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Newbie n6208g
Newbie
8,130 Views
Registered: ‎01-21-2011

High Speed ADC FFT ysubg Spartan

Spartan 3E 100 MSPS ADC/FFT

I want to build a device to take 32268 16-bit analog samples at approximately 100MSPS and perform an FFT. The FFT does not have to be done in real-time but it would be nice. I was thinking of using a Spartan-3E with some sort of Parallel LVDS ADC and adding some external RAM and taking samples, then transferring the data to some other processor to do the FFT (SHARC, XMOS, ???).
Is a Spartan 3E or 3A a bood choice for this peoject?  If not, what would you recommend?
Any suggestions would be greatly appreciated.

Thanks

John Battle\
John Battle
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13 Replies
Professor
Professor
8,117 Views
Registered: ‎08-14-2007

Re: High Speed ADC FFT ysubg Spartan

If I were going to do this I'd use a PNX1702 processor from NXP and feed the parallel data into

the video capture port using the "FGPI" (fast general purpose input) unit.  If you're already

married to some other DSP processor, I would guess you know more than I do about interfacing

the signals to that processor.  If you're not going to do the FFT in the FPGA, and this is a newer

design I'd look first at the Spartan 3A series (a bit cheaper for small parts with higher IO count).

You could also use a Spartan 3A DSP device and do the FFT in the FPGA, but again that

depends on what you plan to do with the data after that, and whether your system needs

a processor anyway.  For just taking data and storing / forwardning it to the processor,

almost any device would be adequate.  For external memory, the choice will depend on how many

samples you need to buffer, but a single 16-bit wide DDR2 memory part has more than

enough bandwidth.

 

Regards,

Gabor

-- Gabor
Instructor
Instructor
8,112 Views
Registered: ‎07-21-2009

Re: High Speed ADC FFT ysubg Spartan

1.  If you don't need the FFT result in realtime, have you considered simply buffering the data locally and transmitting the data to another number-crunching device for processing (for example, a PC)?  Designing number crunchers in PC SW (or even use Excel) (rather than hardware) is much cheaper and simpler.

 

2.  If you decide to perform FFT locally, using a static RAM (rather than DRAM) will simplify your FPGA and circuit board designs considerably.

  • Access latency is completely predictable.  This makes number-crunch pipeline design simpler.
  • If storing matrices or tables, row vs. column access latency is identical
  • No termination supply
  • Much simpler memory controller (or none at all)

3.  When selecting a local memory, remember that you will need sufficient bandwidth for all concurrent operations

  • buffering incoming data
  • fetching operand data
  • storing (final and interim) result data
  • reading result data for transmission

 

- Bob Elkind

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Observer learner1
Observer
7,996 Views
Registered: ‎11-12-2010

Re: High Speed ADC FFT ysubg Spartan

As far as I know Spartan 3E, 3AN supports 1.5 MHz sampling on board for ADC and DAC.

That is signals up to 750 KHz can be sampled.

I think you need to have external ADC then try to operate it on the board.

 

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Historian
Historian
7,990 Views
Registered: ‎02-25-2008

Re: High Speed ADC FFT ysubg Spartan

 


@learner1 wrote:

As far as I know Spartan 3E, 3AN supports 1.5 MHz sampling on board for ADC and DAC.

That is signals up to 750 KHz can be sampled.

I think you need to have external ADC then try to operate it on the board.

 


 

Your statement makes absolutely no sense.

It's a piece of cake to use a 50 MHz converter with a Spartan 3AN device.

----------------------------Yes, I do this for a living.
Instructor
Instructor
7,986 Views
Registered: ‎07-21-2009

Re: High Speed ADC FFT ysubg Spartan

As far as I know Spartan 3E, 3AN supports 1.5 MHz sampling on board for ADC and DAC.

That is signals up to 750 KHz can be sampled.

I think you need to have external ADC then try to operate it on the board.

This is one of the most incorrect and misleading posts I've read in quite a while, including my own posts -- and that says quite a bit.

 

+1 to bassman's response.

 

- Bob Elkind

SIGNATURE:
README for newbies is here: http://forums.xilinx.com/t5/New-Users-Forum/README-first-Help-for-new-users/td-p/219369

Summary:
1. Read the manual or user guide. Have you read the manual? Can you find the manual?
2. Search the forums (and search the web) for similar topics.
3. Do not post the same question on multiple forums.
4. Do not post a new topic or question on someone else's thread, start a new thread!
5. Students: Copying code is not the same as learning to design.
6 "It does not work" is not a question which can be answered. Provide useful details (with webpage, datasheet links, please).
7. You are not charged extra fees for comments in your code.
8. I am not paid for forum posts. If I write a good post, then I have been good for nothing.
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Xilinx Employee
Xilinx Employee
7,961 Views
Registered: ‎11-28-2007

Re: High Speed ADC FFT ysubg Spartan

I guess learner1 was referring to the ADC/DAC on the Spartan3E/A starter kits, which indeed are low speed converters. It looks like the OP (n6208g) is building his own board, so there shouldn't be any problem choosing converters with much higher sampling rate.

Cheers,
Jim
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Highlighted
Visitor sa_penguin
Visitor
7,418 Views
Registered: ‎03-04-2012

Re: High Speed ADC FFT ysubg Spartan

bassman59:

I'm sure its a piece of cake for You to design a PC board, with Spartan 3AN and 50Msps 12+ bit ADC. Me? Not so much.

 

I found this thread by searching for FPGA's with ADC's. Most kits don't seem to have one - they expect you to buy an extra card. Fair enough - except the ADC cards seem to START at over 200Msps, have multiple channels, and cost $$_Lots_$$.

 

As learner1 noted, thet kits that DO have onboard ADC tend to "wimp out" at about 750KHz. The few that don't - go straight to top end again, with prices to match.

 

At the "sweet Spot" of 50Msps, before clock speeds etc. make PCB design a major headache, there is... a vacancy.  If you know of such a FPGA/ADC board, I'd like to buy one. Even if it's a barebones board I have to populate myself.

Tags (1)
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Instructor
Instructor
7,414 Views
Registered: ‎07-21-2009

Re: High Speed ADC FFT ysubg Spartan

penguin,

 

For discussing selection of low-cost boards with converters, this is the wrong thread and the wrong forum.

 

-- Bob Elkind

SIGNATURE:
README for newbies is here: http://forums.xilinx.com/t5/New-Users-Forum/README-first-Help-for-new-users/td-p/219369

Summary:
1. Read the manual or user guide. Have you read the manual? Can you find the manual?
2. Search the forums (and search the web) for similar topics.
3. Do not post the same question on multiple forums.
4. Do not post a new topic or question on someone else's thread, start a new thread!
5. Students: Copying code is not the same as learning to design.
6 "It does not work" is not a question which can be answered. Provide useful details (with webpage, datasheet links, please).
7. You are not charged extra fees for comments in your code.
8. I am not paid for forum posts. If I write a good post, then I have been good for nothing.
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Historian
Historian
7,405 Views
Registered: ‎02-25-2008

Re: High Speed ADC FFT ysubg Spartan


@sa_penguin wrote:

bassman59:

I'm sure its a piece of cake for You to design a PC board, with Spartan 3AN and 50Msps 12+ bit ADC. Me? Not so much.

 


 

Well, it's what I do for a living.

 

 


I found this thread by searching for FPGA's with ADC's. Most kits don't seem to have one - they expect you to buy an extra card. Fair enough - except the ADC cards seem to START at over 200Msps, have multiple channels, and cost $$_Lots_$$.

 

As learner1 noted, thet kits that DO have onboard ADC tend to "wimp out" at about 750KHz. The few that don't - go straight to top end again, with prices to match.

 

At the "sweet Spot" of 50Msps, before clock speeds etc. make PCB design a major headache, there is... a vacancy.  If you know of such a FPGA/ADC board, I'd like to buy one. Even if it's a barebones board I have to populate myself.


I think the lack of such products is based on the intended use. For the most part, the users of these converters (and the higher-speed ones, too) have custom packaging and interconnection and such requirements. A "kit" doesn't help us, so boards are designed.

 

Another reason is that a lot of these high-end converters may end up being used as part of a proof-of-concept design, as a way for the engineer to work out getting the parts to work in an application (or at least a bench-test approximation) before committing to an expensive PCB.

 

But remember that converter speeds are always increasing. Once upon a time, that 50 Ms/s converter was the high end for which expensive evaluation kits were produced. Now they are mainstream, and the kit vendors realize that the users of such parts understand them and can plop them onto a board and go with it. I haven't seen an FPGA board with that sort of conveter on it in ages.

The low-speed stuff is included on kits because it's easy. The point of the starter kits is for people to learn how to use FPGAs, and running a simple, inexpensive, relatively low-speed converter with an FPGA is pretty common. 

So, I'm sorry if it's difficult for you to design a board with a 50 Ms/s 12-bit converter on it. if that's a requirement, then you have two choices: build it yourself, or hire someone to build it for you. Expect to pay for it. This is the way it goes.

----------------------------Yes, I do this for a living.
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Instructor
Instructor
3,930 Views
Registered: ‎07-21-2009

Re: High Speed ADC FFT ysubg Spartan

So, I'm sorry if it's difficult for you to design a board with a 50 Ms/s 12-bit converter on it. if that's a requirement, then you have two choices: build it yourself, or hire someone to build it for you. Expect to pay for it. This is the way it goes.

 

Bassman is correct on all points.  Unless you need your own production design, it's a big savings in time, money, and risk to purchase a ready-made ADC board which is too fast and too good for your purposes.

 

And this is still the wrong thread and forum for this discussion!

 

-- Bob Elkind

SIGNATURE:
README for newbies is here: http://forums.xilinx.com/t5/New-Users-Forum/README-first-Help-for-new-users/td-p/219369

Summary:
1. Read the manual or user guide. Have you read the manual? Can you find the manual?
2. Search the forums (and search the web) for similar topics.
3. Do not post the same question on multiple forums.
4. Do not post a new topic or question on someone else's thread, start a new thread!
5. Students: Copying code is not the same as learning to design.
6 "It does not work" is not a question which can be answered. Provide useful details (with webpage, datasheet links, please).
7. You are not charged extra fees for comments in your code.
8. I am not paid for forum posts. If I write a good post, then I have been good for nothing.
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Advisor joelby
Advisor
3,918 Views
Registered: ‎10-05-2010

Re: High Speed ADC FFT ysubg Spartan

If this is for a prototype or proof of concept, you're not already an experienced PCB designer, you don't have an unlimited amount of free time, and you have a few hundred dollars to spare, don't even think about designing your own PCB. I would recommend having a look at Linear Technology's range of ADC demo boards. The LTC2194 seems to fit your criteria (105 MSPS, 16-bit, dual channel - though you said both 12-bit and 16-bit and didn't say how many channels you needed) and its corresponding demo board DC1532A-B costs $200. I guarantee that this is less than it will cost to design and build one yourself, if you've never done it before.

 

These demo boards have an FMC interface, which is compatible with boards like the SP601 ($295). This board includes a Spartan-6 which is a fair bit nicer than the Spartan-3* parts for high speed deserialisation, and you can easily perform the FFT on the FPGA rather than using a separate processor. The only other thing you'll need is a good ADC clock source. LTC also sell these, or you could use a signal generator or something.

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Visitor sa_penguin
Visitor
3,909 Views
Registered: ‎03-04-2012

Re: High Speed ADC FFT ysubg Spartan

Thanks for the clarification, bassman59.

 

Like the OP, I suspect I'll have to make my own board - eventually. As an FPGA newbie, I reckon I should START with one of the cheaper, low-end boards and try tacking an ADC on that. 

 

Papilio looks like a good starting point. Wish me luck.

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Advisor joelby
Advisor
3,906 Views
Registered: ‎10-05-2010

Re: High Speed ADC FFT ysubg Spartan

Actually, I would strongly advise against the Papilio if you want to use it with a high speed ADC. Your task will be far easier if you use a development board that has matched (in length and physical position) differential pairs, controlled impedance traces, and a high speed connector with lots of grounds. The Papilio has none of these as it is not really designed for high speed applications.

 

Some recommendations:

 

  • SIOI FPGA boards - $49/$59. Fairly bare bones with no on-board peripherals, but it's no worse than the Papilio in this regard. It is superior in all other ways (faster, better FPGA, DDR memory), though it doesn't have an integrated JTAG programmer.
  • Digilent Nexys3 - $119 (academic price). Larger FPGA than the SIOI boards, and a range of nice on-board I/O.

 

The Digilent Nexys2 is slightly cheaper but I would spend more for the newer board, especially if you're doing high speed deserialisation. If your budget will stretch to it, the Digilent Atlys is even better and has enough on-board toys to keep you busy for a long time.