On 5/2/07, Michael Rigby-Jones wrote: > > > >-----Original Message----- > >From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] > >On Behalf Of Vasile Surducan > >Sent: 01 May 2007 18:16 > >To: Microcontroller discussion list - Public. > >Subject: Re: [PIC] [EE] Signal Generator > > > > > >On 5/1/07, Michael Rigby-Jones wrote: > >> > >> > >> >-----Original Message----- > >> >From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On > >> >Behalf Of PAUL James > >> >Sent: 30 April 2007 20:15 > >> >To: Microcontroller discussion list - Public. > >> >Subject: RE: [PIC] [EE] Signal Generator > >> > > >> > > >> > > >> > > >> > > >> >Or go to Analog Devices and get a DDS (Direct Digital > >> >Synthesis) generator chip. Add a high frequency master oscillator, > >> >and a microcontroller with keypad and LCD, And get any frequency > >> >between DC and master oscillator/2 in sub hertz increments. > >> > > >> >I've done this and it works great. > >> > >> > >> Unfortunately the output at Fosc/2 is a square wave, you really need > >> to have either a tracking filter or limit output frequency > >to a small > >> fraction of the master clock. > > > >That makes the spurious free dinamic range (SFDR) with a > >perfect second order low pass filter can be around > >-55...-60dB while without filter around -40dB. It's that enough ? > > Depending on phase, the output of a DDS generator at Fosc/2 could be DC, no low pas filter can fix that! Also implementing a tracking filter adds to the complexity, it's easier if the DDS clock can me made much higher than the maximum required output frequency. There is another trick: changing the clock frequency will produce different spurios for the same output frequency. Choosing the right clock (not necesarily the highest one) for the better SNR of the unfiltered output signal. On the AD site there is a simulator reproducing pretty well the real life. greetings, Vasile -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist