Just to clarify one point: How do you define the start of the transition region? Do you go from the 3 dB corner frequency, or something else, like the equiripple bandwidth of the FIR filter?
On Wed, 19 Feb 2025 at 13:11, Marcus D. Leech <patchvonbr...@gmail.com> wrote: > On 18/02/2025 21:45, Brendan Horsfield wrote: > > Point taken. At this stage we are mainly interested in straight IQ > recording & playback with minimal processing. However, in the future it > would be desirable to be able to display a real-time spectrum trace & > waterfall plot during recording/playback, using GNU Radio or something like > it. > > As you suggest, I am assuming our host machine will need a dual-10Gbe > adaptor card and a high-spec CPU, memory, SSD etc. This is a complex > procurement exercise all by itself. > > My understanding (and I haven't played with them) is that NVME SSDs are > among the fastest. Performance up to a few GByte/Sec > on write is possible, although I don't know if it can be sustained at > those rates, or whether it's "bursty". > > I've been able to produce "real-time" spectral displays on 10yo dual-Xeon > hardware at 100Msps, but only using the > kind of "stuttered" display approach that Gnu Radio FFT displays often > use, where most of the data is discarded. Often, > that's all that's needed to show a quick summary of the spectrum. > > On your other question, about transition bandwidth, I don't have a direct > answer, but on an N310 I measured the roll-off > as a fraction of the overall bandwidth, and it is about 12.5%. That > doesn't necessarily translate to the X310, but the > DDC implementation is of the same generation. > > > > > > On Wed, 19 Feb 2025 at 10:58, Marcus D. Leech <patchvonbr...@gmail.com> > wrote: > >> On 18/02/2025 19:26, Brendan Horsfield wrote: >> >> I thought your name sounded familiar! 🙂 >> >> Overall the X310+UBX-160 appears to be a good fit to our requirements. >> My original question was really about ensuring that our host PC & network >> interface have sufficient bandwidth to ingest the IQ data from a pair of >> UBX-160s. It would be nice (although not essential) if we could run one >> channel at 100 Msps, and the other at 200 Msps, to reduce the bandwidth >> requirements on the backend hardware. >> >> You'd need to have separate streamers to support two different sample >> rates, and two 10Gbe interfaces. >> >> But in terms of "what kind of PC hardware do I need?". There's no >> closed-form answer to that question. There's no >> handy-dandy "engineering worksheet" that tells you how much "grunt" you >> need for different DSP "flows" at >> a given sample-rate--so very much depends on what you're doing, and how >> you're doing it. Generally, as you scale up >> in sample-rate, you have to scale up in: >> >> o CPU base clock rate >> >> o Memory bandwidth >> >> o Number of CPUs >> >> >> >> >> >> >> >> On Wed, 19 Feb 2025 at 10:17, Marcus D. Leech <patchvonbr...@gmail.com> >> wrote: >> >>> On 18/02/2025 19:13, Brendan Horsfield wrote: >>> >>> Thanks for the suggestion about the noise source -- that's what I would >>> normally do. Unfortunately I haven't actually purchased the hardware yet >>> -- I was hoping to clarify this issue before raising a purchase order. >>> >>> Perhaps I should follow this up with one of the application engineers at >>> NI? They might have access to an X310+UBX-160 system that they can use to >>> answer my question directly. >>> >>> Thanks again for your help in this matter. >>> >>> Regards, >>> Brendan. >>> >>> I actually do work for NI on USRP devices (on a very very very part-time >>> basis). My X310 is currently elsewhere, and not populated >>> with a UBX-160. >>> >>> >>> >>> >>> On Wed, 19 Feb 2025 at 09:55, Marcus D. Leech <patchvonbr...@gmail.com> >>> wrote: >>> >>>> On 18/02/2025 18:45, Brendan Horsfield wrote: >>>> >>>> Yes, I assumed that was the case. However, it is not clear from the >>>> X300 documentation how sharp those filters are. Can you tell me how wide >>>> the transition band is at the lower sample rates? >>>> >>>> To give you some context, I would like to use an X300 (or X310) with a >>>> UBX-160 daughterboard to digitise the entire 2.4 GHz Wi-Fi band, which is >>>> 83.5 MHz wide. Ideally I would like to use a sample rate of 100 Msps to >>>> minimise the data rate between the USRP and the host PC. However, before I >>>> do this I need to be certain that the usable bandwidth at this sample rate >>>> will be greater than 83.5 MHz. Is this information documented somewhere? >>>> >>>> >>>> It somewhat depends on the decimation. If the decimation has a factor >>>> of two or 4, the edge roll-off is fairly sharp. Otherwise, >>>> there's a half-band filter in-place that causes a less-desirable >>>> pass-band. >>>> >>>> But I don't know, precisely, what the transition band is in the "nicer" >>>> filter shapes. >>>> >>>> >>>> If you have an X310+UBX-160, you can always just use a noise source, >>>> and measure it yourself to see if it's appropriate for >>>> your application. >>>> >>>> >>>> >>>> >>>> On Tue, 18 Feb 2025 at 23:11, Marcus D Leech <patchvonbr...@gmail.com> >>>> wrote: >>>> >>>>> There will always be some edge roll off. Decimation includes filtering >>>>> and those filters cannot be infinitely steep. >>>>> Sent from my iPhone >>>>> >>>>> > On Feb 18, 2025, at 2:12 AM, Brendan Horsfield < >>>>> brendan.horsfi...@vectalabs.com> wrote: >>>>> > >>>>> >  >>>>> > Hi All, >>>>> > >>>>> > I have a question about the usable bandwidth of the X300 USRP / >>>>> UBX-160 daughterboard combo at sampling rates below 200 Msps: >>>>> > >>>>> > As I understand it, the UBX-160 receiver has an analog (hardware) >>>>> filter before the ADC that limits the usable bandwidth to 160 MHz, while >>>>> the ADC runs at 200 Msps. Therefore the usable bandwidth is around 80% of >>>>> the sample rate. >>>>> > >>>>> > My question is: What is the usable bandwidth at lower sampling >>>>> rates? Does the 80% factor always apply? >>>>> > >>>>> > For example, if I set the decimation factor to 4, so that my >>>>> sampling rate is 50 Msps, does this mean that the usable bandwidth will be >>>>> 40 MHz? >>>>> > >>>>> > Thanks & Regards, >>>>> > Brendan. >>>>> > >>>>> > _______________________________________________ >>>>> > USRP-users mailing list -- usrp-users@lists.ettus.com >>>>> > To unsubscribe send an email to usrp-users-le...@lists.ettus.com >>>>> >>>> >>>> >>> >> >
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