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Hi,
I am trying to better understand the fast sweep mode on the SM435C device (using the SDK).
My understanding is that in this mode, the device captures a single frame (max. 16384 samples) at each LO step, computes the windowed FFT, and stitches the results from each step. Since the internal sampling rate is 250MSPS, but the actual signal bandwidth is 160MHz (probably lower if the optional preselector is used under 645MHz), only 16/25 * N_FFT bins are used/valid.
Based on my experiments, the device has 6 discrete N_FFT options (power-of-two, 512 – 16384). Thus, the FFT bin frequency widths are also just a few discrete options (i.e. 250 MHz/ N_FFT). The RBW is calculated by multiplying these with the selected window’s noise bandwidth (thus, given a specific window, we also have 6 discrete truly different RBW options).
My questions:
– Is the detector setting relevant in this mode (I assume not, if a single FFT frame is created at each LO step)?
– When I configure the device with different RBWs (smSetSweepCoupling), and I query the actual results (smGetSweepParameters), it seems that many more RBWs can be set/used (the actual RBW is close to an arbitrary requested value). But, the sweep size only changes according to the discrete options I described above. Is there something I completely misunderstand in this mode?Thank you!
(BTW, I am new to this, but I enjoy the easy-to-use device and very accessible API. Kudos for making the software and documentation publicly accessible).Hi Volgy,
Nice work for being new at this! You have largely properly characterized the functionality of the fast sweep mode. There is only one more thing to know about. To accomplish “arbitrary” RBWs, you can perform FFT zero-padding. This basically gets us the ability to configure for RBWs as if we have FFT sizes between 16 (or whatever we use as the smallest) and 16384, in increments of 1, and not just 512,1024,…,16384.
And to address your other question, you are correct, the detector is not relevant for the reason you listed.
If you need more flexibility than what fast sweep mode offers, take a look at the I/Q sweep list. This mode lets you preconfigure a list of freq steps and I/Q capture sizes, and executes them at the same speed as fast sweep mode. This gets you the time domain I/Q samples as opposed to the processed spectrum data. On the SM435C, the I/Q bandwidth for each “step” is 160MHz, so you can achieve > 1THz sweeps with the right settings. For the SM435B models, the I/Q bandwidth is only 40MHz, so it falls short of 1THz per second, and in that case the fast sweep mode is providing functionality that I/Q sweep lists cannot. The downside of I/Q sweep lists is that now you have to do your own processing, but it will let you reproduce most of the sweep capabilities offered through the sweep modes.
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