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/
