- PMargulis October 2, 2021 at 4:49 am
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.
Justin Crooks October 4, 2021 at 9:44 am
- This reply was modified 9 months ago by Justin Crooks.
The 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.PMargulis October 5, 2021 at 6:42 am
Thank you for details. I believe that knowing the right device setup for my measurement should help me, although measurement itself became more complicated.
It is very strange that the device, which are positioned and defined in its data sheet as a spectrum analyzer, can’t do well one of the typical tasks of spectrum analyzers, a measurement of noise.
I performed a measurement of noise with the SA44B setup as you described and with a correction of 3 dB for image energy.
I got values, which are by about 1 dB higher than values that I get with a span of 200 MHz and with no any correction.
1. Could you help me to understand this effect, please?
2. Which one of measurement results should I believe more?
Thank youJustin Crooks October 5, 2021 at 9:12 am
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.PMargulis October 6, 2021 at 10:44 am
One more question.
I did measurements with following setup (as you suggested): span 100 KHz, RBW 1 KHz, VBW 30 Hz, power, average, marker noise is at the center frequency, reducing result of marker noise by 3 dB.
Measurements were done at following center frequencies [MHz]:
10, 20, 30, 40, 50, 60, 70, 80, 90, 100
Please, provide a center frequency of the image (that noise energy at it is added) for all of them (10 values).
This will help me in interpretation of my results.
Thank youJustin Crooks October 6, 2021 at 4:13 pm
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.PMargulis October 7, 2021 at 10:54 am
OK, I am advancing in understanding of what happens there and it helps me to understand why I got results as I got..
Some more, please:
1. What IF/s frequency is/are used for spans of more than 200 KHz? Is it/them remain the same for spans up to 200 MHz?
2. What is the reason to select for spans below 200 KHz the better (lower) measured value between results with two different IF-s, why lower reading value is declared to be more truthful, what philosophy stands behind this approach?
Thank youJustin Crooks October 8, 2021 at 9:49 am
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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