I'm working with an Altera Stratix ep1s10 on a development board. The data sheet says the IO can operate at rates up to 800 MSPS. If I have a look on the internet, I see DAC and ADC technology going up to 400 MSPS.
My application is software defined radio, where the general mantra is to do as little analog front-end as possible, i.e. sample as fast as you can.
What are the limits on my conversion rates? I doubt it's even remotely possible for me to do data conversion at 400 MSPS? My main concern is the PCB layout, with about 15cm of track and a header between the FPGA and the conversion chips. Crosstalk, EMI and impedance matching are all things I know very little about.
Crossposted to s.e.d. There's no reason you can't sample at 400MS/s with the right parts and proper layout. At those speeds you do have to be very careful about layout, and differential sampling is probably a *very* good idea.
More details are really needed:
What type of header are you talking about? I have run signals at 5Gb/s through connectors designed for the task with little loss and very low crosstalk, but they aren't cheap. Is this an existing header?
Does this mean you want to run wires, or are you making a circuit board for the A-D? You are unlikely to get high data rates with handwired circuitry (you can get reasonably high speeds, but not too high unless you are an expert in wirewrap and routing ;).
You say you are using a development board. What documentation is there for it? Point us at some details if you have them.
What is the interface from your A-D converter(s)? Serial? Parallel? Parallel may take more lines, but it also gives the lowest data rate. There are also tricks of synchronised sampling (using multiple converters, suitably calibrated) to increase the effective sample rate, should that be necessary. On that subject, is there a particular A-D device you have in mind, or are you open to suggestions?
What signal frequency range are you particularly interested in getting? If you are not sure of what to use, this will let others help you choose an appropriate device.
Even if you sample with a decent A-D, you should still (in my opinion) put an anti-aliasing filter on the front end, and buffering to prevent loading. (Some parts have them internally). Unless you have a buffered output from something, you are probably going to have to put down at least some analog circuitry apart from the A-D(s).
Speaking of that, what RF device did you intend to use to get the signal in the first place?
Answer those (reply to both groups) and some of the denizens of s.e.d. will no doubt dispense some ideas, and no doubt ask more questions.
Software radio makes life a lot more interesting than regular A/D conversion.
The sorts of circuits that run at 400MSPS tend to be quite potent radio transmitters, unless you are very careful about circuit layout - classical radios had to bury the local oscillator in a shielded metal box to stop it radiating into your antenna.
Presumably this is a soluble problem, but the OP doesn't seem to know it exists, which could complicate life.
The headers are existing. They are standard beak-away headers.
I plan to have a PCB made. We have facilities for two layer PCB manufacturing.
I'm working with an Altera dev kit:
There is tons of documentaion for the NiosII embedded CPU on this dev kit but almost no documnetation on the board itself.
I'd find it simpler to do parallel, there's no shortage of pins.
I'd like to capture and generate signals anywhere from 10kHz to 100MHz. It would be great if I could reach 450MHz with some mixing at the front end. I don't really need more than 1MHz at a time. So a lower rate with a NCO + mixer might be a better bet. The basic idea to to be able to receice and transmit commercial FM and AM, plus some walkie-talkie FM, and maybe some HAM radio, all with as little HW as possible.
This is fine, I simply want to keep it to a minimum.
On a sunny day (10 Jul 2006 15:42:18 -0700) it happened firstname.lastname@example.org wrote in : classical radios had to bury the local oscillator in a shielded metal
Dunno what you call 'classical', but even my tube communications receiver in the sixties had no 'separate box' for the LO.
A RF pre-amp, folowed by a mixer would give plenty isolation. The good old 'ECH81 ?? osc mixer tubes had it all in obne tube and no RF pre. Those were commercial radios. No normal transistor radio has a boxed LO.
Modern TV tuners using dual gate MOSFETS have it all on one PCB.... Very old UHF / VHF tuners had a separate section for the LO. But the reason was likely one of tuning, after all there was 38 MHz or so offset.
It is true that with many transistor radios you can jam a AM MW station by tuning 455 kHz (or whatever) next to it, but it only works on a few meters distance.
I'll bet that there was some internal shielding within the ECH81 oscillator/mixer tube.
I'll take your word for it - I haven't opened up a transistor radio for some years now. Careful printed circuit layout, and tricks like generating complementary/balanced signal pairs and routing them close together can obviously minimise the radiation from the local oscillator to the point where the shielded metal box is no longer necessary.
The point is that you have to know you need to be this careful,and you ought to read up on the tricks of the trade. Reinventing the wheel takes time and gives loads of opportunities to invent a whole clas of constant cross-section rollers that don't work as well.
The OP would find it easier to get a good layout on a four or six layer board with a buried ground plane or two, but a double-sided board should be practical, with careful layout.
It is rather difficult to keep the electronics of a radion more than a few metres away from its antenna.
On a sunny day (11 Jul 2006 03:33:50 -0700) it happened email@example.com wrote in :
Well, point is, it will not jam 'itself' (for all I know). You can jam an OTHER radio, try it, it is easy, works on FM too. Way of topic anyways, I have looked in Tek scopes with fast AD, not very special layout I could see, just keep it logical...
1G samples/second is normal these days. As soon as it is 'digital' many problems become simpler.
But anyways, maybe I am a bit old-fashinoned (say older), IMO some RF pre (selectivity and gain) before any digitising in radio makes some sense, especially for weak signals. I do notice a trend to connect the antenna directly to the AD chip ;-)
Have you considered undersampling? You could do any part of your band without needing any mixers and/or excessively high sampling rates. Covering whole band would require switchable anti-aliasing filters...
I don't have any information on the buffers, but the header you have probably runs out of steam at around a few MHz. If you have the schematic for your dev. board , send it to me (you can find my email via my profile) and I'll check the ratings on the buffers (if they exist). I'll consider this one of my good deeds[tm] for the week :)
A two layer PCB (as noted by the esteemed Bill) may be ok with careful layout. At a minimum, you'll have to have clean power to the A-D and the appropriate clocks/framing etc., and then run the actual data back across. Of course, keeping the signal to be sampled clean is an absolute must (pretty pointless if you don't).
What resolution did you want to get? There are a number of solutions at reasonably high speeds up to 24 bits (although they aren't cheap; say $30 to $100s per unit depending on features, and that's in 1k qtys)
How up are you on A-D theory and practise? A device may say 24 bits but you'll typically get an equivalent 21 bits or so due to the various errors and noise (some manufacturers are good about putting such things in their data sheets, others less so), and then there's some tricks of the trade in the layout to ensure you get the best out of the device (indeed, get anything useful at all).
The RF signal is going to be pretty low (to say the least) although some newer devices designed for the task can deal with it - in that case, you are looking at a different class of A-D with low noise PGAs on the front end. A wideband RF amp and perhaps some band shaping may be in order. There are simple solutions for that around, but it will increase the complexity of the design somewhat.
Perhaps a digitally controlled mixer / IF stage might be a solution; after all, you can do all sorts of band selection in hardware, and that can be set up for the band you desire.
A lot of this will depend on exactly what you really need, of course. If you want to do pre-amplification and selection, life becomes much simpler (use a single IF output stage set by a mixer perhaps) that is then sampled.
If you try to do a 2-layer PCB for high-speed ADC/DAC you are on your own with zero support from manufacturer. The chip pinouts are designed in the assumption of existence of GND and PWR planes. I don't beleive one can get advertised performance of a high-speed ADC/DAC using a 2-layer board. One will be lucky if it will function at all...
There no ADCs on the market with more than 14 bits that can cover the requested band of 450 MHz. If oversampling is a requirement then you won't even find a 12 bit device that can do it. Perhaps 10, or 8 for sure, but not better.
That was a 'wish'. I said 'reasonably high speeds' simply because the OP seems to be open to suggestions. That said, I haven't seen better than 10 bit resolution at 450 MHz (which, unless you are undersampling, implies almost 1GS/s rates).