Hi My assumption is that the DDSs run with a sample clock at 2x the 37 MHz. The chip is rated to go to 75 MHz on the sample clock input. That would give you a maximum clock output frequency of 75 / 2 = 37.5 MHz.
Normally you can't actually use an output that close to the Nyquist frequency and you are limited to something like 60 to 80% of the 37.6 MHz frequency. The limitation is based on the complexity of the filter you want to use after the output of the DDS and how much you object to the interference from the spur. Bob On Jan 24, 2010, at 11:37 AM, Stephan Schaa wrote: > > Hi Bob! > > I'm not a native english speaker, thats why I maybe not understand what you > ment?! > > > From what I know is: > > - you need (normally) double sampling rate from that frequency that you want > to sample (Nyquist, 60 MHz sampling rate for 30 mhz frequency) > > - in addition to the above said for a I/Q mixer you need two VCO frequencys > of the same frequency with a 90 degree shift. > (this is sometimes created by taking a frequency 4 times high than the rx > frequency and put it into a johnson counter to get, but there are sure > different ways of doing this, 2 DDS outputs with the shift , ... ) > > - you can do a trick in order if you haven't got a that high going frequency > clock that fits the nyquist frequency: take a single balanced mixer which > doen't attenuate lower harmonics that much and use the rx for that > frequencies. the problem is that in this case you are loosing some > information from the real signal because you are sampling only every second > time of the sine wave. > > > Stephan > >
