Hi Hari, Good point regarding RFI below 25 MHz, though in coming years more and more AM stations may go off air ! It may be hard to find high quality, inexpensive mixers for this application having a very high dynamic range (>60dB). Perhaps Dan's suggestion of using quadrature sampling is a more cost-effective way to implement this, provided red pitaya supports that mode.
Jishnu From: "Hariharan Krishnan" <vasanthikrishh...@gmail.com> To: "casper" <casper@lists.berkeley.edu> Sent: Monday, February 10, 2020 4:33:32 PM Subject: Re: [casper] Solar Spectrometer Channeliser Hi Jishnu, Down-conversion is inevitable with red pitaya given the max. sampling frequency is 125 MSPS for the on-board ADC (for the 20 - 80 MHz RF band here). The dynamic range of the mixer used in the down-conversion becomes rather important here primarily because of the RFI channels (AM transmission below 25 MHz) that will introduce IMD products. Otherwise I wouldn't expect the dynamic range for a relatively clean band to go down at single-stage down-conversion. Regards, Hari On Mon, Feb 10, 2020 at 1:00 AM Jishnu Nambissan T < [ mailto:jis...@rri.res.in | jis...@rri.res.in ] > wrote: Hi Mugundhan, With red pitaya, can you sample 20-80 MHz without downconversion ? And if analog down conversion is required, wouldn't that restrict the usable dynamic range (unless a high level mixer is used) ? Jishnu From: "Mugundhan vijayaraghavan" < [ mailto:v.vaishnav151...@gmail.com | v.vaishnav151...@gmail.com ] > To: "casper" < [ mailto:casper@lists.berkeley.edu | casper@lists.berkeley.edu ] > Sent: Thursday, February 6, 2020 1:06:09 AM Subject: Re: [casper] Solar Spectrometer Channeliser Hi Dan, We have done integration times from 20ms upto a second. There are bursts that last for hours, minutes and a few ms to seconds as well. We have not tried AGC on these bursts since we were aiming to study them only. In the 8 bit design, we had 3 bits for background and remaining 5 to accommodate bursts. The red pitaya can still be used if Colm can restrict the band to 20-80 mhz, because the ionosphere starts cutting of anything below 20mhz mostly(depending on the location of course). Then this can be down converted to fit into 0-62.5 mhz base band of the red pitaya. Thanks, Mugundhan On Wed, 5 Feb 2020, 22:23 Dan Werthimer, < [ mailto:d...@ssl.berkeley.edu | d...@ssl.berkeley.edu ] > wrote: BQ_BEGIN hi mugundhan, what's the time scale for these bursts? rise and fall times? can you use a AGC circuit (automatic gain control), eg: computer controlled attenuator to turn down the power going into the ADC during the bursts, so you could keep the levels going into the ADC relatively constant? if the rise and fall times are longer than 1ms (the integration time of the spectrometer), then you could adjust the power level for each spectrum, and write down where you set the attenuator for that spectrum, so you could still know the absolute power. if not, there are some 14 bit 200 Msps ADC boards, and i think the new RFSOC boards/chips have 14 bit ADC's, but you'll have to write a casper yellow interface block for this ADC, as we don't have a 14 bit 200 Msps ADC in the casper library. another possiblity is to multiplex your 80 MHz band, 40 MHz at a time into a red pitaya board, ping ponging back and forth between bands: 0 to 40 MHz for 1 ms, then 40 to 80 MHz for the next ms. best wishes, dan On Wed, Feb 5, 2020 at 8:33 AM Mugundhan vijayaraghavan < [ mailto:v.vaishnav151...@gmail.com | v.vaishnav151...@gmail.com ] > wrote: BQ_BEGIN Hi Dan, Usually quiet sun doesn't show such abrupt changes, but bursts do (easily 40-50dB or more) for bright bursts. We have built 8 bit spectrometers in 40-80Mhz, but have found then when the burst is pretty strong, saturation effects starts kicking in. Thanks, Mugundhan On Wed, 5 Feb 2020, 21:52 Dan Werthimer, < [ mailto:d...@ssl.berkeley.edu | d...@ssl.berkeley.edu ] > wrote: BQ_BEGIN hi colm, regarding dynamic range --------------------------------- for your solar spectrometer, do you need 14 bits of ADC dynamic range? it's very unusual in radio astronomy to need that much instantaneous dynamic range on the input. does the sun vary on short time scales in the radio band by factor of 1000 in voltage (1,000,000 in power) ? or do you have very strong bursting RFI that is 1000 times the average noise voltage (1M in power) in the whole band? as you probably know, you'll have lots more dynamic range in the output power spectrum than the dynamic range of the ADC: if you are building a 1024 channel spectrometer with 1 ms integration, you'll get about 8 bits more bits of dynamic range above your ADC dynamic range in frequency domain voltage, which is 16 bits more of dynamic range above your ADC dynamic range in power spectra. so you'll have about 20 bits of spectral dynamic range if you use an 8 bit ADC, (power spectrum dynamic range of about 1 million in 1 ms with an 8 bit ADC, setting noise at 3 bit RMS). and 24 bits of spectrral dynamic range for a 10 bit ADC, 28 bits for 12 bit ADC, and 32 bits for for 14 bit ADC). regarding boards for your spectrometer --------------------------------------------------- 1) as adam pointed out, the red pitaya is cheap, but sample rate and bandwidth don't quite get the specs you need. 2) another possibility is to use a snap board, which costs more, but can sample 3 inputs at 950 Msps, or 6 inputs at 500 Msps, or 12 inputs at 250 Msps with 8 bit ADC's. most people populate the snap board with 8 bit ADCs, but a few people have populated it with 12 bit ADC's, although the sample rate goes down by 8/12. 3) another possibility is to use a xilinx RFSOC board. the first gen has a bank of 12 bit ADC's (8 inputs at 4 Gsps, or 16 inputs at 2 Gsps), but i think the new generation has 14 bit ADC's ? the RFSOC boards cost more than snaps, but RFSOC was designed in dublin, so you can probably get one from xilinx dublin.... the ZCU111 board has not been fully casperized yet though. best wishes, dan Dan Werthimer Marilyn and Watson Alberts Chair Astronomy Dept and Space Sciences Lab University of California, Berkeley On Wed, Feb 5, 2020 at 5:06 AM Colm Bracken < [ mailto:colmbrac...@gmail.com | colmbrac...@gmail.com ] > wrote: BQ_BEGIN Hello CASPER people, We are looking to build a spectrometer with not too demanding requirements. Based on the specs below, would the Red Pitaya be up to the job do you think? Or, is there another, better suited (but similarly affordable) solution? Chanel widths: ~< 100 kHz Time sampling: ~< millisecond Polarisation: 2 channels Antenna freq range: 10-85 MHz (total bandwidth of 75 MHz) Digitisation: 14 bit Any advice on this would be great! Thanks in advance, Colm -- Dr Colm Bracken Lecturer Maynooth University Experimental Physics Maynooth University, Maynooth, Co. Kildare, Ireland. T: +353 1 708 3641 E: [ mailto:colm.brac...@mu.ie | colm.brac...@mu.ie ] W: [ http://www.maynoothuniversity.ie/ | www.maynoothuniversity.ie ] Follow my work on [ https://nuim.academia.edu/ColmBracken | https://nuim.academia.edu/ColmBracken ] And Research Associate Astronomy & Astrophysics Section School of Cosmic Physics Dublin Institute for Advanced Studies 31 Fitzwilliam Place Dublin 2, D02 XF86 T: +353 1 440 6656 ext 352 E: [ mailto:cbrac...@cp.dias.ie | cbrac...@cp.dias.ie ] W: [ https://www.dias.ie/2017/06/22/dr-colm-bracken | www.dias.ie/2017/06/22/dr-colm-bracken ] Follow my work on [ https://nuim.academia.edu/ColmBracken | https://nuim.academia.edu/ColmBracken ] -- You received this message because you are subscribed to the Google Groups " [ mailto:casper@lists.berkeley.edu | casper@lists.berkeley.edu ] " group. 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