mikesParticipantmikes February 7, 2017 at 10:34 am
We have a custom application that performs swept analysis from about 20 MHz to 6 GHz once / second (currently 25 kHz resolution) and hands off signals of interest to IQ streaming for durations from just a few seconds to several minutes. The IQ streaming is typically 10 MHz BW or less, but we’re interested in potentially larger BW (20 MHz) recordings. We currently have one of the BB60C devices and are evaluating its performance for swept analysis, which must meet certain performance requirements. Assuming the swept analysis performance is adequate (which we believe it will be), what is the practical limit of multiple BB60C IQ streams on the same PC? It appears that the IQ streaming rate is fixed from the device to the PC (80 Msps of 14 bit real? or about 160 MB/s?). We are assuming that 2 BB60C streams could be handled by a single USB 3.0 port, with 4 USB 3.0 ports being able to handle 7 simultaneous BB60C IQ streams and 1 BB60C sweeper. We also know that there are significant CPU resources required, along with storage I/O bandwidth to sustain the recording (7 10 MHz recordings requiring 8 bytes / sample at 20 MSps rate ~= 1.12 GB/sec). Has this type of configuration (or similar) been tested in your facility or by others? We’re attempting to understand what the maximum number of IQ streams can be supported with a single PC before actually purchasing several BB60C + expensive storage subsystem. Thanks!
AndrewModeratorAndrew February 7, 2017 at 11:07 am
Good question. We have several customers who have developed 2 receiver systems. The most common issue users see is high CPU usage. Each IQ stream is going to consume ~10% usage on a desktop Intel processor and customers report that it doesn’t scale linearly, i.e. 2 receivers will generally consume more CPU usage than 2 X 1 receiver. We don’t have much experience above 2 receivers. I suspect you will run into stability issues regarding USB transfer and will start experiencing disconnects/data loss. I’m guessing, that a very high end desktop system could support 4 receivers with minimal issue.
Let me know if you have more specific questions. I apologize I can’t be more concrete here, these types of solutions are going to be very dependent on the hardware used and software requirements in the final configuration.
You can also email me at aj at signalhound dot com if you would like to discuss your ideas in more detail.
catalin_roParticipantcatalin_ro April 7, 2017 at 8:09 am
I’m probably one of the guys that Andrew mentioned by “several customers”! I’ve just seen your question, so sorry for the delay in my reply!
Briefly said, we currently have a platform involving 2 BB60Cs and a simple (FTDI serial port basically!) device that we operate through a 10m USB3 active cable and a 4-port USB3 hub and we saw no major issues. But in order to get to the current structure, that can be connected to several PCs and shows no significant performance differences, we ended up with a box of useless USB3 hubs!
With two BB60Cs there is a rather small performance regression with latest APIs. At some point in the past it was terrible but Andrew sorted it out! And for 7 BB60Cs you need to consider a pretty powerful CPU.
I will do some tests with even more BB60Cs connected at once to the same PC as I am involved in a project where I need 14 units. Clearly I won’t go that far, as the project doesn’t require them in just one place, but I will certainly do some tests with as many as possible connected to the same PC.
Just as a warning, depending on the motherboard chipset and/or USB hub, you might see periodic errors while streaming, with the end result of missing samples. With similar structures I testes on several platforms and so far I could not find a general rule. But, on the platform where I see it happening, it tends to happen also when only a single BB60C is connected directly to the PC.
Anyway, our “nightmare” is that we need those two BB60Cs to be in sync!
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