Re: Agilent 346A and N9069A reading high?
John Makes a good point - NF meters can typically measure in bandwidths of several MHz and are assuming the noise figure, gain and match is constant across this bandwidth.toggle quoted messageShow quoted text
When you design a preamp for VHF it is normally a good idea to make it narrow band, especially in the UK where we have very strong pager signals either side of 144MHz. To a DXer, there is nothing of importance outside the 144-145MHz range and a preamp may have a 3dB bandwidth of 2MHz or less to cover this range plus a bit. What this means depends on the filter shape and the NF meter bandwidth, but its quite possible to be missing nearly half the noise due to the filter.
As John says this will mean the ENR calibration won't be right so the difference between noise on and off will be lower than expected resulting in a higher recorded noise figure. You can fix this with a filter which is difficult, or by doing the maths if you know the relative bandwidths, which may be easier.
The negative noise figure issues is probably just noise. If you expect to get better than 0.1dB accuracy after calibration you will need to be very careful, do a lot of integrating and keep everything at a constant temperature and do it all in a screened room. You should see a mean value of zero. The fact it is reading -0.1dB consistently could just be the instrument needs recalibrating, or maybe you have not got the settings right for temperature, or indicate external noise is getting into it from local RF sources and disturbing the calibration. If you are doing this at home I would go with the QRM. Since they installed a mobile phone mast at RAL near the RF lab, we haven't been able to measure VHF/UHF noise figures accurately in the RF lab.
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