Variable clock using Virtex 4?

John,

Well, in Virtex 4, it is a simple matter to instantiate an accumulator with feedback, which is then a digitally controlled oscillator (a direct digital synthesizer). The frequency is from nearly 0 to 1/2 the clock at which the DDS is run with.

The resolution of the DDS is just how many bits you use. For example,

To generate an analog voltage, pulse width modulation, or pulse position modulation may be used with any output. A RC filter is needed to smooth out the result, and provide a nice clean analog signal. Rate multipliers are often used to spread out the corrections, and make the steps smaller in time (no long periods without any pulses at all) so that a small RC may be used.

Rise and fall time of the IO is one factor in causing error.

If the Vcco of the bank with that IO pin is referenced to a voltage standard, the accuracy of such a circuit may be as much as 14 bits, but it will not be very fast (D/A good up to perhaps ~ 10 KHz?). I would say that 8 to 10 bits of control would be all I would expect.

Austin

Virtex 4? If not clock, how about variable analog voltage?