The DDS does in fact divide the clock down. Let me give an example. To make it easier for me to explain, let's take the case of outputting a square wave.
In this case the phase to amplitude lookup table has the first half of the table filled with ones and the second half filled with zeros. Now if you set the tuning word 'M' so that it jumps through the table twice the output is a square wave at the 1/2 the clock freq. If you set to tuning word so that it jumps through the table 4 times the output is 1/4 the clock rate and so on. Therefore the phase accumulator is actually a modulo-M counter that increments its value by 'M' each time it receives a clock pulse. The basic tuning equation is Fout = Fclk*(M/(2**n)), where M is the tuning word and n is the length in bits of the phase accumulator. Regards, k2ox -----Original Message----- From: richard allen [mailto:[EMAIL PROTECTED] Sent: Wednesday, January 04, 2006 5:39 PM To: John Ackermann N8UR Cc: [EMAIL PROTECTED]; FlexRadio@flex-radio.biz Subject: Re: [Flexradio] DIP OCXO for SDR Gentlemen, The DDS does Direct Digital Synthesis hence the name. It does not divide back to anything but runs an phase accumulation engine at 200 MHz (or whatever the clock rate) that produces output values to the dac at that rate. No division is performed. Richard W5SXD John Ackermann N8UR <[EMAIL PROTECTED]> wrote: (01/04/2006 16:09) >[EMAIL PROTECTED] wrote: >> First of all, any osc multiplied up widens its sidebands >> (phase noise) by the multiplication factor and the inverse is >> also true. The DDS does a 20X to 200MHz and a divide by 20 to >> get back to 10MHz. I guess it's academic at this point how >> much jitter is added by the DDS until someone measures it. > >You're only dividing back to 10MHz if that's the operating frequency. >At higher operating frequencies, the division doesn't equal the >multiplication (and of course, at lower ones it exceeds it). Measuring >the DDS output lets us see the phase noise where it counts, taking into >account both multiplication and division, rather than just at the >fundamental frequency of the reference. > >> Second, Rubidium standards are not intended to be used as local >> oscillators. They have terrible phase noise. They are intended >> to be used in timekeeping. It is their long term drift that >> excels, not short term phase noise. > >Agreed in general -- though there's a wide difference in performance >between different types of Rb; some use FM modulation of the xtal, which >results in horrible phase noise, while others, like the 5065A, don't. >My post wasn't suggesting that you use an Rb as the primary reference. >However, the DDS output when driven by 10MHz shows the effect of the >multiplication, which is all I was trying to do. My web page also has a >plot of the HP 5065A phase noise at 10MHz, so you can see the difference >in noise between the raw and multiplied frequencies. > >John > >> http://www.febo.com/geekworks/sdr1k/sdr1k_phase/index.html has >> screenshots that show the phase noise at the output of the DDS for the >> standard 200MHz oscillator, and an HP Rubidium frequency standard at >> 10MHz multiplied by 20 in the DDS. You can clearly see the phase-noise >> hit caused by the multiplication. >> >> John >> >> _______________________________________________ >> FlexRadio mailing list >> FlexRadio@flex-radio.biz >> http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz >> >> > > >_______________________________________________ >FlexRadio mailing list >FlexRadio@flex-radio.biz >http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz
