<div dir="ltr">Hi Jayce,<div><br></div><div>Yep, so if you give the LWA 392 channels and an LO offset of -0.00000588, and the VLA 320 channels, then you should have everything matched up.</div><div><br></div><div>Cheers,</div><div>Adam</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, 9 Feb 2021 at 04:18, Jayce Dowell <<a href="mailto:jdowell@unm.edu">jdowell@unm.edu</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi Adam and Chris,<br>
<br>
The VLA is outputting 8 MHz of bandwidth centered at 76 MHz and the LWA <br>
is 9.8 MHz of bandwidth centered at (76 MHz - 5.588 mHz). To answer <br>
Chris' earlier question, yes, both data sets are in VDIF format (if I <br>
can figure out how to correctly convert the LWA data).<br>
<br>
Jayce<br>
--<br>
On 2/7/21 3:33 PM, Adam Deller wrote:<br>
> * [EXTERNAL]*<br>
> <br>
> **<br>
> Hi Jayce,<br>
> <br>
> Just to expand on what Chris wrote, if you give the exact centre <br>
> frequency and bandwidth for the LWA and VLA, it would be easy to let you <br>
> know how DiFX can be set up to support correlation of the over-lapping <br>
> portion of the bands. As Chris said, this would be implemented using <br>
> "zoom" bands (which select a subset of channels) after configuration the <br>
> spectral resolution of the VLA and LWA to match. If the centre <br>
> frequencies are convenient, then this can be done pretty easily. If <br>
> they are separated by an unfortunate amount, though, you need to either <br>
> go to very high spectral resolution (in order to get the centre <br>
> frequencies of the channels to match, in addition to their widths) or <br>
> alternatively one can use the "LO offset" functionality in DiFX to <br>
> effectively tweak the centre frequency of one of the bands slightly to a <br>
> more convenient value, enabling lower spectral resolution to be used.<br>
> <br>
> Cheers,<br>
> Adam<br>
> <br>
> On Fri, 5 Feb 2021 at 09:11, Phillips, Chris (CASS, Marsfield) <br>
> <Chris.Phillips@csiro.au> wrote:<br>
> <br>
> Hi Jayce<br>
> <br>
> I'm working the data from the Long Wavelength Array and I am trying<br>
> to see if we can move from our homegrown correlator to DiFX. The<br>
> two modes I need to support are a LWA-only mode with homogeneous<br>
> data and an "eLWA" mode with heterogeneous data that combines the<br>
> LWA stations with the VLA 4-band system. For the LWA-only mode I<br>
> have 4+4-bit complex voltage data from two 19.6 MHz wide spectral<br>
> windows. I am currently converting these data from the LWA format<br>
> into two VDIF files, one per window, for correlation with DiFX. My<br>
> .input file describes the data as:<br>
> ><br>
> > DATA FORMAT: INTERLACEDVDIF/0:1<br>
> > QUANTISATION BITS: 4<br>
> > DATA FRAME SIZE: 7872<br>
> > DATA SAMPLING: COMPLEX_DSB<br>
> > DATA SOURCE: FILE<br>
> ><br>
> <br>
> I have never personally tested complex_dsb data with interlaced<br>
> vdif. I have no idea if there could be any “interaction” with the<br>
> VDIF interleaving and the specific data type (I would have thought<br>
> not, but you never know).<br>
> <br>
> The issue could also be with interpreting the interlaced VDIF, not<br>
> the complex double sideband.<br>
> <br>
> Once you have tested using trunk, not the released version, I would<br>
> try and use some of the vdifio utilities to try and convert the<br>
> interlaced VDIF into plain VDIF - either extracting out one thread,<br>
> or merging into multichannel single thread VDIF. You would obviously<br>
> need to update the DIFX .input (and or .v2d) file and vexfile to<br>
> match the new setup.<br>
> <br>
> I would also be using the m5access tools to do things like make the<br>
> autocorrelation (both before and after fiddling with the data<br>
> layout). Check DIFX gives the same auto shape as you get with<br>
> m5spec. If you use a modulated noise source, folding the data<br>
> (m5fold) on the period of the noise cal can be very informative<br>
> also. Also look at the headers and check that the # frames/sec etc<br>
> are what you expect (before and after merging threads etc). If you<br>
> decide it is the interlacing is the issue, I suspect you will need<br>
> to discuss with Walter.<br>
> <br>
> <br>
> > I am less sure about DiFX supporting our eLWA mode. For this we<br>
> have the same 4+4-bit complex voltage data from the LWA stations but<br>
> with a bandwidth of 9.8 MHz. We are combining this with 4-bit real<br>
> voltages from the VLA with 8 MHz of bandwidth and a slightly<br>
> different frequency setup. We currently process these data by<br>
> shifting the center frequency of the LWA data to match that of the<br>
> VLA, running two F-engines with different channelization to arrive<br>
> at the same channel width for both data sources, and then cross<br>
> correlate what overlaps. Is this mode something that DiFX supports?<br>
> ><br>
> <br>
> Can you represent this data as VDIF? If you you can probably handle<br>
> the different setup using zoom bands. However if this can be done<br>
> will really depend on the EXACT setup. Basically you need to be able<br>
> to channelise the two setups with the same final frequency<br>
> resolution (ie different FFT sizes will be needed, with the ratio of<br>
> the FFT size matching the ratio of the bandwidth difference. But the<br>
> sky frequency of the frequency points out of the FFT need to match<br>
> exactly (ie even if you could achieve the same frequency resolution,<br>
> if the fine channels are offset by 1/3 of a channel you will have<br>
> problems (if that offset is a simple integer ratio, you can just use<br>
> a higher frequency resolution)<br>
> <br>
> I hope this makes sense.<br>
> <br>
> Cheers<br>
> Chris<br>
> <br>
> <br>
> <br>
> -- <br>
> !=============================================================!<br>
> A/Prof. Adam Deller<br>
> ARC Future Fellow<br>
> Centre for Astrophysics & Supercomputing<br>
> Swinburne University of Technology<br>
> John St, Hawthorn VIC 3122 Australia<br>
> phone: +61 3 9214 5307<br>
> fax: +61 3 9214 8797<br>
> <br>
> office days (usually): Mon-Thu<br>
> !=============================================================!<br>
<br>
</blockquote></div><br clear="all"><div><br></div>-- <br><div dir="ltr" class="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr" style="font-size:12.8px"><div dir="ltr" style="font-size:12.8px"><div dir="ltr" style="font-size:12.8px"><div dir="ltr" style="font-size:12.8px"><div dir="ltr" style="font-size:12.8px">!=============================================================!<br><div dir="ltr" style="font-size:12.8px">A/Prof. Adam Deller </div><div dir="ltr" style="font-size:12.8px">ARC Future Fellow</div></div><div style="font-size:12.8px">Centre for Astrophysics & Supercomputing </div><div dir="ltr" style="font-size:12.8px">Swinburne University of Technology <br>John St, Hawthorn VIC 3122 Australia</div><div style="font-size:12.8px">phone: +61 3 9214 5307</div><div style="font-size:12.8px">fax: +61 3 9214 8797</div><div style="font-size:12.8px"><br></div><div style="font-size:12.8px">office days (usually): Mon-Thu<br>!=============================================================!</div></div></div></div></div></div></div></div></div></div></div></div></div></div></div>