18-04-2013, 10:17 AM
Thanks Al, according to the way I read the specs on these DDS VFO's they seem to be precise and accurate, precise being that the frequency can be set (on some) down to Micro Hertz levels, I am guessing that any shift in accuracy would be down to variations in the clock frequency over time eg: thermal drift within the clock xtal.
It does not seem to matter if the clock xtals natural resonant frequency is out by a few ppm from it's designated frequency as that in itself does not determine the final frequency output from the DDS VFO once all the parameters have been entered, that is done digitally within the various registers within the chip.
It is only when the xtals frequency drifts that accuracy becomes affected.
So, if the manufacturer says that the DDS VFO clock xtals frequency is 100 Mhz + - 50 ppm it does not mean that the output from the DDS VFO is tied to that + - 50 ppm or any divisor of that tolerance.
That's my initial interpretation.
Anyone else read the guff on these devices?
I did find something on the web about a bloke who had one of these and covered it in foam panels to see how far it drifted over time due to temperature shift, I seem to remember that it was less than 1 Hertz over several hours, can't remember the test frequency but if I can find the page again I will post it.
Lawrence.
It does not seem to matter if the clock xtals natural resonant frequency is out by a few ppm from it's designated frequency as that in itself does not determine the final frequency output from the DDS VFO once all the parameters have been entered, that is done digitally within the various registers within the chip.
It is only when the xtals frequency drifts that accuracy becomes affected.
So, if the manufacturer says that the DDS VFO clock xtals frequency is 100 Mhz + - 50 ppm it does not mean that the output from the DDS VFO is tied to that + - 50 ppm or any divisor of that tolerance.
That's my initial interpretation.
Anyone else read the guff on these devices?
I did find something on the web about a bloke who had one of these and covered it in foam panels to see how far it drifted over time due to temperature shift, I seem to remember that it was less than 1 Hertz over several hours, can't remember the test frequency but if I can find the page again I will post it.
Lawrence.
