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I'm seeing some odd-looking distortion when I get much past 10 MS/s, and even at 10 I think there may be some. 8 MS/s is very clean.

Some illustrations of what's happening:
8 MS/s. Note the signals at ~932 MHz. They look pretty sharp and clean.
10 MS/s. Still looks pretty good, though the signals look a bit "wider" than before.
11 MS/s. Getting some very obvious (and problematic) distortion here. The signal is very spread out in frequency.
12 MS/s. It's really bad now. You can see lots of harmonic (?) peaks spreading out from the central peak.

Past that, it gets no worse (or better). But it's not usable at these rates.

I don't think it's any kind of bug in my visualization or anything. I see the same harmonics when I dump the raw samples and view them in Audacity. For instance:
16 MS/s in Audacity spectrum tool

Any thoughts? Thanks.

Ahh! That's gotta be it--thanks. I was indeed on USB 2.0. I didn't realize that the data was packed into a 32-bit word instead of 24 (I know the data gets to me that way, but that's not necessarily how its transmitted across the bus...).

Offhand, do you know if the timestamp field accompanying the BLADERF_FORMAT_SC16_Q11_META mode is effectively a sample count? That would allow me to detect if I'm getting dropped samples.

Thanks. I took a look at this last night and I do see that the counter works, but I noticed that the docs say that the metadata is only decoded automatically in synchronous mode, not async (like I'm using). Well, that's no problem, I can deal with that.

The bigger concern though is that the header docs say that the first four samples out of every 1024 are overwritten with the metadata. Is that right in that the samples are lost completely? I certainly observe that out of the 32768 samples in a packet, each block of 1024 has the metadata at the beginning. It doesn't write a metadata block and then another 1024 samples; it writes the metadata and then 1020 samples.

So am I really just losing the samples? If so, that seems to be unacceptable. If instead the samples are just skipped, and I get slightly more packets per second than before, then that's fine by me. But that doesn't seem to be what the docs say (and I can't tell given my current software setup).