But were some with the RX1 Loop in line and some without it?
Eric On Wed, Apr 15, 2009 at 12:57 AM, Clay W7CE <w...@curtiss.net> wrote: > Hi Eric, > Yes. All measurements were made using antenna port 1. > > 73, > Clay W7CE > > ----- Original Message ----- > *From:* Eric Wachsmann <e...@flex-radio.com> > *To:* Clay W7CE <w...@curtiss.net> > *Cc:* Bob McGwier <rwmcgw...@gmail.com> ; flexradio@flex-radio.biz > *Sent:* Tuesday, April 14, 2009 8:56 AM > *Subject:* Re: [Flexradio] PowerSDR S-Meter Power Readings > > Clay, > > Are you using the same antenna setting for all of the measurements? If > not, this could account for the difference you are seeing. We offset the > meter and display values to account for the loss through the switches in the > antenna circuitry depending on the selected antenna. > > > Eric Wachsmann > FlexRadio Systems > > On Tue, Apr 14, 2009 at 1:12 AM, Clay W7CE <w...@curtiss.net> wrote: > >> Hi Bob, >> It's certainly possible that I'm doing this wrong, so let me explain my >> procedure and then you can tell me if I'm making a mistake. All of the >> following is performed on 6M at 50.200 MHz with a 50 ohm termination >> connected to antenna port 1 and the bandwidth set to 500 Hz (mode CWU). >> Also, the multimeter average time is increased to 5000 mS and the Digital >> Refresh is increased to 2000 mS. The external preamp I am using is an ARR >> P50VDG preamp with a spec'd noise figure of 0.5 dB and a measured gain of 26 >> dB. Gain was measured on an HP 4396A Spectrum/Network Analyzer and >> confirmed using the ADC L and ADC R meters. >> >> First, I measured the receiver noise floor using the RX1 meter set to "Sig >> Avg". With the internal preamp turned on and no external preamp, the >> average noise level is -125.0 dBm (the value fluctuates some, but seems to >> be centered here). This indicates a receiver noise figure of 22.0 dB. Now >> if I enable the external preamp the average noise level decreases to -143.9 >> dBm, corresponding to a receiver noise figure of 3.1 dB. However if I do >> the math, adding my preamp and assuming connector, coax and internal relay >> losses of 0.3 dB in addition to the 0.5 dB preamp noise figure, I should see >> a receiver noise figure of 2.0 dB. For the receiver noise figure (including >> external preamp) to be 3.1 dB the external preamp noise figure and passive >> losses would need to add up to 2.2 dB which is much larger than I would >> expect. >> >> Next, I computed the receiver noise figure by measuring the receiver MDS >> using an external HP RMS volt meter at the speaker connector and the HP >> 4396A as a signal generator. Without the external preamp the measured MDS >> is -126 dBm and with the preamp the MDS is -145 dBm, corresponding to noise >> figures of 21 dB and 2 dB, respectively. If I do the math, adding my preamp >> in front of a 21 dB NF receiver, should yield a system NF of about 1.8 dB, >> so the measurements agree closely with the math. >> >> Based on the results, my MDS measurements (using the voltmeter) with the >> preamp enabled are more consistent with the calculated values than those >> using the internal meter. I suspect that measurements this close to the >> noise floor will not be as accurate and that I'm asking too much. I'm >> certainly not complaining though. The accuracy of the metering rivals that >> of my HP spectrum analyzer on measurements that I've made from -100 dBm to >> -30 dBm. >> >> 73, >> Clay W7CE >> >> >> >> >> You are not reading this correctly. >>> >>> The power displayed on the meter is integrated power in the receive >>> filter, just as it should be. The computation is accurate to much more than >>> 0.1 dB irrespective of what the display is doing. But as in all cases of >>> such compromises, it is designed to work for the use case of 99.9% of >>> amateurs (and others) and that is to measure the strength of the signals >>> that are well above the noise floor. That is how it is calibrated. >>> >>> I do not believe your 1-2 dB number and will not without demonstration. >>> You provide proof, I will believe and see what we can do. >>> >>> Bob >>> N4HY >>> >>> Clay W7CE wrote: >>> >>>> I've been looking through the PowerSDR code to see how the RX meter >>>> signal strength is computed. Based on what I've observed it appears to >>>> based on the maximum signal found within the currently selected bandwidth. >>>> So if I'm receiving two CW signals within the current bandwidth, it will >>>> show the strength of the stronger signal. Am I reading this correctly? If >>>> so, I'd like to add a new RX meter option that displays the true RMS power >>>> for the selected bandwidth and increases the display resolution to 0.1 dB. >>>> I think this addition will allow PowerSDR to accurately display the noise >>>> floor of the receiver when the antenna port is terminated with a 50 ohm >>>> load. The current implementation seems to give close, but not totally >>>> accurate results (my results seem to be off by about 1-2 dB) >>>> >>>> I have Microsoft Visual Studio .net 2003 and can build the source, but >>>> would appreciate it if one of the PowerSDR gurus could help point me in the >>>> correct direction in the source code. I think I've found most of the code, >>>> but I'm not sure where the average signal is computed. >>>> >>>> Thanks & 73, >>>> Clay W7CE >>>> >>>> _______________________________________________ >>>> FlexRadio Systems Mailing List >>>> FlexRadio@flex-radio.biz >>>> http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz >>>> Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ >>>> Knowledge Base: http://kc.flex-radio.com/ Homepage: >>>> http://www.flex-radio.com/ >>>> >>>> >>> >>> -- >>> (Co)Author: DttSP, Quiktrak, PowerSDR, GnuRadio Member: ARRL, AMSAT, >>> AMSAT-DL, TAPR, Packrats, >>> NJQRP, QRP ARCI, QCWA, FRC. >>> "It is human nature to think wisely and act in >>> an absurd fashion.", Anatole France. >>> Twitter:rwmcgwier >>> Active: Facebook,Myspace,LinkedIn >>> >>> >>> >> >> _______________________________________________ >> FlexRadio Systems Mailing List >> FlexRadio@flex-radio.biz >> http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz >> Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ >> Knowledge Base: http://kc.flex-radio.com/ Homepage: >> http://www.flex-radio.com/ >> > > _______________________________________________ FlexRadio Systems Mailing List FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ Knowledge Base: http://kc.flex-radio.com/ Homepage: http://www.flex-radio.com/
