Technical Discussion Topic

"DDS-60 Signal Purity"

I was intrigued with the "ratty-looking signal" being produced by the DDS-60 when running the wrong (DDS-30) software for it, either on the Micro908 or on the PIC-EL platforms.  Others had commented similarly, and the general thought was "gee, the only difference between these two chips from a programming sense is that upper bit of the control words that enables the PLL on the AD9851 to give a 6x reference clock.  So why aren't we see a nice-looking signal when keeping the fundamental frequency below 10 MHz, which is 1/3 the refclock of 30 MHz??"

Well, to accommodate an easy comparison, I added an experimental mode to the AA-908 configuration menu that selects the DDS chip that one has installed in his system.  The experimental mode allowed selection of the AD9851 *without* the PLL turned on so as to efectively use the 30 MHz clock oscillator as the main refclock for the DDS chip.  In fact, this experiment was also designed to let me see how "clean" the direct oscillator could be as the refclock, without the dreaded phase noise that is so often talked about with the use of PLLs.  (Note that this same experiment of turning off the PLL control bit could also be done on the PEGen software or the PIC-EL Test Suite program on the PIC-EL platform ... I just happened to be in the bowels of the AA-908 code and decided to do it there.)

Okay, so here's the simple observations as seen on a spectrum analyzer ...

1) With a fundamental of (say) 1 MHz, a pair of sampling artifacts are also seen at 29 MHz and 31 MHz, and another pair are seen at 59 MHz and 61 MHz.  The artifact pairs "converge" to 30 MHz and 60 MHz, respectively, when the frequency dial on the AA-908 reduces the signal to 0.  (Craig and I saw this "mirroring" while debugging the PIC-EL platform trying to control the DDS-60 card.)  These artifact pairs would been seen out farther and farther at 30 MHz spacings if it were not for the LPF on the DDS card rolling off at about 70 MHz, per design.  THIS is the reason for the "ratty" signals being seen on an oscilloscope in the time domain ... multiple frequencies contribute to distorting the nice sine wave we like to see coming out of the DDS cards.

2) The part that has me still stumped is that I do NOT see a clock signal (feedthru, or whatever) at 30 MHz, but I DO see a (small) constant one at 60 MHz.  In other words, the artifact pairs converge to an empty (signal-less) point at 30 MHz, yet the next pair converges to a small signal at 60 MHz. I'm sure the answer for this lies in the mathematics for the sampling ... but seeing the clock "feedthru" at 60 Mhz but not at 30 MHz is not intuitive.

3) The signal quality of the fundamental signal all by itself is very nice and clean ... there was no discernable difference between that "direct reference clock-generated" signal and a signal created by the 6x PLL. But the sampling artifacts when using the 30 MHz clock with a 70 MHz LPF will start becoming visible above 10 MHz (i.e., 1/3 of 30 MHz refclock), so this operating mode is really quite unusable.  One would need to move the LPF down to 10 MHz in order to use the 30 MHz oscillator as the 1x refclock for the AD9851.  (I commented out the selection of this 1x mode in the production release of the AA908 v5.0i code, but it is easy to re-enable by just uncommenting the two lines of code in the DDS Settings routine in Config_m.asm and then recompiling.)

4) As a final observation, I explored just *what* signal(s) I was seeing while running AD9850 code on the AD9851 DDS chip.  The 10 MHz signal commanded by the AA-908 upon initial power-up is actually interpretted as a 9 MHz signal by a DDS-60 daughtercard.  The differences are caused by two reasons: (1) the AD9850 software computes a control word based on using a 100 MHz refclk and the AD9851 is using a 30 MHz refclk; and (2) the different weighting/scaling of the displayed frequency digits when computing the actual control word (plus the PLL control bit being turned off, of course).  When turning the dial lower to approach 0 Hz, the signal (and its artifacts on the SA screen) reacted similarly to what I described above.

Anyway, I thought all this was interesting and perhaps others would find it so too.

73, George N2APB

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Page last updated: 
January 2, 2006