fpga complex digital signal generator direct digital synthesis and dsp

[ant17]

hi guys i have problem i want to build a fairly complex test instrument signal/ function generator and i want be able to achieve 1ghz output signal of square  wave sine wave and sawtooth and ramp and pwm  but i am finding in dds chips seem to only go up about 500mhz output frequency max due the nyquist frequency i was wondering if i would be better using Fpga if someone can advise i would be greatful thanks

[nctnico]

First you have to determine the output bandwidth and number of harmonics of non-sinusoidal signals you want to generate. Secondly: DACs are notorious for generating glitches when changing between code so that is something to look into when selecting a DAC (and it may not be specified!).

[ant17]

sorry what do you mean output bandwidth do you mean do say i want the output signal from 150hz to 1gzh

[rstofer]

No, he is saying that anything other than a pure sine wave is made up of various harmonics of the fundamental frequency.

If you want to have a square wave at 1 GHz, you probably want to include at least the 7th harmonic at 7 GHz.  Maybe even higher if you want a really square square wave.  If you want to think about it in the time domain, you need to be able to generate very sharp rising and fallling edges.

Play around with the series expansion for a square wave and graph it using a graphing calculator or Microsoft Mathematics (free).

There's a reason that commercial signal generators output a much higher frequency for the sine wave than they do for any of the others.

[ant17]

yes thankyou for that i will look into it but if i want a output of a 1ghz or more am i better of with a fpga which i can modify to my requirements or just a off the shelf dds chip

[exmadscientist]

Neither is going to do the job!

Commercial "DDS" chips do not go up to 1GHz square waves for very good reasons. Unless and until you can identify those reasons, you're wasting your time.

[ale500]

Maybe you only need say 500 MHz and 1 GHz square waves, and nothing in between: two fixed oscillators would do.
"I wish I had every possible frequency between 500 and 100 MHz" may not be even required... think about what you need, not what you wish you need

[ant17]

 are spurs on the dac signal the main problem because i believe some dds chip have spur killer or attenuators

[Kalvin]

Of course you can use an FPGA for that. You just have to think in parallel world. Eight DDS blocks running in parallel and interleaved fashion, running at 250MHZ each can provide sample rate of 2GHz. The system will require quite a few output pins, though. The PCB layout will be also challenging. Just an idea.

[ant17]

thanks for all your great suggestions
about how many logic blocks  or how many pin device would you need to create that type of thing

[XFDDesign]

The problem statement is not well defined here.

What are you trying to achieve?
What do you have to start with?
What are the constraints between what you have to start with, and your end goal?

[ant17]

hi i need to build a accurate  signal generator that goes up to 2GHz sine wave and a adjustable ramp pulse to use as a time mark generator and adjustable square wave with pulse width adjustment

[dmills]

The usual approach to this sort of thing is to start with a good sine wave source, maybe a DDS or such, then mix and filter it to get where you want to be.

Once you have the sine, you can use fast comparators (Pericom, Hittite and such people) to square it up for a square output.

It is important to do the reconstruction filtering before you feed whatever circuit generates your square wave, as otherwise you will have massive jitter.

Generate say DC - 400MHz with a DDS directly.
Lowpass the snot out of it.
Mix with a 300MHz XO and highpass to get 400MHz - 700MHz (DDS tunes from 100Mhz to 400MHz, leaving plenty of space to the filter skirts).
That output could then be mixed with say  600MHz or so to get you 700Mhz - 1.3G.... Well you get the idea.

Mixing is better then multiplying because it hurts your PN numbers less.

I don't really see a need for a FPGA here, a micro and some mixture of Analog devices and Minicircuits stuff should get it done for the sine wave, with some added Hittite or such for the fast square wave. 
 
Note that a decent square wave needs about 10 harmonics or so, so a 200MHz 'square' wave will have components you need to preserve out to ~2GHz, a 2GHz square wave is into that space where the instruments to measure it in the time domain are the price of a nice house.

HP (Keysight) were always the gods of this stuff and back in the day were not at all shy about publishing some very interesting papers and block diagrams.

73 Dan.

[nctnico]

For a high frequency square wave you can also use an ADF4350 or ADF4351.