Thanks everyone for the quick and detailed explanation. I now understand the S meter displays one sum across the entire width of the green filter, while the panadapter displays hundreds of sums across the width of each individual pixel. When I set the filter very narrow (approaching one pixel) the S metter reading approaches the panadapter reading.
My old Kenwood was similar in that selecting the narrow CW filter caused a drop in S meter noise floor. This behavior makes perfect sense for any radio. I guess if someone asks about the noise floor, they need to specify at what filter width. 73, Doug, VE3EFC On 6/14/07, Jim McLester <[EMAIL PROTECTED]> wrote: > > I noticed the same thing and was going to ask when I ran out of other > questions, but, with antenna = 50r dummy, I just set bandwidth to absolute > minimum, played with zoom, preamps etc. > > The best I could do was get the meter down to 6 to 8db above panadapter. > Varies, of course with zoom. > > Interestingly, the difference INCREASES as the preamp is clicked to lesser > values. > > I don't consider this an issue as I use the panadapter as a relative > device > anyway. running in average mode almost all the time. > > Jim - W4YXU > ----- Original Message ----- > From: "Eric Wachsmann" <[EMAIL PROTECTED]> > To: "'Tim Ellison'" <[EMAIL PROTECTED]>; "'Doug McCormack'" > <[EMAIL PROTECTED]>; <flexradio@flex-radio.biz> > Sent: Thursday, June 14, 2007 11:04 AM > Subject: Re: [Flexradio] Noise Floor S meter vs Panadapter > > > > This is close, but technically, a bit different. The multimeter > actually > > calculates the power within the filter. You'll notice that if you half > > the > > filter width, you will get half the power (with just noise in the > > passband). > > Try setting the filter to a bandwidth like 50Hz and I'd bet that you see > > something similar to the noise floor shown on the display. > > > > The display, on the other hand, takes the maximum bin within the range > > represented by each pixel. Depending on the sample rate and the zoom > > setting, this may be just one bin or many bins. Each bin represents the > a > > block of frequency calculated by Sample Rate / 4096. So at 192kHz, each > > bin > > represents just ~47Hz. > > > > All of this to say that it is basically an issue of peak bin (display) > vs. > > a > > true power "sum" of bins (multimeter). > > > > > > Eric Wachsmann > > FlexRadio Systems > > > >> -----Original Message----- > >> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] > >> radio.biz] On Behalf Of Tim Ellison > >> Sent: Thursday, June 14, 2007 10:47 AM > >> To: Doug McCormack; flexradio@flex-radio.biz > >> Subject: Re: [Flexradio] Noise Floor S meter vs Panadapter > >> > >> Do you have averaging turned on with the Panadapter? The S meter will > >> display the highest value it sees for the samples (bins) it is > >> analyzing. Within that sample of data it is analyzing, there are noise > >> peaks that correspond to the higher value you are seeing (-117 dB > rather > >> than the -145 dB). I use the average of the noise floor, or what the > >> Panadapter shows when averaging is on to determine what the noise floor > >> is. > >> > >> -Tim > >> ---- > >> > >> > >> -----Original Message----- > >> From: [EMAIL PROTECTED] > >> [mailto:[EMAIL PROTECTED] On Behalf Of Doug McCormack > >> Sent: Thursday, June 14, 2007 11:30 AM > >> To: flexradio@flex-radio.biz > >> Subject: [Flexradio] Noise Floor S meter vs Panadapter > >> > >> I recently build the Elecraft kit to provide a reference signal for > >> calibration of my SDR1000. With the 50 uV reference my S meter shows > >> -73 dBm (S 9.0). At 1 uV reference, the meter shows -107 dBm ( S > >> 3.3). These two numbers show the S meter is perfectly calibrated. > >> > >> When I remove the Elecraft unit and no antenna is connected, the meter > >> shows a noise floor of -117 dBm (S1.3). But the panadapter shows > >> -145 dB. I wonder why the panadapter does not agree with the S meter. > >> Maybe I am should ground the antenna connector when measuring noise > >> floor? When people ask me about the noise floor, do I say -117 or > >> -145? > >> > >> I have always suspected my 5 year-old Dell 1.8 Ghz has high internal > >> noise possibly on the PCI bus. Perhaps this Dell system noise is > >> getting into my M44 sound card. I hope to have a new dual-core Intel > >> computer later this week. > >> > >> 73, Doug VE3EFC > >> -------------- next part -------------- > >> An HTML attachment was scrubbed... > >> URL: > >> > http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/attachment > >> s/20070614/d3f5da98/attachment.html > >> _______________________________________________ > >> FlexRadio mailing list > >> FlexRadio@flex-radio.biz > >> http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz > >> Archive Link: http://www.mail-archive.com/flexradio%40flex-radio.biz/ > >> FlexRadio Knowledge Base: http://kb.flex-radio.com/ > >> FlexRadio Homepage: http://www.flex-radio.com/ > >> > >> > >> _______________________________________________ > >> FlexRadio mailing list > >> FlexRadio@flex-radio.biz > >> http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz > >> Archive Link: http://www.mail-archive.com/flexradio%40flex-radio.biz/ > >> FlexRadio Knowledge Base: http://kb.flex-radio.com/ > >> FlexRadio Homepage: http://www.flex-radio.com/ > > > > > > > > _______________________________________________ > > FlexRadio mailing list > > FlexRadio@flex-radio.biz > > http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz > > Archive Link: http://www.mail-archive.com/flexradio%40flex-radio.biz/ > > FlexRadio Knowledge Base: http://kb.flex-radio.com/ > > FlexRadio Homepage: http://www.flex-radio.com/ > > > > > > > > -- > > No virus found in this incoming message. > > Checked by AVG Free Edition. > > Version: 7.5.472 / Virus Database: 269.8.15/847 - Release Date: > 6/12/2007 > > 9:42 PM > > > > > > -------------- next part -------------- An HTML attachment was scrubbed... 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