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Hi,
When I bought SA44B my goal was to measure the noise at the output of my detector (50 Ohm), with no any signal. I was attracted by low DANL and low price of SA44B.
Here are measurement conditions:
– I expect to take measurements of noise density in dBm/Hz in about 20 points from 100Hz and up to 200 MHz and process them mathematically in order to get an average noise density over the bandwidth of interest and the noise rms value
– expected noise density levels are from -135 dBm/Hz and up to -110 dBm/Hz
– Att=0, PreAmp=On, selected Gain2
– SA44B noise floor is below -164 dBm/Hz.
When I perform measurements, I see that noise density doesn’t go below -118 dBm/Hz and results depend on the selected frequency span and RBW (with difference of up to 4 dB). And the noise density spectral distribution on the Spike’s screen doesn’t meet my expectations (design is proved to be better than SA44B shows by other expensive desktop SA).
I need some insight into how SA44B works and what limits its capabilities in my specific mode of operation.
Thank youThe SA44B is not ideal for broadband measurements. We generally only recommend it for narrow-band signal analysis (<250 kHz occupied bandwidth is ideal) due to the limitations of the SA-series software image rejection.
Because we use a software image rejection algorithm, making noise measurements can be tricky. A span of 200 kHz or less should be used. Good settings might be 1 kHz RBW, 30 Hz VBW, power, average. Then you have to factor in that the noise from the image will add to the reading, typically 3 dB.
Hope this helps. The BB60C might be a better choice for noise measurements, as it uses hardware image rejection so it can be used like a normal spectrum analyzer (the BB60C should not be used for noise measurements below about 100 kHz).
Another thing to note is that the preamplifier is off for sweeps that start below 500 kHz or so. So you will not have maximum sensitivity at low frequencies.Unless video bandwidth is much lower than resolution bandwidth, our image rejection algorithm (a minimum of 2 measurements function) will not preserve average noise energy.
My recommended settings are what I would recommend using. And if you decide to try a BB60C, you can avoid the problem entirely.We use two different IF frequencies when software image/spur reject is active and span is 200 kHz or less: 2.9 MHz and 10.7 MHz. The image frequency is [RF frequency] + 2 * [IF frequency]
Internal measurements will be made with the 2.9 MHz IF, followed by the 10.7 MHz IF. The lower reading for each frequency bin is displayed.
If you disable spur reject, only the 10.7 MHz IF will be used.The BB60 or SM200 is a better choice for noise measurements, since the image is rejected in hardware, and software spur reject is off by default.
Above 200 kHz span, only the 10.7 MHz IF is used, and the LO is injected first low side, then high side. The noise figure of the 10.7 MHz IF is maybe 5 dB higher, so this can affect measurements where the level of noise present is low.
As long as your VBW is much lower than your RBW, you can still make decent noise measurements, with the caveat that you’re measuring the combined noise from RF and image.
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