB and that the minimum detectable signal (MDS)
is 0.25 uV. That's probably about where the MDS of the K3 is going to come in,
plus or minus a few hundredths. How many dB between 0.25 uV and 50 uV? The
formula is dB = 20log(V1/V2), so dB = 20log(200) = 46 dB. At 6 dB per S unit,
that's 7.67 S-units below S9, or about S1-and-a-third. That means that a
signal that you can just barely distinguish from the receiver's own internal
noise floor -- i.e., essentially the amplitude of the receiver's internal
noise floor itself -- is over S1 on the meter. That doesn't make intuitive
sense to me.
A more reasonable S-unit is about 5 dB. Why? You put the 0.25 uV MDS at S0 so
that you get 46 dB / 5 dB/S-unit = ~9 S-units. This means that anything above
the MDS moves the S-meter, including any atmospheric noise level that rises
above MDS. A no-signal condition (i.e., the antenna is disconnected) is S0, no
meter deflection -- where it should be, IMHO. 6 dB per S-unit might have been
somebody's standard way back when, but as you can see, it doesn't really make
sense. If you really want a calibrated S-meter, I think you have to go with 5
dB, unless you want to number the scale from S1 "on the pin" (or actually a
little below the "pin"). Maybe that's acceptable. Depends on how you think
about it. I like 5 dB better because it makes better technical and intuitive
sense to me.
My guess is that the K3 will come with some default value for an S-unit, but
that it will also be user-settable in the menu. Based on a conversation I had
with Wayne a couple weeks back, that default will NOT be 6 dB. I'm hoping I
convinced him that 5 dB (as opposed to something lower) is a good number, by
the logic given above. 5 dB also works well because it is a modulus of 10, 15,
20, etc., which gives a more pleasing symmetry to "dB over S9".
In short -- Since there really isn't an existing S-meter calibration standard
that anybody agrees on and is currently using, we might as well set a standard
that makes some sense. I suspect that the K3 will be setting standards all
over the place. :-)
Bill
Brian Lloyd wrote:
> On May 16, 2007, at 5:01 PM, Bill W5WVO wrote:
>
>> When you're talking about trying to copy this 0.5 uv S1 signal in
>> the presence of 40-over-9 signals 5 and 10 kHz away (again, a
>> common real-world scenario on 6m during a contest), you're looking
>> at a range of 40 dB + ( 8 S-units x 5 dB/S-unit ) = 80 dB.
>
> 6dB per S-unit is the standard. That would make it a 48dB+40dB or
> 88dB. That takes a really good receiver.
>
> 73 de Brian, WB6RQN
> Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com
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