K3 S-Meter and Noise
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Here's my look for what its worth. ;-)
I have been following AGC, S-meter, and threshold/slope values since the beginning - K2 #272 over two years ago. Until recently, I have found that K3 sensitivity varied between serial numbers and that could not be changed. I suspect this value was not measured before units left the factory. I had about 10db difference between K3 #272 and #2308. The s-meter presentation was tied to that difference. A hotter receiver would have a higher resting value on a given band that the more moderate receiver. Happily, in the newest limited beta release this has been addressed. The RF Gain can be calibrated to the user's satisfaction. It works great. For example - tonight on 40m I see the same resting band noise that I'd see on IC756p, or OMNI VI+. It is S3. In the past, the K3 always showed at least 2 points higher, sometimes 3, assuming identical sensitivity with my other sets. So, in the past my K3 would be showing S6 as resting. That was troublesome because I could not see how strong anyone was, if it were less than S6. At my location the single biggest plus with my K3's was getting the updated DSP board. That took all the critical adjustment out of THRESHOLD and SLOPE in CONFIG and made the default values good everywhere for me. The next major upgrade was this latest firmware and DSP that the guys have been working on. It allows RF Gain calibration. Very nice indeed. I did write custom s-meter code so that when the first bar on the K3 is solid and the next one is flickering, I can actually see on my remote meter the precision of the value as it is swinging up from S1 and down from S2. For the most part I find flickering bars to be useless. ;-) Cheers, Don [Elecraft] [K3] RE: K3 S-Meter and Noise Don Wilhelm w3fpr at embarqmail.com Sun Oct 4 19:43:06 EDT 2009 Previous message: [Elecraft] [K3] RE: K3 S-Meter and Noise Next message: [Elecraft] [K3] K3 S-Meter and Noise Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] Jim, There are 2 reasons I can think of to make that condition 'normal'. First is if you have the preamp on for a band where it is not needed - that would amplify the atmospheric noise when the receiver has no signal present and no AGC is active. If your preamp is on, try tuning it off and see if it makes a difference. A good test for whether the preamp is needed or not is to disconnect the antenna and reconnect it. If the noise level increases when you connect the antenna, there is more than enough gain in the receiver - turn off the preamp and try the same thing again - if the noise level still increases when the antenna is connected, try again but use the attenuator this time. If you can just barely hear the band noise, that is the proper gain setting for that band at that time and your antennas. The noise level on a particular band will vary, perhaps even on an hourly basis (i.e. 80 meters is noisy during the day, but will quiet down some at night). ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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Don,
I notice that if I have the RF gain less than full CW the S Meter bars are progressively lit and it looks like the noise level has increased when it has not. The latest beta is a work of art and should make folks happy but the S Meter / RF gain interaction puzzles me. Out of habit I do not run RF gain full out, but for a more accurate meter I guess I will. Can you explain the interaction? Dan AB3EN
Dan AB3EN |
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Dan,
That behavior is common for many receivers - the K2 and K3 included. Admittedly, a few do it differently and the RF Gain will reduce the S-meter reading rather than sliding the scale upward. The reason is that the RF Gain works in 'parallel' with the AGC. In other words, the developed AGC voltage and the RF Gain voltage is applied to the same circuits, and the way the S-meter is implemented, there is no way to distinguish between them. Each operator must make a decision - if the S-meter reading is the most important consideration, the RF Gain must be set at the full CW position, but if maximum receiver dynamic range is more important, the RF gain will be reduced and the S-meter reading will be a relative reading rather than an accurate scale. The difference between (say) S-5 and S-7 will still be 2 bars, but they will appear at the S-7 and S-9 marks instead of the S-5 and S-7. If you are doing weak signal work or working on a crowded band, I suggest you opt for the maximum dynamic range by reducing the RF Gain (and advance the AF Gain) so you can just barely hear the band noise. But if your current operation is more casual, run the RF Gain at max and you will have valid S-meter readings. 73, Don W3FPR AB3EN wrote: > Don, > > I notice that if I have the RF gain less than full CW the S Meter bars are > progressively lit and it looks like the noise level has increased when it > has not. The latest beta is a work of art and should make folks happy but > the S Meter / RF gain interaction puzzles me. Out of habit I do not run RF > gain full out, but for a more accurate meter I guess I will. Can you explain > the interaction? > > Dan > AB3EN > > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by AB3EN
Holding my tongue firmly in my cheek, I suggest that we can give
'proper' S reports without an S-meter at all. S-3 - I can barely hear you S-5 - You have a weak, but copyable signal S-7 - Good copy, but not the strongest signal S-9 - armchair copy S-9+ - you are really booming in here. For contests, there is no problem, everyone is 599 or 59 anyway unless it is a QRP contest where S-7 is the norm. 73, Don W3FPR ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by Don Wilhelm-4
I recommend backing off RF Gain until your S-meter is right at the point where noise is flickering the display. For example if noise is S3-S4, back off RF Gain so that the S-meter reads a constant S4. Then AGC will not be trying to function on weaker signals so you will be maximizing dynamic range above the noise threshold. Of course this also assumes you're already using the least gain that will give you band noise (i.e. ATTenuator, OFF and PREamplifier preferred in that order) when an antenna is connected. This is the way it was done it in the pre-DSP analog radio era. Elecraft makes it very easy to set the proper RF Gain setting in a DSP rig by backing off the S-meter when reducing gain. 73, Bill |
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In reply to this post by Don Wilhelm-4
Don,
Thanks for the explanation. I too use the method you describe such as "S-9 - armchair copy" but the interaction with the RF-gain had me looking all over the manual trying to figure out why the S Meter calibration didn't work. Dan
Dan AB3EN |
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