I'm seeing something strange which has me puzzled... If, when on 6 meters, I turn Spur Reduction OFF (and set my Clock Offset to '0'), I see a picket-fence of spurs whose spacing becomes smaller and smaller as I tune until, at 50.011625 MHz, the picket-fence disappears altogether and the spectrum on the panadapter looks spur-free (except for an itsy-bitsy one at -108dB/-8200 Hz).
I'm assuming the IF frequency is 11625 Hz, so my tuning frequency at this point would actually correspond to the DDS IC set to exactly 50 MHz, (i.e. an FTW of 0x400000000000 for the 9854). What's strange is that, if I turn Spur Reduction ON, as I tune around this frequency I *never* see a Spectrum that looks as clean as the one that I see at 50.011625 MHz with Spur Reduction OFF. In other words, it almosts seems that the DDS's FTW is never set to 0x400000000000, otherwise, I would expect to see a "tuning block" that's about 3052 Hz wide that has this clean spectrum, but...I never seem to see it. Just wondering if anyone can help explain what I'm seeing (or not seeing). Thanks & 73, - Jeff, WA6AHL P.S. Even with spur reduction ON, there are quite a few spurs in this region of 6 meters (ditto on 10). Per my understanding of DDS chips, even though the phase-accumulator is hitting its marks with no phase "remainder" bits (i.e. only the top 4 nibbles of the 9854's 48-bit word are non-zero), quantization introduced by the phase-to-amplitude converter (e.g. the 12-bit approximation of Sin(0.0054931640625 degrees) ) also creates spurs. Is this amplitude-quantization the source of the spurs that I see around 50.011625 MHz when I tune with Spur Reduction ON (rather than mixing-products or DAC non-linearities)? -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Robert McGwier Sent: Tuesday, December 06, 2005 10:23 AM To: Jiri Sanda Cc: flexradio@flex-radio.biz Subject: Re: [Flexradio] Interesting behavior when connected to a dummy load There is a lot of confusion here. Let me attempt to straighten it out. 1) The extra noise occurs (as the original note said) when SPUR REDUCTION IS <<<OFF>>>. This is because the hardware is tuned EVERY time the dial is changed. 2) Spur reduction ON, does NOT move the spur out of band. It does something quite different and clever. There are "good frequencies" where the spur generation which is caused by truncation of the phase word and by the number of bits that can be applied to the DAC are minimized. If you are not on the good frequencies the phase accumulator has fractional parts that are not exact values to give the DAC in the synthesizer. This leads to "walking on and off" an exact DAC value in the DDS. This "walking on and off" of the good points is a periodic process and and because the amplitude and phase are just a little off when you are not exactly on a table phase (DAC bits are nonzero but the rest are zero), this little bit of amplitude and phase distortion generated spurs. Spur reduction recognizes this process and limits the HARDWARE DDS oscillator settings to these good frequencies. The remainder of the tuning is then done in the "perfect" software oscillator inside the code. So when spur reduction is ON, and the frequency request changes by 3051.7578125 Hz from one of these good frequencies, we move the hardware frequency only then. This approach has pluses and minuses but it is felt the pusses outweigh the minuses. You can still hear spurs from the DDS but these are due to DAC nonlinearities and clock leakage and mixing in the AD9854. These are almost gone in the AD9954 and really gone in the AD9958. Analog is learning along with the rest of us. Bob N4HY
