[P3] Power measurement with the P3
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I note my P3's amplitude readings are about 6 dB low-- that's with the IF output
switched off. I'm not sure how to calibrate it. Even so, the P3 gives great insight into signals and their amplitude. Let's say you calibrated your S-meter to read S9 with a 50 uV input signal. Fifty microvolts is -73 dBm in 50 ohms. A CW signal which is S9 on your S-meter should peak at around -73 dBm (plus or minus 3 dB) on the P3. But an SSB signal which is S9 on your S-meter will not. It will be several dB less than -73 dBm on the screen of the P3. Why? A CW signal is one pure tone, and all of the power is concentrated in that one tone, whereas the human voice contains many different tones or frequencies within the 2 or 3 kHz bandwidth of the SSB signal. To measure the power in the SSB signal requires integrating or summing all of the power of all those different tones to come up with the total. Any one frequency falls far short of -73 dBm, but the total is far greater and should approach -73 dBm. This is easy to see on the P3. As an example of the above, if you had (10) -100 dBm in-phase tones in a 3 kHz bandwidth, the total channel power would actually be -90 dBm, 10 dB higher due to the summation. If the number of tones is greater than 10 as it can be with the human voice, the total can be even higher, that is, the difference between any single frequency and the total becomes greater. Fascinating. One final thing: my son is studying for his Technician license, and I plan to show him some real AM, SSB and CW signals on the P3 to help him see the difference. Do you suppose I can write the P3 off as an education expense? :^) Al W6LX ______________________________________________________________ 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 |
Re: [P3] Power measurement with the P3
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Amplitude calibration is not yet implemented in the firmware.
As for SSB peaks do you have peak hold turned on when you are viewing this? You need to be looking at the signals with averaging disabled (and preferably with peak hold on) in order to see the power peaks of an SSB signal. ~Brett (N7MG) On Fri, Aug 13, 2010 at 12:29 AM, Al Lorona <[hidden email]> wrote: > I note my P3's amplitude readings are about 6 dB low-- that's with the IF output > switched off. I'm not sure how to calibrate it. > > Even so, the P3 gives great insight into signals and their amplitude. > > Let's say you calibrated your S-meter to read S9 with a 50 uV input > signal. Fifty microvolts is -73 dBm in 50 ohms. > > A CW signal which is S9 on your S-meter should peak at around -73 dBm (plus or > minus 3 dB) on the P3. But an SSB signal which is S9 on your S-meter will not. > It will be several dB less than -73 dBm on the screen of the P3. Why? > > A CW signal is one pure tone, and all of the power is concentrated in that one > tone, whereas the human voice contains many different tones or frequencies > within the 2 or 3 kHz bandwidth of the SSB signal. To measure the power in the > SSB signal requires integrating or summing all of the power of all those > different tones to come up with the total. Any one frequency falls far short of > -73 dBm, but the total is far greater and should approach -73 dBm. This is easy > to see on the P3. > > As an example of the above, if you had (10) -100 dBm in-phase tones in a 3 kHz > bandwidth, the total channel power would actually be -90 dBm, 10 dB higher due > to the summation. If the number of tones is greater than 10 as it can be with > the human voice, the total can be even higher, that is, the difference between > any single frequency and the total becomes greater. Fascinating. > > One final thing: my son is studying for his Technician license, and I plan > to show him some real AM, SSB and CW signals on the P3 to help him see the > difference. Do you suppose I can write the P3 off as an education expense? :^) > > Al W6LX > ______________________________________________________________ > 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 > 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 alorona
Have you done the IF OUT Gain Mod mentioned at the top of page 5 in the manual? http://www.elecraft.com/K3/mods/IF_Output_Buffer_Gain_Mod_Rev_A.pdf Earliest P3 firmware versions had a gain error of ~10 dB but the latest version (beta 00.30) is better. My P3 now reads -74 dBm for all combinations of PRE/ATT on and off (using the XG1's -73 dBm setting). Once Alan implements Amplitude Calibration we should be able to correct the remaining errors. 73, Bill |
Re: [P3] Power measurement with the P3
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I have done the mods and I'm running 00.30 and I'm still a bit off.
My XG2 puts out ~-74dBm give or take a few tenths last time I measured it on our 80K spectrum analyzer and my combination reads -81dBm on the K3 with the ATU in bypass and no pre or att. PRE or ATT are correctly accounted for and the signal does not change if either are activated individually. Activating both at the same time causes the signal to increase by 1dB but operating in this condition would just be plain silly so I don't consider that a real world scenario. Engaging the tuner which is setup for my antenna at that particular frequency actually makes a change of 3dB. Again thats no fault of any of the equipment just something of interesting note... ~Brett (N7MG) On Fri, Aug 13, 2010 at 3:56 AM, Bill W4ZV <[hidden email]> wrote: > > > Al Lorona wrote: >> >> I note my P3's amplitude readings are about 6 dB low-- that's with the IF >> output >> switched off. I'm not sure how to calibrate it. >> > > Have you done the IF OUT Gain Mod mentioned at the top of page 5 in the > manual? > > http://www.elecraft.com/K3/mods/IF_Output_Buffer_Gain_Mod_Rev_A.pdf > > Earliest P3 firmware versions had a gain error of ~10 dB but the latest > version (beta 00.30) is better. My P3 now reads -74 dBm for all > combinations of PRE/ATT on and off (using the XG1's -73 dBm setting). Once > Alan implements Amplitude Calibration we should be able to correct the > remaining errors. > > 73, Bill > > -- > View this message in context: http://elecraft.365791.n2.nabble.com/P3-Power-measurement-with-the-P3-tp5419099p5419585.html > Sent from the Elecraft mailing list archive at Nabble.com. > ______________________________________________________________ > 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 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 alorona
Comparing your S meter lights on the K3 with the P3 is comparing a
length measured with your shoes with a length measured with a measuring tape, and that should include the fact that your left shoe is size 6 and your right shoe size 5, as S9 and down are 6 db increments and all above are 5 db. And just how much accuracy would one really be looking for in a 1.25 inch segment display that covers 108 dB. If this were put on an analog meter with the same length of display which by some miracle were driven with a perfectly linear circuit, the needle would be 2 or 3 dB wide on the scale, not to mention parallax error. If you are properly calibrated on the K3, the S9 light will be turned on if the signal is anywhere between -73.0 dBm (when the S9 segment comes on) and -62.1 dBm (when the +5 segment comes on). The light should mean anything at least S9 but less than 5 over. When I calibrated mine carefully I was able to have the correct light come on in the correct dB advancing a precision attenuator dB by dB. The tenth of a dB spot wandered around a bit. This makes the segment display more accurate than the fictitious (and unattainable) analog meter above, if one ignores the fact that the segment display is "blind" for 4 or 5 dB spots between the flip points. There is a distinct usefulness to knowing the absolute noise level, and whether a signal is in fact 40 over 9, no kidding, no sh*t. Most people clearly and consistently recognize sound level changes at 3 to 6 dB levels, and 5 or 6 dB increments is a nice match to this general heel-to-toe order of magnitude scale. It's as accurate as measure by counting strides, which one relative can get to within a yard or two across the length of a football field. When I9*** is 40 over 9 on a calibrated K3 S meter, meaning his signal is -33 dBm, regardless of how good the 5 element 40m quad is, I know that he's as loud as the BC stations up above 7.2 MHz, and his power for the contest is the majority of his power bill for the month. I don't own one, but I believe that the W2 has the cute extension to the LED display of coming on weakly as the segments are crossed increasing to full brilliance as the next segment value is approached, which if that were possible on an LCD display would give the display perhaps 2 dB accuracy presuming the operator would perceive low, medium and high brightness. But this would only remove the blind spots between the flip points. The K3 S meter is an "order of magnitude" measure-by-stride indicator. The P3 is a ruler where you can see less than 1 dB increments on signals. 73, Guy. > Let's say you calibrated your S-meter to read S9 with a 50 uV input > signal. Fifty microvolts is -73 dBm in 50 ohms. > > A CW signal which is S9 on your S-meter should peak at around -73 dBm (plus or > minus 3 dB) on the P3. But an SSB signal which is S9 on your S-meter will not. > It will be several dB less than -73 dBm on the screen of the P3. Why? > ______________________________________________________________ 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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Hi, Guy,
I *think* we're in agreement... the point I was trying to make was that a spectrum display tells you soooo much more than an S-meter. That's my way of saying how much better it is to have a P3. I tried to express my enthusiasm with a little mini-lesson in the way power gets integrated in a bandwidth. Can't wait for amplitude cal on the P3. It'll be awesome to be plus or minus just a few dB! Regards, Al W6LX > Comparing your S meter lights on the K3 with the P3 is comparing a > length measured with your shoes with a length measured with a > measuring tape... ______________________________________________________________ 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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