Arbitrary Waveform

[Altera_Forum]

Hi, 

 I need to improve the Linearisation of a VCO, For that i need to provide arbitrary waveform see attached between voltage 0 to 5VDC (it is like Triangle waveform but not the same).   I think to insert the values to a PROM (look-up table) and with D/A the CPLD/FPGA will produce the waveform.   Please advice.   Regards,   Doron

[Altera_Forum]

Yes, it surely works. To select the design details, a specification for resolution, accuracy, speed would be useful. You also didn't mention what's the source of the control signal.

[Altera_Forum]

--- Quote Start ---  Yes, it surely works. To select the design details, a specification for resolution, accuracy, speed would be useful. You also didn't mention what's the source of the control signal.  --- Quote End ---     The period of the waveform is 250uSec and it is cyclic. I would like that the Altera device will provide will provide this waveform or to put a D/A at the output of the device.   Can CPLD Do the the job ?   Please advice which simplified device will do the job ?   Thanks,   Doron

[Altera_Forum]

Hi, 

 if the voltage waveform is between 0VDC to +5VDC then lets calculate the resolution with 2^8:   5000mV/(2^8) = 19.5mV.   This the voltage Resolution.   Let me know if you need more data.   Thanks,   Doron

[Altera_Forum]

You will need a D/A converter on the board - FPGA only have digital output pins.

[Altera_Forum]

Yes, I took as granted, that an external DAC would be necessary with any programmable logic device. 8-Bit amplitude resolution defines the number of data bits, a second design parameter is the number of LUT entries. 

 A CPLD must implement a ROM in logic cells, most smaller CPLD won't have enough resources to represent an arbitrary look-up table. But the logic is minimized by the design compiler, in some cases it may be sufficient. I suggest to define the table coding and let Quartus try to fit it to different CPLD.   FPGAs have internal memory, that can be used as a ROM, so it's much easier with it. As another point, at low and medium speed, a PWM or SD modulator 1-Bit DAC can be used with a low-pass to generate the waveform.