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Owen Duffy wrote:
Dave Oldridge wrote in 9: Near as I could measure it, the NF of the receiver after my mod was 1.2db. I had to resort to boiling and freezing water and a tiny dummy load to measure it at all. I haven't tried hot/cold tests using ice and boiling water, I didn't think it was practical. You finally measured a receiver noise temperature of 50K with hot and cold loads of 270 and 370. That means a Y factor of 1.059dB. If Y were just 0.1dB greater, NF would be 0.78dB, 0.1dB lower and, NF would be 1.66dB. With this configuration the sensitivity of NF to changes in Y are extreme, 0.4dB change in NF per 0.1dB change in Y around that point. If you made the Y measurements using the audio output of a narrow band receiver, it is very hard to make high resolution measurements (eg to 0.01dB resolution) with say, a multimeter. I have done these tests with a liquid nitrogen cooled load and room temperature load, and that gives more practical Y ratios, 3.7dB for a 1.2dBNF, and the sensitivity in NF is 0.08dB per 0.1dB change in Y. This still demands high resolution measurement of noise power. Owen Indeed, this would be a very challenging measurement, because you also have to take into account the match of that load, and if it's just a resistor that you're plunging into hot and cold, its resistance will almost certainly change. At microwave frequencies, a more common technique for radiometers is to use a flat plate absorber that has been characterized for changes in absorption over temperature. One might want to take a look at how NIST does this kind of thing. Here's the slides from a talk by Jim Randa http://www.boulder.nist.gov/div818/8...t%20Crs_06.pdf he's a noise measurement guru at NIST.. check out the website: http://www.boulder.nist.gov/div818/81801/Noise/ I've had a precision noise source (used to do Y factor measurements on a precision 13.402 GHz receiver) measured in their lab over a week. The measurement uncertainty (for a system with waveguide connections) was in the few Kelvins range (out of a noise power of 7000K or so), and the connect/reconnect uncertainty dominates. I doubt one could get this kind of performance with a coaxial connector (the uncertainty in the mismatch). By the way, a good noise diode source is probably a better standard for the hot side than heating a resistor. They're very, very stable over time, once calibrated, and if properly designed, have a very stable match as the noise is turned on and off. (that's what we were using in the above system, a temperature controlled Noise/Com style source). http://www.boulder.nist.gov/div818/8...ability_IM.pdf http://www.boulder.nist.gov/div818/8...ility_CPEM.pdf describes the performance Another useful link might be: http://www.boulder.nist.gov/div818/8...97_Amps_IM.pdf D.F. Wait, J. Randa, "Amplifier Noise Measurements at NIST", IEEE Trans on Inst. and Meas., v.46, n.2, Apr 1997 They give measurement uncertainties of 0.04dB on a 0.5 dB NF for 2-4 GHz.. |
#2
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Owen Duffy wrote in news:Xns999955EE72868nonenowhere@
61.9.191.5: Dave Oldridge wrote in 9: Near as I could measure it, the NF of the receiver after my mod was 1.2db. I had to resort to boiling and freezing water and a tiny dummy load to measure it at all. I haven't tried hot/cold tests using ice and boiling water, I didn't think it was practical. Only just and you need a good 4-digit or better AC voltmeter to do it at all. I wasn't after accuracy, just a ball-park estimate and I know I got it fairly close because the receiver did show a marked increase in noise when any decent antenna was connected. You finally measured a receiver noise temperature of 50K with hot and cold loads of 270 and 370. That means a Y factor of 1.059dB. If Y were just 0.1dB greater, NF would be 0.78dB, 0.1dB lower and, NF would be 1.66dB. Yep...the most I'd be willing to commit to with that measurement would be that it was below around 2.5 and PROBABLY fairly close to my measurement. I measured the voltages alternately 25 times and took a mean to try to smooth out the errors. With this configuration the sensitivity of NF to changes in Y are extreme, 0.4dB change in NF per 0.1dB change in Y around that point. If you made the Y measurements using the audio output of a narrow band receiver, it is very hard to make high resolution measurements (eg to 0.01dB resolution) with say, a multimeter. It is. You need a good AC voltmeter with decent digital accuracy and resolution and you have to average a bunch of readings. I have done these tests with a liquid nitrogen cooled load and room temperature load, and that gives more practical Y ratios, 3.7dB for a 1.2dBNF, and the sensitivity in NF is 0.08dB per 0.1dB change in Y. This still demands high resolution measurement of noise power. Yes, anything less than 4 digits is just about useless. -- Dave Oldridge+ ICQ 1800667 |
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